mhc i assembly and peptide editing chaperones, clients

MHC I assembly and peptide editing — chaperones,clients, and molecular plasticity in immunityChristoph Thomas and Robert Tampe

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Peptides presented on MHC I molecules allow the immune

system to detect diseased cells. The displayed peptides

typically stem from proteasomal degradation of cytoplasmic

proteins and are translocated into the ER lumen where they are

trimmed and loaded onto MHC I. Peptide translocation is

carried out by the transporter associated with antigen

processing, which forms the central building block of a

dynamic assembly called the peptide-loading complex (PLC).

By coordinating peptide transfer with MHC I loading and

peptide optimization, the PLC is a linchpin in the adaptive

immune system. Peptide loading and optimization is catalyzed

by the PLC component tapasin and the PLC-independent

TAPBPR, two MHC I-dedicated enzymes chaperoning empty

or suboptimally loaded MHC I and selecting stable peptide-

MHC I complexes in a process called peptide editing or

proofreading. Recent structural and functional studies of

peptide editing have dramatically improved our understanding

of this pivotal event in antigen processing/presentation. This

review is dedicated to Vincenzo Cerundolo (1959–2020) for his

pioneering work in the field of antigen processing/presentation.

Address

Institute of Biochemistry, Biocenter, Goethe University Frankfurt,

Max-von-Laue Str. 9, Frankfurt, 60438 Main, Germany

Corresponding authors:

Thomas, Christoph ([email protected]),

Tampe, Robert ([email protected])

Current Opinion in Immunology 2021, 70:48–56

This review comes from a themed issue on Antigen processing

Edited by Scheherazade Sadegh-Nasseri

https://doi.org/10.1016/j.coi.2021.02.004

0952-7915/ã 2021 Elsevier Ltd. All rights reserved.

IntroductionNucleated cells of jawed vertebrates employ peptide-

MHC I (pMHC I) complexes on their surface to alert

the immune system about intracellular and extracellular

changes caused by pathogenic infection or malignant

transformation. Peptides bound to MHC I are recognized

by T-cell receptors (TCRs) on CD8+ (cytotoxic) T cells

and by natural killer (NK) cell receptors, allowing the

immune system to mount appropriate responses against


compromised cells without harming healthy cells [1,2].

The peptides displayed on MHC I are typically gener-

ated by proteasomal degradation of cytosolic proteins and

defective ribosomal products (DRiPs) and include even

spliced and post-translationally modified peptides [3].

The heterodimeric ATP-binding cassette (ABC) trans-

porter TAP1/2 (ABCB2/B3), which forms the cornerstone

of a dynamic, supramolecular assembly called the PLC,

selectively transports a small fraction of these peptides

into the ER where they can be trimmed by aminopepti-

dases such as ERAP1/2 to a length of 8–10 amino acids

that is optimal for fitting into the MHC I groove [4–6]

(Figure 1). In the ER, the peptides are loaded onto MHC

I complexes, which are heterodimers of an N-glycosy-lated, highly polymorphic heavy chain and the invariant

light chain b2-microglobulin (b2m). The loading of pep-

tides onto MHC I is catalyzed and subject to quality

control mechanisms: the MHC I-specific chaperones

tapasin (Tsn), a key component of the PLC, and

TAPBPR, a Tsn homolog that operates outside the

PLC, stabilize intrinsically unstable unliganded MHC I

complexes and function as peptide-exchange catalysts,

discriminating against pMHC I complexes that carry low-

affinity peptides [7–16] (Figure 1). By accelerating the

dissociation of suboptimal peptides, Tsn and TAPBPR

are also expected to facilitate peptide trimming by

ERAP1/2. Properly folded and kinetically stable pMHC

I complexes are eventually able to egress from the ER via

the Golgi compartment to the cell surface where they are

scanned by TCRs of cytotoxic T lymphocytes and by

killer cell immunoglobulin-like receptors (KIRs) of NK

cells [17] (Figure 1). It is important to note that peptide

editing by Tsn and TAPBPR is of crucial importance for

immunosurveillance, as it generates a repertoire of

pMHC I complexes that are stable enough to be scruti-

nized by immune cells and prevents the display of low-

affinity epitopes that would be exchanged for extracellu-

lar peptides and lead to a misrepresentation of the cellular

health status.

Besides the two pMHC I quality inspectors Tsn and

TAPBPR, several additional chaperones and folding

assistants shepherd MHC I molecules on their way from

translational birth to their final destination, the cell sur-

face: After the core N-glycan Glc3Man9GlcNAc2 is co-

translationally transferred to a conserved asparagine in the

MHC I heavy chain by oligosaccharyltransferase (OST)

and the outer two glucose residues are subsequently

cleaved off by glucosidase I and II, the resulting mono-

glucosylated dodecasaccharide is recognized by the

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MHC I chaperones in antigen processing Thomas and Tampe 49

