Presentation on theme: "Mitochondria Plasma Membrane Nucleus Lysosome ER Golgi."— Presentation transcript:
Mitochondria Plasma Membrane Nucleus Lysosome ER Golgi
A variety of coat complexes participate in vesicle formation
COP-II Coat Components 1)GDP-Sar1p binds to Sec12p 2)GTP/GDP exchange 3)GTP-Sar1p anchors to membrane Sec refers to secretory mutants in yeast develop by Randy Scheckman. Coat Assembly
Monomeric G-proteins Regulate COP-II Coat Assembly Sar1 = ras-like G-protein Sec12 = Sar1-specific GEF Sec23 = Sar1-specific GAP GEF = guanine nucleotide exchange factor GAP = GTPase activating protein
COP-II Coat Components 1)GDP-Sar1p binds to Sec12p 2)GTP/GDP exchange 3)GTP-Sar1p anchors to membrane 4)Sec23p-Sec24p complex binds to GTP-Sar1p 5)Sec13p-Sec31p complex binds next Sec refers to secretory mutants in yeast. Coat Assembly
Vesicle Formation driven by coat assembly cargo is concentrated SNAREs implicated p24 family? ER resident proteins are excluded (Sec61) and/or retrieved (BiP, SNARE)
GTP-Sar1p converted to GDP-Sar1p following vesicle release activated by Sec23p GDP-Sar1p dissociates promotes coat disassembly uncoating exposes SNAREs (SNAP receptor) mediate docking and fusion 2 types: vesicle and target v-SNARE binds t-SNARE Transport Vesicles Uncoat and Dock with Destination Compartment
t-SNARE (=syntaxin family) 8 members in yeast all in different compartment (except 2 on plasma membrane) each binds specific v-SNARE (eg., Sed5p/Sft1p) rab checks fit between SNAREs monomeric G-protein GTPase locks complex SNAREs Determine Specificity of Vesicle Docking
NSF = NEM-Sensitive Fusion Protein (Sec18) Sec18 required at all steps in secretory and and endocytic pathways NSF binding requires cytosolic factor SNAP (Soluble NSF Attachment Protein) Membrane Fusion Machinery SNAP binds to v/t- SNARE complex NSF only binds to SNARE-SNAP complex activation of NSF associated ATPase fusion mechanism not known
vesicle formation at ER driven by COPII COPII vesicles fuse to form ERGIC (ER-Golgi Intermediate Compartment) aka VTC (Vesicular-Tubular Clusters) return of ER components?
COP-I vesicles responsible for retrograde transport KDEL signal (eg., BiP) analogous to COP-II
ARF1 (ras-like G-protein) + 7 COPs (coat proteins) coatomer (,, ',,,, and ) GTP-ARF1 binds to membrane anchored by myristic acid ARF1 receptor unknown brefeldin A (BFA) inhibits GEF membrane bound ARF1 recruits coatomer budding and vesicle formation GTP hydrolysis leads to dissociation of coatomer docking and fusion (SNARE, SNAP, and NSF) COP I Components and Assembly
COP-I also in Golgi originally ascribed to both anterograde and retrograde transport targeting dictated by SNARES BFA: loss of Golgi dilation of ER Golgi markers in ER rapidly reversible coats prevent premature fusion Golgi and beyond?
Problems with Vesicular Transport Model requires additional t-SNARES or mechanisms for COPI bidirectionality no evidence for anterograde movement of COPI vesicles resident Golgi proteins demonstrate gradient-like distribution across cisternae large structures like algal scales or procollogen precursors A recent rebirth of cisternae maturation model