Protein Engineering of a Viral Cage forConstrained Nanomaterials Synthesis**

By Trevor Douglas,* Erica Strable, Deborah Willits,Abdelaziz Aitouchen, Matthew Libera, and Mark Young*

Biomimetic chemistry offers a new approach to nanomate-rials synthesis and assembly.[1±6] Protein cages,[7±10] and in par-ticular viral protein cages,[11,12] provide attractive systems asconstrained reaction environments. Viral protein cages occurin a wide variety of sizes and shapes and have structural tran-sitions that allow controlled access to the protein cage interi-or. We have demonstrated that assembled viral protein cagescan be altered by design to provide an organic scaffolding forinorganic nanomaterials synthesis constrained within the cagestructure. The selective nanomaterial synthesis relies on a spa-tial and electrostatic registry between the organic and theinorganic phases to initiate nucleation of the mineral. Alteringthe charge on the interior of the protein cage imparts a com-pletely new chemical reactivity. These modified cage struc-tures catalyze the oxidative mineralization of iron oxide nano-particles. This demonstrates the chemical plasticity of the viralprotein cage and its utility as a template for nano syntheticreactions. Furthermore, it is likely that altering, by design, thechemical nature of other protein cages will impart the capaci-ty to direct the synthesis of a broad range of nanomaterials.

The formation of solids in biological systems, biomineraliza-tion, has provided inspiration for the controlled formation ofnovel inorganic materials.[13] In biomineralization, self-organi-zation of organic-based templates, such as ferritin[14] or col-lagen, provides scaffolding for the assembly of inorganicmaterials. In addition, there exists a chemical and spatialinteraction between the organic and the inorganic phases. Wehave previously shown that self-assembled virus protein cages,devoid of their nucleic acid, can be used for the size-depen-dent encapsulation of non-native materials.[11,12] This host±guest relationship between the protein cage and the encapsu-lated material is based primarily on a complementary electro-static interaction. In the native viral protein cage the cationicinterior of the virus interacts with the polyanion of RNA. We

subverted this host±guest encapsulation and packaged other,non-native, polyanions. These included the mineralization ofnanoparticles of paratungstate and decavanadate that wereboth size- and shape-constrained by the dimensions of the vir-al protein cage. Also, anionic organic polymers can be encap-sulated within the protein, based on a strong electrostatic in-teraction with the protein interface.[11,12] In principle, self-assembled viral architectures (Fig. 1) can be functionalizedthrough protein design and genetic engineering to impart anew and specific functionality into a pre-existing architecturefor spatially constrained materials synthesis.

Fig. 1. a) Ribbon diagram of the 180-subunit protein cage of the cowpea chloro-tic mottle virus and b) cut-away view showing the central cavity of the proteincage.

In the present work, we have used the protein cage of thewell-characterized cowpea chlorotic mottle virus (CCMV;Fig. 1), devoid of nucleic acid, as the starting material forcreating a new cage with specific chemical functionality. Spe-cifically, one goal was to create a synthetic mimic of the ironstorage protein ferritin (L-chain), which sequesters a nanopar-ticle of iron oxide within a protein cage structure through spa-tially constrained oxidative hydrolysis of FeII.[14] Conceptually,we sought to alter the electrostatics of the protein cage interi-or surface while maintaining the overall architecture of theconstrained reaction environment. We assumed a minimal setof criteria for spatially constrained mineralization and usedthese criteria as the basis for protein design. This includes acage-like architecture to spatially constrain the size and shapeof the mineral particle, to create chemically or electrostati-cally distinct interior and exterior surfaces, and to achieve thepossibility for small molecules to access the surfaces. To dothis we needed to alter the recognition specificity of the guestmolecules, favoring the encapsulation of cationic species.