Figure 1

Current Opinion in Immunology

Assembly and quality control of MHC I molecules and complexes. MHC I complexes are recruited to the PLC by the scaffolding chaperone

calreticulin (step 1) where the MHC I-dedicated chaperone tapasin stabilizes empty and suboptimally loaded MHC I complexes and facilitates

loading of high-affinity peptides in a peptide-rich environment generated by the ABC transporter TAP1/2 (step 2). The peptide-rich environment of

the PLC that tapasin is operating in is depicted by a red cloud. The tapasin homolog TAPBPR represents a second pMHC I quality inspector,

which accelerates peptide loading outside the PLC (step 3). In addition, TAPBPR aids the folding sensor and glucosyltransferase UGGT1 in

detecting and re-glucosylating peptide-deficient MHC I (step 4). This enables the mono-glucosylated MHC I to re-engage calnexin/calreticulin and

to (re-)visit the PLC in order to capture a suitable peptide (step 5). Stably loaded pMHC I complexes can leave the ER and migrate via the Golgi

compartment to the cell surface (step 6) where they are scanned by cytotoxic T cells (step 7) and natural killer cells (step 8).

scaffolding lectin chaperone calnexin (Cnx), which

recruits the thiol oxidoreductase ERp57 [18,19]

(Figure 1). In general, ERp57 supports folding of glyco-

proteins by facilitating disulfide bond isomerization. After

binding of b2m and dissociation of Cnx, the inherently

labile empty MHC I is escorted by calreticulin (Crt), the

soluble homolog of Cnx. In the next step, Tsn-dependent

allomorphs assemble together with Crt, Tsn-ERp57, and


TAP into the PLC, with TAP1/2 forming the central

assembly platform [1,20] (Figure 1). Through its action as

a peptide transporter with pronounced substrate specific-

ity, TAP creates an environment that is characterized by a

high local concentration of peptides suitable for Tsn-

catalyzed MHC I loading within the PLC. Important

insights into the conformational dynamics of the antigen

translocation complex within the PLC have been gained


50 Antigen processing

by investigating the bacterial ABC transporter TmrAB,

which can serve as a model system for TAP, as the two

transporters share structural as well as functional homol-

ogy [21]. Recently, nine different high-resolution cryo-

EM structures allowed us to dissect the full conformation

cycle of TmrAB. In conjunction with single-turnover

studies that delineated the power stroke of substrate

transport, these structures substantially enhanced our

mechanistic understanding of peptide-translocating

molecular machines [22��,23�]. Those pMHC I com-

plexes, which are properly folded and charged with a

high-affinity cargo when their remaining glucose moiety

is removed by glucosidase II, are licensed to exit the ER

and travel through the Golgi apparatus to the cell surface

(Figure 1).

In addition to Tsn, TAPBPR operates downstream of the

PLC in peptide-deficient environments as a second pMHC

I quality inspector. TAPBPR is not restricted to the ER but

alsofoundintheGolgicompartment.Initsactionasasecond

quality inspector, TAPBPR likely cooperates with UDP-

glucose:glycoprotein-glucosyltransferase 1 (UGGT1).

UGGT1 is the major folding sensor of glycoproteins in

theERand cis-Golgi. It recognizes incompletefoldingstates

of its client proteins and reglucosylates their Man9GlcNAc2glycan. This crucial decision point prevents misfolded

proteins from leaving the ER by enforcing a re-engagement

with Crt/Cnx and associated chaperones. In the case of

incompletely folded MHC I complexes, reglucosylation by

UGGT1 allows them to (re-)visit the PLC in order to be

loaded with a suitable peptide. Interestingly, TAPBPR has

recently been demonstrated to aid UGGT1 in detecting

peptide-deficient MHC I [24] (Figure 1). Thus, TAPBPR

supports pMHC I maturation in two different ways: first, by

directly acting as a peptide editor, and secondly, by pro-

moting UGGT1-catalyzed reglucosylation and recycling of

the MHC I to Tsn in the peptide-rich environment of the

PLC, if loading in the peptide-scarce environment

TAPBPR operates in is not successful.

In this review, we present the latest insights into the inner

workings of the PLC, the mechanism of peptide editing

by the two MHC I-dedicated chaperones Tsn and

TAPBPR, including the significance of molecular plas-

ticity in this process. These insights have been gained by

single-particle cryogenic electron microscopy (cryo-EM)

and X-ray crystallography of the native PLC and

TAPBPR-MHC I complexes [25��,26��,27��], respec-

tively, but also by carefully designed nuclear magnetic

resonance (NMR) studies, biochemical investigations,

and cell biological experiments.

The PLC as a central hub in MHC I loading andquality controlThe PLC is a supramolecular assembly consisting of

TAP, a heterodimeric type IV ABC peptide transporter

[4,28–30], the disulfide-bridged conjugate of Tsn and


ERp57, and Crt-bound MHC I [5]. The PLC is embed-

ded in the ER membrane and represents a crucial ele-

ment of the antigen presentation pathway by translocat-

ing short peptides from the cytosol into the ER and by

facilitating peptide loading and proofreading on MHC I.

To hide from the immune system, several DNA viruses

such as Herpesviridae and Poxviridae block effective

immunosurveillance by targeting PLC components.

