Regulation of Small GTPases PDF

Regulation of Small GTPases cycle between the cytosol and membranes in the cytosol, the GTPases are bound to GDP and guanine nucleotide dissociation inhibitor (GDI) GTPases are recruited to the membranes by a GDI displacement factor (GDF) the binding to the membrane is through prenyl groups...

Regulation of Small GTPases cycle between the cytosol and membranes in the cytosol, the GTPases are bound to GDP and guanine nucleotide dissociation inhibitor (GDI) GTPases are recruited to the membranes by a GDI displacement factor (GDF) the binding to the membrane is through prenyl groups on the membranes, the GTPases are activated by a Guanine Nucleotide Exchange Factor (GEF) through the release of GDP and binding to GTP GTPases are inactivated by GTPase Activating Proteins (GAP) through GTP hydrolysis to GDP The Rab Family Rab effectors are proteins recruited by active Rab proteins localization is dictated by divergent C-terminal sequences Rabs and effectors collect membrane proteins into domains by regulating protein-protein interactions; they link molecules (e.g., SNAREs) to the vesicles vesicle formation, budding from the donor compartment, transport to the acceptor compartment, vesicle fusion, release of the vesicle content into the acceptor compartment after a vesicle fusion, Rabs hydrolyze their bound GTP inactive Rab–GDP complexes return to the cytoplasm The ADP Ribosylation Factor (Arf) Family GTP-Arf at donor membranes activate coat proteins; facilitate cargo sorting, vesicle formation, release formation of Arf-GDP dissociates the Arf–coat protein complex, allowing for vesicle fusion with acceptor membranes Arf1 controls the formation of COPI vesicles in retrograde transport (Golgi to ER), and clathrin vesicles at the trans-Golgi network Sar1 controls the assembly of the COPII-coated vesicles at the ER The Rho Family Cdc42, Rac1, RhoA Rho plays a role in endocytosis/exocytosis, adhesion and motility, cell proliferation, and death Rac1 contributes to the formation of actin meshworks that result in protrusive structures for cell spreading or motility Cdc42 promotes the formation of actin-rich filopodia The Ras Family “relay switches” in signaling ligand binding at some receptors leads to phosphorylation of Tyr on the receptors phospho-tyrosines are binding sites for adaptor proteins, SOS (a Ras-specific GEF) and Ras active Ras-GTP promotes the translocation of Raf serine/threonine kinase to the plasma membrane, where phosphorylation promotes full Raf kinase activation Raf phosphorylates/activates MEK1/2 kinase that phosphorylates/activates ERK1/2 kinase (referred to as the MAP kinase cascade) active ERK moves to the nucleus to regulate gene transcription The Ran Family molecules > 40 kD and mRNAs require carrier proteins for energy-dependent nuclear transport the energy is supplied by GTP hydrolysis and facilitated by importins and exportins (“cargo“ receptors) Ran is regulated by a Ran-GEF in the nucleus and a Ran-GAP in the cytosol review the nuclear export/import from the previous lectures Putting It Together the Ras superfamily consists of several families: Ras, Rho, Ran, Rab, Arf, and Rad small GTPases (remember Ras, Ran, Rab, Arf) small GTPases have an active membrane-linked GTP-bound state and an inactive cytosolic GDP-bound state Guanine Nucleotide Exchange Factors (GEFs) replace bound GDP with GTP and activate small GTPases; GTPase Activating Proteins (GAPs) stimulate the hydrolysis of the bound GTP to GDP and inactivate the small GTPases membrane anchorage of small GTPases is prevented by Guanine nucleotide Dissociation Inhibitors (GDIs) that mask the prenyl groups promoting membrane attachment Ras family promotes cell proliferation, Ran - nuclear transport, and Rab and Arf - vesicle transport Ras is an intermediary in the signaling pathway of some cell surface receptors; active Ras stimulates the MAP kinase cascade that results in specific gene transcription