The CCMV protein cages are composed of 180 identical20 kDa subunits that self assemble into an empty virion hav-ing icosahedral symmetry (Fig. 1a). Structural analysis indi-cates the viral protein cage should allow access to small mole-cules through pores created at the protein subunit interfaces.This empty protein cage has an outer diameter of 28 nm andan inner diameter ranging from 18±24 nm[15] (Fig. 1b). Thus,CCMV has a cavity approximately twice the diameter of ferri-tin. Structural analysis of CCMV has demonstrated that thehighly basic N-termini (6 Arg, 3 Lys) project into the interiorof the protein cage. These 180 N-termini (a total of 1620 basicamino acids lysine or arginine, 9 from each of 180 subunits)

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±[*] Prof. T. Douglas

Department of Chemistry and Biochemistry, Montana State UniversityBozeman, MT 59717 (USA)E-mail: tdouglas@chemistry.montana.edu

Prof. M. Young, D. WillitsDepartment of Plant Sciences, Montana State UniversityBozeman, MT 59717 (USA)E-mail: myoung@montana.edu

E. StrableDepartment of Chemistry, Temple UniversityPhiladelphia, PA 19122 (USA)

A. Aitouchen, Prof. M. LiberaDepartment of Chemical, Biochemical, and Materials EngineeringStevens Institute of TechnologyHoboken, NJ 07030 (USA)

[**] This work was funded by a grant from the NSF. We thank Prof. J. E. John-son for permission to use the image in Figure 1 and Sue Brumfield for as-sistance with electron microscopy.

are required to package and condense the anionic RNA viralgenome. However, genetic analysis has demonstrated that theN-terminus is not required for the self-assembly of the emptyprotein cage in vitro. We explored the potential of using a vir-al protein cage for biomimetic design and nanomaterial syn-thesis. Conceptually, we wanted to demonstrate that we canalter the chemical properties of the cage through rational de-sign without disrupting the overall architecture of the cage.This has been accomplished by genetically engineering the N-terminus of the protein. The consequence is that the electro-static characteristics of the interior surface of the cage arechanged from cationic to anionic. This does not alter the over-all architecture of the cage. The altered electrostatic characterof the interior of the protein cage interface favors strong inter-action with ferrous and ferric ions, which promotes oxidativehydrolysis leading to size-constrained iron oxide formationwithin the viral protein cage.

The CCMV coat protein was genetically modified by re-placing 9 basic residues at the N-terminal with glutamic acid(subE mutant). The mutant assembled readily into a cage-likearchitecture similar to the wild type. This electrostatically al-tered viral protein cage catalyzed the rapid oxidation of FeII

leading to the formation of a spatially constrained iron oxidenanoparticle within the cage. The purified subE protein cageswere treated with aliquots of FeII at pH 6.5 and allowed tooxidize in air. In the presence of the anionic, empty, subEcage, the reaction proceeded to form a homogeneous orangesolution. In contrast, reactions in the presence of the wild typeempty protein cages resulted in the bulk precipitation of anorange solid. This was identical to bulk precipitation reactionsobserved in protein-free controls. The lack of precipitate inthe reactions containing subE, and the strong color present inthese solutions suggested that the oxidative hydrolysis of FeII

occurred in a spatially selective manner within the confines ofthe electrostatically altered protein cage of subE. Comparisonof initial rates as measured by the change in absorbance at420 nm shows that iron oxidation in the presence of subE isslightly faster than autooxidation in protein-free control reac-tions.

Mineralized subE preparations negatively stained with ura-nyl acetate (Fig. 2) demonstrated the intact protein cages. Theiron oxide cores can be effectively imaged in unstained miner-alized subE using high-angle annular dark field (HAADF)scanning transmission electron microscopy (STEM). Thehigher atomic number Fe gives rise to substantially more inco-herent high-angle electron scattering and hence provides forstrong HAADF image contrast between the mineral core andthe protein cage. Initiating the synthesis with a loading factorof 2000 iron atoms per protein cage resulted in the formationof spherical 8.2 ± 1.6 nm high atomic number cores (Fig. 2a).Performing a second mineralization on these premineralizedspecimens, such that the iron loading factor was increased to6000 iron atoms per cage, resulted in an increase of the aver-age particle size to 24.0 ± 3.5 nm diameter (Fig. 2b). This isconsistent with the upper limit of inner diameter of theCCMV virion.[15] The double mineralization process substan-

tially increases the amount of mineral contained within theprotein cage (histogram in Fig. 2c). High-resolution phase-contrast TEM imaging (Fig. 3) shows that the single-mineral-ized cores are single crystals. Filtered Fourier transforms fromsuch high-resolution images display 2mm symmetry consistentwith the lepidocrocite structure of c-FeOOH.[16] This is consis-tent with the bulk precipitate from control reactions, which

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Fig. 2. HAADF STEM images of single (a) and double (b) mineralized CCMV-derived protein cages and corresponding size-distribution histogram (c).The scale bars in (a) and (b) correspond to 100 nm. The inset figure (scalebar = 25 nm) shows a bright-field TEM image of a uranyl acetate negativelystained specimen.