The immune evasion strategies range from TAP inhibi-

tion to induced degradation of TAP and Tsn [31��,32].For cryo-EM studies, the immune evasin ICP47 of

human herpes simplex virus was utilized as a bait to

isolate native human PLC from a lymphoblastoid cell

line [25��]. Remarkably, the PLC preparation not only

contained HLA-A, HLA-B, and HLA-C, but also the non-

classical, oligomorphic HLA-E, HLA-F, and HLA-G

(and MR1, unpublished results) [25��]. The PLC cryo-

EM reconstructions showed that the fully assembled

complex is composed of two editing modules formed

by Tsn-ERp57 and MHC I-Crt, which are arranged along

a pseudosymmetric axis around the centrally positioned

TAP (Figure 2a). The two TAP subunits (TAP1/2)

possess an extra N-terminal transmembrane domain

called TMD0 that is known to serve as interaction

platform for the transmembrane helix of Tsn [33–35].

The central lumenal framework is built from the two

Tsn molecules, whereas Crt is anchored by its lectin

domain to the N-linked glycan of the MHC I and

interacts with ERp57 via the tip of its elongated, arm-

like, proline-rich (P)-domain (Figure 2b). The long C-

terminal a-helix of Crt contacts the membrane-proximal

IgC1 domain of Tsn. The arrangement of the two edit-

ing modules gives rise to a cavity-like structure above

the lumenal gate of TAP, which has two lateral openings

and might function as a reservoir for translocated pep-

tides (Figure 2a). Notably, subclasses of the cryo-EM

data uncovered asymmetric PLC particles containing

one incomplete editing module, with either MHC I,

Crt, or both being absent, reflecting the transient nature

of the PLC (Figure 2c). Metaphorically speaking, the

PLC might be regarded as a busy cargo loading station

where TAP brings in the supplies, Tsn is the loading

master, and peptide-receptive and peptide-loaded MHC

I complexes come and go. Based on the experimentally

determined reconstruction, the highly dynamic nature of

the PLC has been studied in a solvated lipid bilayer by

all-atom molecular dynamics (MD) simulations [36�](Figure 2c). The simulations on the microsecond time

scale confirm the architectural role of Tsn in forming a

rigid core together with MHC I, while the remaining

constituents are more flexible. Furthermore, the simula-

tions unveil that Crt dampens conformational dynamics

in Tsn and thereby facilitates MHC I recruitment into

the PLC. These results will serve as a base for future

studies on the allosteric crosstalk in this fascinating,

highly dynamic machinery and on the intriguing molec-

ular sociology during viral immune evasion.



Figure 2

ER membrane

cytosol

ER lumen

(a) (b)

Peptide-loading complex (PLC)

2m

MHC hc

Tsn

ERp57

Crt

TAP1

ICP47

peptide

cavity

TMD0 TMD0

(c)

Tsn

MHC I hcERp57

Crt

2m

view from ER lumen


Architecture of the peptide-loading complex (PLC). (a) Single-particle cryo-EM reconstruction of the PLC (PDB codes 6ENY and 5U1D; EM density

maps: EMD-3904-3906) [25��,29]. The fully assembled PLC comprises two editing modules that are arranged around the central ABC peptide

transporter TAP1/2. Each editing module consists of the scaffold chaperone calreticulin (Crt), the oxidoreductase ERp57, the MHC I-specific

chaperone tapasin (Tsn), and the MHC I heterodimer of heavy chain (hc) and b2-microglobulin (b2m). Neither the N-terminal transmembrane

domain 0 (TMD0) of TAP1 and TAP2 nor the transmembrane helices of Tsn and MHC I hc were visible in the cryo-EM map. (b) View from the ER

lumen onto the cryo-EM reconstruction of the PLC. The two Tsn molecules form the central lumenal framework, while Crt is anchored through its

lectin domain to the N-linked glycan of the MHC I hc and interacts with ERp57 via the tip of its arm-like proline-rich (P)-domain. (c) Assembly and

disassembly of the PLC. Peptide-deficient MHC I complexes are escorted by Crt to the asymmetric PLC to assemble into the symmetric PLC

comprising two editing modules. Cytosolic antigenic precursor peptides are translocated by TAP1/2 into a cavity formed by the two editing

modules. After diffusing into the ER lumen through two lateral openings, the peptides are trimmed by ERAP and loaded onto MHC I complexes.

Once the peptide-MHC I complexes have passed Tsn-catalyzed proofreading, they exit the PLC and are licensed to travel to the cell surface via

the Golgi apparatus. The panel figure was prepared using the coordinates of a MD simulation model of the PLC (ModelArchive DOI: https://doi.

org/10.5452/ma-whom2) [36�], and the program Illustrate [71]. The MD model is based on the experimental cryo-EM reconstruction shown in (a)

and (b) [25��] and experimentally determined structures of (homologous) subcomplexes and individual components (homologs), including the

TAPBPR-MHC I complex [26��,72–76]. The lipid bilayer is from the MemProtMD database (PDB code 6RAN) [22��,77].

www.sciencedirect.com Current Opinion in Immunology 2021, 70:48–56

https://doi.org/10.5452/ma-whom2

https://doi.org/10.5452/ma-whom2


Chaperone function and catalytic mechanismof Tsn and TAPBPRThe two MHC I-dedicated, IFNg-inducible chaperones

