Monomeric G proteins

Alice Skoumalová

1. General features of chemical messengers - types of receptors

2. Endocrine, paracrine and autocrine actions

3. Examples of chemical messengers

Ion-channel receptors

Protein kinase receptors

Heptahelical receptors (G protein-coupled receptors)

G proteins = GTP binding proteins They bind GTP (active conformation) They regulate a wide variety of processes in the cell

Heterotrimeric G proteins

• 3 subunits (α, β, γ)

• α-subunit binds GTP, dissociates and interacts with an effector

Monomeric G proteins

= small G proteins or small GTPases

• bind GTP (active GTP-bound conformation), interact with effectors, promote various cellular responses

• catalyze the hydrolysis of the terminal phosphate group (inactive GDP-bound conformation)

GTP

Small GTPases• GDP/GTP-regulated molecular switches in signaling pathways („on“ and „off“)

• two-state structural change that is fundamental for their biological function:

GTP-protein „on“

GDP-protein „off“

Regulatory proteins:

GEFs (guanine nucleotide exchange factors): increase the rate of nucleotide exchange

GAPs (GTPase-activating proteins): increase the rate of the GTP hydrolysis

GDIs (guanine nucleotide dissociation inhibitors): prevent nucleotide exchange

Classification of small GTPases

• Over 150 mammalian proteins

• The „Ras superfamily“ related proteins, further divided into families:

Ras, Rab, Rho, Arf, Rad, Ran, Rheb, Rit

1. Farnesyltransferase (Ras): the covalent addition of a C15 farnesyl isoprenoid (F) lipid to the cysteine residue of the CAAX motif

2. Geranylgeranyltransferase I (Rho): the covalent addition of a C-20 geranylgeranyl isoprenoid (GG) lipid to the cysteine residue of the CAAX motif

3. Geranylgeranyltransferase II (Rab): the covalent addition of a C-20 geranylgeranyl isoprenoid (GG) lipid to the cysteine residue of the CC, CXC, CCX, CCXXX

Lipid post-translational modification (prenylation)

Guanine nucleotide exchange factors (GEFs)

• regulate activation of small GTPases and their downstream effects

• responsible for exchanging free cytosolic GTP for bound GDP

Diverse extracellular signals promote Ras activation by stimulating GEFs

• common CDC25 homology domain, catalyzes nucleotide exchange

• different GEFs can be activated by different signals

The mutated Ras proteins found in human cancers - insensitive to GAP stimulation - are persistently GTP-bound - deregulated effector activation

GTPase activating proteins (GAPs)

• accerelate the small GTPase hydrolysis• contain the GAP catalytic domain and additional sequences (involved in regulation)

Small GTPase structureH-Ras functional domains

1. The G domain: four sequences involved in GTP-binding and GTPase activity (structurally similar among small GTPases)

2. The CAAX motif: Ras and Rho

3. Core effector domain: effector binding

4. Switches I and II: change in conformation in the GDP- and GTP-proteins

5. RHO insert: additional sequences

Obrázek

Diverse functions of small GTPases

Various cellular functions:

1. Distinct GAPs and GEFs are regulated by different input signals

2. Various effectors resulting in many different cellular responses

Overview of small GTPase functions

Ras: regulators of cell proliferation and differentiation

Rab: regulators of membrane trafficking (the secretory and endocytic pahways)

Rho: regulators of actin cytoskeletal organization, which influences cell morphology and movement

Mutations that block GAP-stimulated GTP hydrolysis result in Ras proteins being locked in their active GTP-bound state (in various human tumors)

Ras GTPasesFunction: cell proliferation and differentiation

• The frequent mutations of Ras proteins in human cancers

Ras effectorsRas proteins serve as signaling nodes

1. Various extracellular signals converge on Ras proteins:

Growth factors (epidermal growth factor, platelet-derived growth factor)

Hormones (insulin)Cytokines (interleukin-1)The extracellular matrix proteins (via integrins)

2. Ras proteins regulate the activities of various effectors with divergent biochemical functions:

The Raf serine/threonine kinases (activate the Raf/Mek/Erk kinase cascade – the growth-regulatory functions)

The phosphatidylinositol 3-kinases (generate PIP3 – regulates various cascades)

Phospholipase Cε (generate DAG and Ca2+)GEFs of other small GTPases (small GTPase cascades – Ral,

Rac, Rab)

Rab GTPases(Ras homologue from brain)

Function: membrane trafficking

• the largest family of small GTPases (at least 60 members)

• localizing to specific organelles of the secretory and endocytic pathway

• ubiquitously expressed; but cell-type specific Rab3a (expressed in neurons) and Rab27 (expressed in melanocytes and platelets)

The cellular localization of Rab GTPases

Rabs and diseasesMutation in Rab genes-membrane trafficking alterations-diseases

Griscelli syndrome (an autosomal recessive disorder)• Pigmentation defects, immunological defects, neurological dysfunction• Mutations in the Rab27a gene (regulates the transport of melanosomes to the

periphery of melanocytes and the secretion of lytic granules in cytotoxic T lymphocytes) result in the loss of pigmentation and in the uncontrolled activation of T lymphocytes

Choroideremia (an X-linked disease)• Degeneration of the pigment epithelium of the eye• Defect in the REP-1 gene (geranylgeranyltransferase that is essential for the

prenylation of Rab27a in the retinal pigment epithelium) results in deficient melanosome transport in the pigment epithelium and lack of protection during light exposure

Rho GTPases

Function: Regulation of actin cytoskeletal organization (cell morphology and movement)

Modulation of various cellular processes:

1. Cell migration: regulation of the actin cytoskeleton, coordinated activation of Rho proteins

2. Secretion and endocytosis: vesicle trafficking

3. Proliferation and transformation: invasiveness of cancer cells

Rho GTPase Effectors Function in cell biology

RhoA, B, C ROCK I, II Actomyosin contractility,

cell migration

Rac1, 2, 3 IRSp53 Actin polymerization, lamellipodia formation, driving force in cell migration

Cdc42 WASP/N-WASP Actin polymerization, filopodia formation, sensor role in cell

migration

Model of effector activation by Rho GTPases

Cdc42-induced WASP activation promotes filopodia

Questions

1. The principle of function of small GTPases, the cycle, regulations, activations

2. The principle of lipid post-translational modification

3. The structure of small GTPases

4. The function of the Ras proteins, the results of Ras mutations in human carcinoma

5. The function of the Rho and Rab proteins