Fig. 3. High-resolution TEM image of the single-crystal iron-oxide core of a sin-gle-mineralized CCMV-derived protein cage. The scale bar corresponds to 3 nm.

was identified as lepidocrocite on the basis of X-ray powderdiffraction.

Compositional mapping using spatially resolved electronenergy-loss spectroscopy (EELS) unequivocally defines thejuxtaposition of the mineral to the protein cage. Maps with1 nm spatial resolution were generated by spectrum imaging(Fig. 4). This approach digitally scans a focused electronprobe across a specimen region of interest. During the dwell

time at each pixel, an electron energy-loss spectrum is col-lected. The result of one such experiment from a single-miner-alized structure is presented in Figure 4. The HAADF imageshows the specimen region where the spectrum dataset wascollected as well as the adjacent region that was used to elec-tronically compensate for specimen drift during the roughlyten minute data acquisition time. The dataset was collectedfrom a specimen area 33 nm � 34 nm, which was just suffi-cient to span a single protein cage structure. Post-acquisitiondata analysis recovers a series of four images that map thespatial distribution of Fe, O, C, and N (Fig. 4, right). Brightcontrast corresponds to a high-count concentration for a par-ticular element. These maps indicate that the Fe and O areco-located in the center of the field, and the C and N are co-located in an annular fashion. The nitrogen signal is due tothe polypeptide cage and is an effective fingerprint to followthe spatial distribution of the protein. Superposition of the Fe(yellow) and N (blue) maps (Fig. 4, lower left) clearly showsthat the mineral is fully contained in the protein cage.

The composite nature of the mineralized subE protein cagewas additionally analyzed by gel filtration chromatographyand gel electrophoresis. Size-exclusion chromatography wasmonitored by either the absorbance due to the protein

(280 nm) or by absorbance due to the mineral particle encap-sulated within the protein (405 nm). The elution profile indi-cated co-elution of the protein cage and the mineral core fromthe column with a retention time identical to the empty subEprotein cage (Fig. 5). This co-elution indicates the composite(protein±mineral) nature of the product and also suggests thatthe overall structure of the protein cage has not been signifi-cantly perturbed by the synthesis. In addition, both the empty

subE protein cage and iron oxide mineralized protein cagewere electrophoretically separated on 0.3 % agarose gels un-der native (non-denaturing) conditions. Gels were stained foriron using Prussian blue or transferred to nitrocellulose forWestern blot analysis. As expected, only the mineralized subEsample was stained with Prussian blue (Fig. 5, inset). Theco-migration of the assembled subE protein cage in bothmineralized and empty subE samples indicated that themineralized virus remained intact.

We have changed the electrostatic nature of the inner pro-tein surface by up to 3240 units of charge. Despite the dra-matic change of 1620 cationic to 1620 corresponding anionicsites, the viral protein cage architecture is still stable. Ourmodel predicts that this change in the electrostatic nature ofthe interior surface allows the direct mineralization of transi-tion metal oxyhydroxides to be carried out. Gouy-Chapmantheory predicts that a surface with a net charge will aggregatecounter ions at the interface. The concentration of these coun-ter-ions decreases exponentially with distance until the bulkconcentration is achieved. This suggests that in the modifiedviral protein cage iron cations such as FeII will aggregate atthe protein interface. We speculate that this aggregation isenough to change the redox potential of FeII and to act as anucleation site by clustering FeII/III cations at the interface. Asin ferritin biomineralization, after nucleation, the initiallyformed crystallite can act as a catalytic site for further oxida-tive hydrolysis. This is consistent with our observation that theinitially formed iron oxide particles increase in size upon sub-sequent addition of further FeII. Therefore, the mineralizationprocess within the protein cage is autocatalytic for mineraliza-

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Fig. 4. Electron energy-loss spectrum imaging generates compositional maps ofFe and O from the mineral and C and N from the protein cage. Energy-loss datawere collected from the 33 pixel � 34 pixel spectrum image area indicated onthe HAADF STEM image (top left) with a 1 nm pixel size and a 500 ms pixeldwell time. Light contrast in the compositional maps (right) indicates highercount intensity. Superimposing the Fe and N maps (lower left, vertical bar =33 nm) indicates that the mineral core is contained within the protein shell.