Tsn and TAPBPR exhibit 21% sequence identity in their

lumenal regions, which fold into an N-terminal composite

domain of a seven-stranded b-barrel and IgV structure,

and a C-terminal IgC1 domain [37]. While Tsn forms a

disulfide-bonded heterodimer with ERp57 and is inte-

grated into the PLC, the solitary TAPBPR performs its

tasks beyond the PLC and is also found in the Golgicompartment, due to the lack of an ER retention motif

[37]. Both Tsn and TAPBPR specifically recognize and

stabilize ‘frustrated’ MHC I species, that is, those MHC I

complexes which are peptide-deficient or loaded with a

suboptimal, low-affinity peptide. At the same time, Tsn

and TAPBPR enable MHC I complexes to swap peptides

more rapidly and sample out high-affinity ligands

[9–11,14,38,39]. The core principles underlying this chap-

erone and peptide exchange activity of TAPBPR and Tsn

were revealed by two crystal structures of the TAPBPR-

MHC I complex [26��,27��]. The structures show that the

N-terminal domain of TAPBPR clutches the a2-1 helix

region of the MHC I heavy chain, whereas the TAPBPR

C-terminal domain interacts with b2m and the a3 domain

(Figure 3a and b). The concave N-terminal interface

includes a b-hairpin, termed ‘jack hairpin’, which con-

tacts the floor bottom of the MHC I peptide-binding

groove and W60, a conserved tryptophan in b2m. Intrigu-

ingly, the conformation of W60 is sensitive to the loading

status of the groove [40,41], suggesting that the ‘jack

hairpin’ contributes to the ability of TAPBPR to sense

MHC I peptide occupancy. Another structural element of

Figure 3

(a) (b)

Crystal structure of the complex between the MHC I-specific peptide proofr

code 5OPI). (a) and (b) Overview of the TAPBPR-MHC I complex, highlighti

TAPBPR is depicted in cartoon representation, whereas MHC I hc and b2m

contacts with its N-terminal composite b-barrel-IgV domain and its C-termin

hc, but also b2m. (c) View onto the chaperoned peptide-receptive MHC I bi

region.


TAPBPR, the so-called ‘scoop loop’, dips into the

F-pocket region of the MHC I [26��], where the C

terminus of bound peptides is anchored and whose role

in governing pMHC I stability has been established

[42,43] (Figure 3c). A highly conserved tyrosine (Y84)

of MHC I near the F pocket that coordinates the C

terminus of the peptide in the ligand-bound state is also

swung out of its normal position and coordinated by the

side chain of E105 in TAPBPR. The Y84-E105 pair could

be regarded as a switch promoting peptide exchange and

selection. Indeed, the glutamate is found in Tsn as well

and has previously been shown to contribute to catalysis.

The ‘scoop loop’ has only been modeled in one of the two

TAPBPR-MHC I complexes, but due to its position in a

key region of the binding groove, the ‘scoop loop’ has

been proposed to be involved in the proofreading activity

[26��]. This notion has been corroborated by several

studies using independent approaches [44��,45��,46��],with one of them highlighting the importance of a specific

leucine residue in the loop for peptide exchange and

showing how the physico-chemical properties of the F

pocket influence loop-mediated TAPBPR activity [44��].A second study demonstrated that the ‘scoop loop’ is

critical to chaperoning empty MHC I and functions as a

selectivity filter during peptide editing [45��]. Further-

more, the ‘scoop loop’ appears to function as a selectivity

filter in Tsn-catalyzed proofreading as well [46��]. The

recent all-atom MD simulations of the PLC signify that

the N-linked glycan of the MHC I affects the conforma-

tional dynamics of the Tsn ‘scoop loop’ and steers it

toward the peptide-binding groove [36�]. However, the

exact conformation of the shorter Tsn ‘scoop loop’ and its

(c)


eader and chaperone TAPBPR and peptide-deficient MHC I (PDB

ng the tight interaction between chaperone and the MHC I complex.

are shown with their solvent-excluded (‘Connolly’) surface. TAPBPR

al IgC1 domain not only the a2-1 region and a3 domain of the MHC I

nding groove, with the TAPBPR scoop loop positioned in the F-pocket



interaction with the MHC I in the context of the Tsn-

ERp57-MHC I complex remains to be determined exper-

imentally. A comparison of the TAPBPR-complexed

MHC I structures with structures of free, peptide-bound

MHC I discloses several conformational changes in the

complexed MHC I: a widening of the binding groove due

to a shift of the a2 helices, a downward movement of the

groove floor, as well as a positional change of b2m relative

to the heavy chain. NMR experiments support these

observations and confirm the concept that conformational

plasticity is a key property of proteins involved in MHC I

peptide loading and proofreading [47��,48–54,55��].Moreover, the NMR investigations indicate that different

sites of the peptide-binding groove communicate alloste-

rically, with strong coordination of an optimal peptide in

the N-terminal pocket being conveyed to the C-terminal

region, leading to TAPBPR dissociation. In essence, the

experimental evidence on catalyzed peptide loading and

editing collected so far suggests that TAPBPR and Tsn

are able to sense the MHC I loading status by sampling

structural elements that mediate high-affinity ligand

interaction. Editor binding stabilizes an ensemble of

high-energy MHC I conformations characterized by an

open binding groove that is associated with accelerated

peptide exchange. The presence of TAPBPR lowers the

affinity of peptides for the MHC I groove 100-fold [47��],and only high-affinity peptides are able to induce disso-

ciation of the editor by competing over critical peptide-

coordinating regions. As a result of the catalyzed editing

process, the MHC I-bound epitope repertoire is opti-

mized such that the pMHC I complexes at the cell surface

are kinetically stable enough to be scanned by the com-

binatorial repertoire of patrolling T and NK cells.

