SIGNAL TRANSMISSION WITHIN THE CELL

Nela Pavlíková

nela.pavlikova@lf3.cuni.cz

G-PROTEIN COUPLED RECEPTOR

- 7 transmembrane segments

G-protein

heterotrimer, composed of 3 different subunits: subunit ~ 33-55kD

- binding place for GDP / GTP

- intrinsic hydrolytic activity for GTP (it binds GTP = activation → hydrolyzing of GTP to GDP = deactivation)

- in deactivated state - it has bound GDP molecule

- it is associated with Gcomplex subunit ~ 35kD

subunit ~ 15kD

- creates Gcomplex

mammals: 20 different G proteins (each contains unique Gsubunit + one of 5 subunits and one of 12 subunits)

Gi

Gs

Gq

Gs

stimulates adenylyl cyclase → synthesis of cAMP → activation of protein kinase A (PAK) → phosphorylation of nearby substrates

cholera toxin: it keeps Gs permanently activated → ↑cAMP

Gi

after activation it inhibits adenylyl cyclase → ↓cAMP pertussis toxin: Gi subunit unable to release GDP → incapable of

activation → cannot inhibit adenylyl cyclase

cAMP is involved in regulation of many ion channels

Gq

not influenced by cholera toxin nor pertussis toxin phospholipase C → inositol-3-phosphate (IP3) → ↑Ca2+ in cell →

CaMKII

→ diacylglycerol (DAG) → protein kinase C both pathways lead to phosphorylation of nearby proteins

Gt

„transducin“ sensitive to pertussis toxin → incapability to release GDP →

inhibition of signalization

Vision: t1 – black-and -white

t2 – colors

occurrence: retina, some stem cells

effector: phosphodiesterase-6 (PDE6)

→ degradation of cGMP

Go

„other“ effectors not known sensitive to pertussis toxin → incapability to release GDP →

inhibition of signalization

occurrence: neural and endocrine tissues, mitotic spindle early development: effect of serotonin on neuron migration

G12/13

activation of REF („Rho guanine-nucleotide exchange factors“) → activation of Rho proteins

occurrence: ubiquitous

RECEPTOR TYROSINE KINASE

RECEPTOR TYROSINE KINASE

growth factors, cytokines, hormones 17 subfamilies of receptors dimerization of receptors necessary (even tetramers) only one transmembrane segment (N-end extracellular, C-end

intracellular) tyrosine phosphorylation = creation of binding domains for cytosolic

signaling proteins → binding → activation of signaling pathways cytosolic signaling proteins: e.g. Src, phospholipase C, PI3-kinase

domains for binding on phosphorylated tyrosines:

SH2 domain (src), PTB domain (PI3-kinase)

RECEPTOR TYROSINE KINASE

endocytosis of activated receptor: destruction of receptor in lysosome ← binding of ubiquitin molecule on

kinase endocytosed receptor signalization – e.g. NGF

RECEPTOR TYROSINE KINASE

Ras, Rho - monomeric GTPases

- anchored in the inner membrane

- a part of receptor tyrosine kinase signaling pathway

- activation: GEF (guanine nucleotide exchange factor)

- inhibition: GAP (GTPase activating factor)

↓

resistance → cancer

Ras → MAP kinase → signalization into nucleus (→ e.g. cell proliferation)

Rho → connect RTK with cytoskeleton (→ cell shape, motility, adhesion)

RECEPTOR TYROSINE KINASE

RECEPTOR TYROSINE KINASE

PI-3 kinase - Akt

RECEPTORS ASOCIATED WITH TYROSENE KINASE

JAK-STAT

RECEPTOR SERINE/THREONINE KINASE

ligand: e.g. TGF- SmaDs → regulatory protein endocytosis → activation

→ degradation

NUCLEAR RECEPTORS

their ligands can cross the cytoplasmic membrane on their own = transcription factors (in activated form they start transcription of

target genes) nuclear receptors activated by ligand vs. „orphan“ receptors

type I – receptor in cytosol → ligand binding → homodimerization

→ transport into nucleus → binding on „response element“ part of DNA → start of transcription

type II – receptor as heterodimer with RXR (and some corepressors) bound in nucleus on „response element“ → ligand into nucleus→ binds to receptor → dissociation of corepressors → start of transcription

answer on the question from the last seminar:synthesis of thyroid hormones