Fig. 5. Size-exclusion chromatography of Fe mineralized subE CCMV (Super-ose 6). Elution was monitored at both 280 nm (protein) and at 405 nm (Fe-ox-ide mineral). Retention time for the mineralized and native protein cages werethe same. Inset: Native gel electrophoresis of subE CCMV on 0.3 % agarose.Western blot of a) unmineralized subE, b) Fe mineralized subE, and Prussianblue stain of c) unmineralized subE and d) Fe mineralized subE.

tion of iron oxide and affords a high level of control in thesynthesis of size-constrained particles.

The synthesized materials are both size- and shape- con-strained by the inner volume of the protein cage. Clearly,small molecules have access from the bulk to the interior ofthe protein cage through pores at the subunit interfaces. How-ever, oxidative hydrolysis selectively entraps the mineralproduct within the protein cage. This spatial isolation withinthe protein cage prevents bulk aggregation of the mineralparticles and results in a stable, mono-disperse colloid. Ourresults mimic the reactivity of ferritin-like proteins,[17,18] notall of which require the presence of an enzymatic ferroxidaseactivity for spatially selective mineralization.[19±21] Instead,they can rely on a highly charged interior protein interface toinduce nucleation. Successful synthetic reactions have utilizedelectrostatics to nucleate a range of non-native minerals with-in the ferritin protein cage.[7±10,22,23] It has also recently beenshown that the native protein cage of lumazine synthase willmineralize iron oxides in a biomimetic synthesis[24] analogousto the ferritin system. Our results suggest the potential for re-designing all of these protein cage systems for specific nano-materials synthesis. Creation of novel chemical environmentsutilizing the inherent host±guest properties of cage architec-tures provides a new direction for nanomaterials synthesis.

Experimental

Polymerase Chain Reaction (PCR)-Based Site-Directed Mutagenesis: Themutagenic oligonucleotide primer (5¢CAGTCGGAACAGGGGAATTAACT-GAAGCACAAGAAGAGGCTGCGGCCGAAGAGAACGAGGAGAA-CACGYGTGTGGTCCAACCTG3¢) and the CCMV coat protein cDNA cloneas a template were used to exchange all nine of the N terminal arginine andlysine codons for glutamic acid residues. This mutation in CCMV coat proteingene, termed subE, was confirmed by DNA sequencing.

Protein Expression: The mutagenized coat protein gene was expressed in aPichia pastoris heterologous protein expression system (to be described in de-tail elsewhere [25]). High levels of coat protein expression was induced andyielded assembled viral protein cages devoid of nucleic acid. These proteincages were purified to homogeneity by lysis of cells followed by either chroma-tography (ion-exchange and/or gel filtration) or by ultra centrifugation.

Iron Oxide Mineralization: Solutions of subE (0.5 mg mL±1, pH 6.51) wereincubated with (NH4)2Fe(SO4)2´6H2O (25 mM) at room temperature and al-lowed to air oxidize. Control reactions, protein-free or with an equivalentamount of native CCMV, were performed under identical conditions. Reactionswere monitored spectroscopically on a PE lambda 20 UV-vis spectrometer.

Electron Microscopy: Transmission electron microscopy data was collectedon a Philips CM20 TEM/STEM equipped with Gatan 666 PEELS and EMi-SPEC Vision system for spectrum imaging.

Received: October 24, 2001Final version: January 10, 2002

±[1] S. Mann, F. C. Meldrum, Adv. Mater. 1991, 3, 316.[2] S. Mann, D. D. Archibald, J. M. Didymus, T. Douglas, B. R. Heywood,

F. C. Meldrum, N. J. Reeves, Science 1993, 261, 1286.[3] W. Shenton, D. Pum, U. B. Sleytr, S. Mann, Nature 1997, 389, 585.[4] E. Braun, Y. Eichen, U. Sivan, G. Ben-Yoseph, Nature 1998, 391, 775.[5] S. R. Whaley, D. S. English, E. L. Hu, P. F. Barbara, A. M. Belcher,

Nature 2000, 405, 665.[6] G. Xu, I. A. Aksay, J. T. Groves, J Am Chem. Soc. 2001, 123, 2196.[7] F. C. Meldrum, V. J. Wade, D. L. Nimmo, B. R. Heywood, S. Mann,

Nature 1991, 349, 684.[8] F. C. Meldrum, B. R. Heywood, S. Mann, Science 1992, 257, 522.