Editor specificities and intrinsic selectorcapabilities of different allomorphsBoth Tsn and TAPBPR recognize empty or suboptimally

loaded pMHC I complexes, and, while some differences

in allomorph and peptide specificities exist [9], the two

editors share overall a common substrate preference and

binding hierarchy [56��]. In general, in its function as

chaperone, TAPBPR seems to exhibit a broader client

specificity than in its role as quality inspector of properly

folded pMHC I [55��]. In order to be recognized by

TAPBPR, pMHC I complexes have to be in an excited

state, and the corresponding conformation is adopted by

only 3–6% of the whole pMHC I population [55��].TAPBPR prefers HLA-A allomorphs, in particular mem-

bers of the HLA-A2 and HLA-A24 superfamilies, over

HLA-B and HLA-C representatives, with HLA-A*68:02

being the strongest binder reported so far [56��]. How-

ever, there are exceptions to the preference for HLA-A,

including A*01:01, A*11:01, A*36:01, A*26:01, and

A*68:01, for which no significant TAPBPR binding was

observed [10,56��]. As far as the general susceptibility to

proofreading is concerned, positions 114, 116, 147, and

156 in human MHC I allotypes have been demonstrated


to be key in governing chaperone dependence as they

determine the stability of empty MHC I [49,57,58�]. An

extreme case are the two allotypes B*44:02 and B*44:05:

they differ only at position 116 on the floor of the binding

groove, but while B*44:02 is Tsn-dependent, B*44:05 has

the intrinsic ability to select peptides independently of

Tsn [38,49,59��]. Some evidence suggests that the ability

of allomorphs to intrinsically select peptides is deter-

mined by their plasticity [49], while other data indicate

that the difference in Tsn dependence of the two B*44

allotypes is due to a more closed, stable conformation of

the F pocket region in the B*44:05 binding groove in the

peptide-deficient state [53,60]. A similar correlation

between stability of the F pocket region and Tsn depen-

dence has been described for B*27:05 and B*27:09 [61],

where B*27:05 is strongly associated with the disease

ankylosing spondylitis. These examples underline the

importance of protein dynamics in intrinsic and assisted

peptide selection on MHC I.

Intriguingly, the substrates of Tsn and TAPBPR are not

restricted to classical MHC I: Surface expression of HLA-

E is TAP- and Tsn-dependent [62,63], and HLA-E,

HLA-F, and HLA-G have been found to be associated

with the PLC [25��], with potential implications for the

immunosurveillance of viruses and vaccine development

[64]. Most remarkably, the two editors also stabilize ER-

resident, unliganded MR1 (MHC class I-related protein 1),

a non-classical MHC I that presents exogenous microbial

vitamin B metabolites as antigens (VitBAg) to innate-like

mucosal-associated invariant T (MAIT) cells [65��].Ligand association is unusual in the case of MR1, as VitBAg

forms a covalent Schiff base with a lysine residue in the

binding groove of MR1 [66,67]. While ligand-receptive

MR1in theER stronglyassociateswith Tsn, the interaction

with TAPBPR is weaker, but both chaperones appear to

cooperate in preventing MR1 degradation and increasing

surface expression [65��]. Notably, the role of Tsn and

TAPBPR in VitBAg presentation is not to facilitate loading

but to increase the half-life of empty MR1 in the ER, and

suitable ligands for MR1 are likely selected without the

help of an editor just by their ability to form the Schiff base

[65��].

ConclusionThe sophisticated cellular machinery of antigen proces-

sing and presentation provides a representative, MHC I-

displayed peptidome that allows for highly specific T cell-

mediated responses which are vitally important for adap-

tive immunity. During their life cycle, MHC I molecules

and complexes are diligently looked after by folding

helpers and conformational sensors, loading assistants,

and quality inspectors. At many stages, defined variants

of the N-linked glycan enable the MHC I clients to be

recognized by binding partners and to be dispatched to

proper cellular destination. The last few years have seen

tremendous progress in our understanding of mechanistic



and physiological principles governing peptide loading,

editing, and quality control on MHC I. It has become

apparent that Tsn and TAPBPR work in tandem in a

complementary fashion and that plasticity of the involved

protagonists is of utmost importance. The crystal struc-

tures of the TAPBPR-MHC I complex have unearthed

key elements of catalyzed peptide exchange, including

the jack hairpin, the Y84-E105 switch, and the scoop loop.

Yet, despite their individual importance, these elements

and other regions in the exceptionally large chaperone-

client interaction network have to interplay and crosstalk

allosterically in a concerted and integrative manner to

bring about the observed catalytic effect. In addition to

Tsn and TAPBPR, a fine-meshed net of ER/Golgi cha-

perones is concerned with pMHC I maturation, including

the major glycoprotein folding sensor UGGT1. The

advanced understanding of pMHC I maturation has

already paid dividends: TAPBPR can be utilized to load

immunogenic peptides onto cells and to generate pMHC

I tetramer libraries to identify antigen-specific T cells in

the development of new diagnostics and cancer immu-

notherapies [68�,69�,70]. We expect that additional fields

that aim to tailor antigen presentation for therapeutic

purposes will be able to leverage the deeper insights.

Declaration of interestNone.

AcknowledgementsWe thank Andrea Pott, Inga Nold, and all members of the Institute forBiochemistry for discussion and many helpful comments. This research wassupported by the German Research Foundation (CRC 807/P16 to R.T. andthe Reinhart Koselleck Project TA 157/12-1 to R.T.) and by an ERCAdvanced Grant (789121 to R.T.) of the European Research Council.

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12. Tan P, Kropshofer H, Mandelboim O, Bulbuc N, Hammerling GJ,Momburg F: Recruitment of MHC class I molecules by tapasininto the transporter associated with antigen processing-associated complex is essential for optimal peptide loading. JImmunol 2002, 168:1950-1960.

13. Fleischmann G, Fisette O, Thomas C, Wieneke R, Tumulka F,Schneeweiss C, Springer S, Schafer LV, Tampe R: Mechanisticbasis for epitope proofreading in the peptide-loadingcomplex. J Immunol 2015, 195:4503-4513.

14. Wearsch PA, Cresswell P: Selective loading of high-affinitypeptides onto major histocompatibility complex class Imolecules by the tapasin-ERp57 heterodimer. Nat Immunol2007, 8:873-881.

15. Natarajan K, Jiang J, Margulies DH: Structural aspects ofchaperone-mediated peptide loading in the MHC-I antigenpresentation pathway. Crit Rev Biochem Mol Biol 2019, 54:164-173.

16. Margulies DH, Jiang J, Natarajan K: Structure and function ofmolecular chaperones that govern immune peptide loading.Subcell Biochem 2019, 93:321-337.

17. Zaitoua AJ, Kaur A, Raghavan M: Variations in MHC class Iantigen presentation and immunopeptidome selectionpathways. F1000Res 2020, 9.

18. Kozlov G, Gehring K: Calnexin cycle - structural features of theER chaperone system. FEBS J 2020, 287:4322-4340.

19. Trowitzsch S, Tampe R: ABC transporters in dynamicmacromolecular assemblies. J Mol Biol 2018, 430:4481-4495.

20. Peaper DR, Wearsch PA, Cresswell P: Tapasin and ERp57 form astable disulfide-linked dimer within the MHC class I peptide-loading complex. EMBO J 2005, 24:3613-3623.

21. Noll A, Thomas C, Herbring V, Zollmann T, Barth K, Mehdipour AR,Tomasiak TM, Bruchert S, Joseph B, Abele R et al.: Crystalstructure and mechanistic basis of a functional homolog of theantigen transporter TAP. Proc Natl Acad Sci U S A 2017, 114:E438-E447.

22.��

Hofmann S, Januliene D, Mehdipour AR, Thomas C, Stefan E,Bruchert S, Kuhn BT, Geertsma ER, Hummer G, Tampe R et al.:Conformation space of a heterodimeric ABC exporter underturnover conditions. Nature 2019, 571:580-583

This work dissects the full functional cycle of the TAP homolog TmrAB bydetermining several high-resolution cryo-EM structures under turnoverconditions.

23.�

Stefan E, Hofmann S, Tampe R: A single power stroke by ATPbinding drives substrate translocation in a heterodimeric ABCtransporter. eLife 2020, 9

This single-turnover analysis of the TAP homolog TmrAB at single-lipo-some resolution provides quantitative and mechanistic insights intopeptide transport.

24. Neerincx A, Hermann C, Antrobus R, van Hateren A, Cao H,Trautwein N, Stevanovic S, Elliott T, Deane JE, Boyle LH: TAPBPRbridges UDP-glucose:glycoprotein glucosyltransferase 1 ontoMHC class I to provide quality control in the antigenpresentation pathway. eLife 2017, 6.

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Blees A, Januliene D, Hofmann T, Koller N, Schmidt C,Trowitzsch S, Moeller A, Tampe R: Structure of the human MHC-I peptide-loading complex. Nature 2017, 551:525-528


http://refhub.elsevier.com/S0952-7915(21)00006-6/sbref0005



















































































This study reveals the architecture of the peptide-loading complex (PLC)using cryo-EM and emphasizes the dynamic nature of this supramole-cular assembly.

26.��

Thomas C, Tampe R: Structure of the TAPBPR-MHC I complexdefines the mechanism of peptide loading and editing. Science2017, 358:1060-1064

Based on the X-ray structure of the TAPBPR-MHC I complex, this paperdescribes the fundamental principles of catalyzed peptide loading andediting.

27.��

Jiang J, Natarajan K, Boyd LF, Morozov GI, Mage MG,Margulies DH: Crystal structure of a TAPBPR-MHC I complexreveals the mechanism of peptide editing in antigenpresentation. Science 2017, 358:1064-1068

Based on the X-ray structure of the TAPBPR-MHC I complex, this paperdescribes the fundamental principles of catalyzed peptide loading andediting.