[9] T. Douglas, D. P. E. Dickson, S. Betteridge, J. Charnock, C. D. Garner,S. Mann, Science 1995, 269, 54.

[10] T. Douglas, V. T. Stark, Inorg. Chem. 2000, 39, 1828.[11] T. Douglas, M. Young, Nature 1998, 393, 152.[12] T. Douglas, M. Young, Adv. Mater. 1999, 11, 679.[13] Biomimetic Materials Chemistry (Ed: S. Mann), VCH, Weinheim 1996.[14] N. D. Chasteen, P. M. Harrison, J. Struct. Biol. 1999, 126, 182.[15] J. A. Speir, S. Munshi, G. Wang, T. S. Baker, J. E. Johnson, Structure 1995,

3, 63.[16] Lepidocrocite, (c-FeOOH) JCPDS file 08±0098.[17] P. M. Harrison, P. Arosio, Biochim. Biophys. Acta 1996, 1275, 161.[18] M. Bozzi, G. Mignogna, S. Stefanini, D. Barra, C. Longhi, P. Valenti,

E. Chiancone, J. Biol. Chem. 1997, 272, 3259.[19] D. M. Lawson, P. J. Artymiuk, S. J. Yewdall, J. M. A. Smith, J. C. Living-

stone, A. Treffry, A. Luzzago, S. Levi, P. Arosio, G. Cesareni, C. D. Thom-as, W. V. Shaw, P. M. Harrison, Nature 1991, 349, 541.

[20] V. J. Wade, S. Levi, P. Arosio, A. Treffry, P. M. Harrison, S. Mann, J. Mol.Biol. 1991, 221, 1443.

[21] X. Yang, E. Chiancone, S. Stefanini, A. Ilari, N. D. Chasteen, Biochem.J. 2000, 349, 783.

[22] F. C. Meldrum, T. Douglas, S. Levi, P. Arosio, S. Mann, J. Inorg. Biochem.1995, 58, 59.

[23] T. Douglas, in Biomimetic Materials Chemistry (Ed: S. Mann), VCH,Weinheim 1996.

[24] W. Shenton, S. Mann, H. Colfen, A. Bacher, M. Fischer, Angew. Chem.Int. Ed. 2001, 40, 442.

[25] S. Brumfield, D. Willets, T. Douglas, M. Young, unpublished.

Low-Temperature Fabrication of Light-EmittingZinc Oxide Micropatterns Using Self-AssembledMonolayers

By Noriko Saito,* Hajime Haneda, Takashi Sekiguchi,Naoki Ohashi, Isao Sakaguchi, and Kunihito Koumoto

We have succeeded in the low-temperature (55 �C) fabrica-tion of ZnO micropatterns, which show patterned cathodolu-minescence images. A photopatterned, self-assembled mono-layer (SAM) with phenyl/OH surface functional groups wasused as the template. The selective, electroless deposition ofZnO was achieved on a Pd catalyst that had been adhered tothe phenyl surfaces only.

ZnO is one of the promising phosphor materials because ofits ability to retain a high efficiency, even at low-voltage exci-tation.[1] If micropatterns of it can be produced by a simpleprocess, then the technique for arranging ZnO phosphorshould enjoy widespread application, such as in a high-resolu-tion field emission display, which is a new, small-sized flatpanel display having higher resolution and better contrastcompared to the liquid crystal version. ZnO patterns are also

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±[*] Prof. N. Saito, Dr. H. Haneda, Dr. N. Ohashi, Dr. I. Sakaguchi

Advanced Materials LaboratoryNational Institute for Materials Science1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan)E-mail: Saito.Noriko@nims.go.jp

Dr. T. SekiguchiNanomaterials Laboratory, National Institute for Materials Science1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan)

Prof. K. KoumotoDepartment of Applied ChemistryGraduate School of Engineering, Nagoya UniversityFuro-cho, Chikusa-ku, Nagoya 464-8603 (Japan)