28. Thomas C, Aller SG, Beis K, Carpenter EP, Chang G, Chen L,Dassa E, Dean M, Duong Van Hoa F, Ekiert D et al.: Structural andfunctional diversity calls for a new classification of ABCtransporters. FEBS Lett 2020, 594:3767-3775.

29. Oldham ML, Grigorieff N, Chen J: Structure of the transporterassociated with antigen processing trapped by herpessimplex virus. eLife 2016, 5.

30. Oldham ML, Hite RK, Steffen AM, Damko E, Li Z, Walz T, Chen J: Amechanism of viral immune evasion revealed by cryo-EManalysis of the TAP transporter. Nature 2016, 529:537-540.

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Harvey IB, Wang X, Fremont DH: Molluscum contagiosum virusMC80 sabotages MHC-I antigen presentation by targetingtapasin for ER-associated degradation. PLoS Pathog 2019, 15:e1007711

This paper characterizes a novel mechanism of immune evasion by ahuman poxvirus.

32. Trowitzsch S, Tampe R: Multifunctional chaperone and qualitycontrol complexes in adaptive immunity. Annu Rev Biophys2020, 49:135-161.

33. Petersen JL, Hickman-Miller HD, McIlhaney MM, Vargas SE,Purcell AW, Hildebrand WH, Solheim JC: A charged amino acidresidue in the transmembrane/cytoplasmic region of tapasininfluences MHC class I assembly and maturation. J Immunol2005, 174:962-969.

34. Hulpke S, Tomioka M, Kremmer E, Ueda K, Abele R, Tampe R:Direct evidence that the N-terminal extensions of the TAPcomplex act as autonomous interaction scaffolds for theassembly of the MHC I peptide-loading complex. Cell Mol LifeSci 2012, 69:3317-3327.

35. Blees A, Reichel K, Trowitzsch S, Fisette O, Bock C, Abele R,Hummer G, Schafer LV, Tampe R: Assembly of the MHC Ipeptide-loading complex determined by a conserved ioniclock-switch. Sci Rep 2015, 5:17341.

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Fisette O, Schroder GF, Schafer LV: Atomistic structure anddynamics of the human MHC-I peptide-loading complex. ProcNatl Acad Sci U S A 2020, 117:20597-20606

This study presents an all-atom MD simulation of the PLC, providinginsights into the dynamics of this supramolecular assembly.

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41. Hee CS, Beerbaum M, Loll B, Ballaschk M, Schmieder P,Uchanska-Ziegler B, Ziegler A: Dynamics of free versus


complexed beta2-microglobulin and the evolution ofinterfaces in MHC class I molecules. Immunogenetics 2013,65:157-172.

42. Abualrous ET, Saini SK, Ramnarayan VR, Ilca FT, Zacharias M,Springer S: The carboxy terminus of the ligand peptidedetermines the stability of the MHC class I molecule H-2Kb: acombined molecular dynamics and experimental study. PLoSOne 2015, 10:e0135421.

43. Hein Z, Uchtenhagen H, Abualrous ET, Saini SK, Janssen L, VanHateren A, Wiek C, Hanenberg H, Momburg F, Achour A et al.:Peptide-independent stabilization of MHC class I moleculesbreaches cellular quality control. J Cell Sci 2014, 127:2885-2897.

44.��

Ilca FT, Neerincx A, Hermann C, Marcu A, Stevanovic S, Deane JE,Boyle LH: TAPBPR mediates peptide dissociation from MHCclass I using a leucine lever. eLife 2018, 7

These papers deliver insights into the role of a structural element in thechaperone and editing activities of TAPBPR and Tsn.

45.��

Sagert L, Hennig F, Thomas C, Tampe R: A loop structure allowsTAPBPR to exert its dual function as MHC I chaperone andpeptide editor. eLife 2020, 9


46.��

Hafstrand I, Sayitoglu EC, Apavaloaei A, Josey BJ, Sun R, Han X,Pellegrino S, Ozkazanc D, Potens R, Janssen L et al.: Successivecrystal structure snapshots suggest the basis for MHC class Ipeptide loading and editing by tapasin. Proc Natl Acad Sci U S A2019, 116:5055-5060


47.��

McShan AC, Natarajan K, Kumirov VK, Flores-Solis D, Jiang J,Badstubner M, Toor JS, Bagshaw CR, Kovrigin EL, Margulies DHet al.: Peptide exchange on MHC-I by TAPBPR is driven by anegative allostery release cycle. Nat Chem Biol 2018, 14:811-820

This NMR study delineates in detail the dynamics of the peptide loadingand proofreading process.

48. van Hateren A, Bailey A, Werner JM, Elliott T: Plasticity of emptymajor histocompatibility complex class I moleculesdetermines peptide-selector function. Mol Immunol 2015,68:98-101.

49. Bailey A, Dalchau N, Carter R, Emmott S, Phillips A, Werner JM,Elliott T: Selector function of MHC I molecules is determined byprotein plasticity. Sci Rep 2015, 5:14928.

50. Thomas C, Tampe R: Proofreading of peptide-MHC complexesthrough dynamic multivalent interactions. Front Immunol 2017,8:65.

51. van Hateren A, Bailey A, Elliott T: Recent advances in MajorHistocompatibility Complex (MHC) class I antigenpresentation: plastic MHC molecules and TAPBPR-mediatedquality control. F1000Res 2017, 6:158.

52. Natarajan K, Jiang J, May NA, Mage MG, Boyd LF, McShan AC,Sgourakis NG, Bax A, Margulies DH: The role of molecularflexibility in antigen presentation and T cell receptor-mediatedsignaling. Front Immunol 2018, 9:1657.

53. Garstka MA, Fritzsche S, Lenart I, Hein Z, Jankevicius G, Boyle LH,Elliott T, Trowsdale J, Antoniou AN, Zacharias M et al.: Tapasindependence of major histocompatibility complex class Imolecules correlates with their conformational flexibility.FASEB J 2011, 25:3989-3998.

54. Ayres CM, Abualrous ET, Bailey A, Abraham C, Hellman LM,Corcelli SA, Noe F, Elliott T, Baker BM: Dynamically drivenallostery in MHC proteins: peptide-dependent tuning of class IMHC global flexibility. Front Immunol 2019, 10:966.

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McShan AC, Devlin CA, Overall SA, Park J, Toor JS, Moschidi D,Flores-Solis D, Choi H, Tripathi S, Procko E et al.: Moleculardeterminants of chaperone interactions on MHC-I for foldingand antigen repertoire selection. Proc Natl Acad Sci U S A 2019,116:25602-25613

This paper thoroughly characterizes the interactions of TAPBPR withdifferent MHC I allomorphs to analyze allele specificity.

























































































































56.��

Ilca FT, Drexhage LZ, Brewin G, Peacock S, Boyle LH: Distinctpolymorphisms in HLA class I molecules govern theirsusceptibility to peptide editing by TAPBPR. Cell Rep 2019,29:1621-1632.e3

This study establishes a hierarchy of TAPBPR binding for a series of HLAclass I allotypes.

57. Park B, Lee S, Kim E, Ahn K: A single polymorphic residue withinthe peptide-binding cleft of MHC class I molecules determinesspectrum of tapasin dependence. J Immunol 2003, 170:961-968.

58.�

Badrinath S, Kunze-Schumacher H, Blasczyk R, Huyton T, Bade-Doeding C: A micropolymorphism altering the residue triad 97/114/156 determines the relative levels of tapasinindependence and distinct peptide profiles for HLA-A(*)24allotypes. J Immunol Res 2014, 2014:298145.

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Bashirova AA, Viard M, Naranbhai V, Grifoni A, Garcia-Beltran W,Akdag M, Yuki Y, Gao X, O’HUigin C, Raghavan M et al.: HLAtapasin independence: broader peptide repertoire and HIVcontrol. Proc Natl Acad Sci U S A 2020, 117:28232-28238

This paper reveals a relationship between tapasin dependence of HLAclass I allotypes, presented peptide repertoire, and AIDS progression inHIV patients.

60. Sieker F, Straatsma TP, Springer S, Zacharias M: Differentialtapasin dependence of MHC class I molecules correlates withconformational changes upon peptide dissociation: amolecular dynamics simulation study. Mol Immunol 2008,45:3714-3722.

61. Abualrous ET, Fritzsche S, Hein Z, Al-Balushi MS, Reinink P,Boyle LH, Wellbrock U, Antoniou AN, Springer S: F pocketflexibility influences the tapasin dependence of twodifferentially disease-associated MHC class I proteins. Eur JImmunol 2015, 45:1248-1257.

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64. Hansen SG, Wu HL, Burwitz BJ, Hughes CM, Hammond KB,Ventura AB, Reed JS, Gilbride RM, Ainslie E, Morrow DW et al.:Broadly targeted CD8(+) T cell responses restricted by majorhistocompatibility complex E. Science 2016, 351:714-720.

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McWilliam HEG, Mak JYW, Awad W, Zorkau M, Cruz-Gomez S,Lim HJ, Yan Y, Wormald S, Dagley LF, Eckle SBG et al.:Endoplasmic reticulum chaperones stabilize ligand-receptiveMR1 molecules for efficient presentation of metaboliteantigens. Proc Natl Acad Sci U S A 2020, 117:24974-24985

This study demonstrates that MR1 is chaperoned by Tsn and TAPBPR.


66. Kjer-Nielsen L, Patel O, Corbett AJ, Le Nours J, Meehan B, Liu L,Bhati M, Chen Z, Kostenko L, Reantragoon R et al.: MR1 presentsmicrobial vitamin B metabolites to MAIT cells. Nature 2012,491:717-723.

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Ilca FT, Neerincx A, Wills MR, de la Roche M, Boyle LH: UtilizingTAPBPR to promote exogenous peptide loading onto cellsurface MHC I molecules. Proc Natl Acad Sci U S A 2018, 115:E9353-E9361

This paper shows how TAPBPR can be applied to manipulate therepertoire of presented epitopes.

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Overall SA, Toor JS, Hao S, Yarmarkovich M, Sara MOR,Morozov GI, Nguyen S, Japp AS, Gonzalez N, Moschidi D et al.:High throughput pMHC-I tetramer library production usingchaperone-mediated peptide exchange. Nat Commun 2020,11:1909

This paper illustrates how TAPBPR can be utilized for generation of MHC Itetramer libraries.

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