Unformatted text preview: titutive and regulated Bonifacino & Glick (2004) Cell 1The membrane-proximal coat components (blue) are recruited to the donor compartment by binding to a
(1) Initiation of coat assembly. 16:153 34 membrane-associated GTPase (red) and/or to a specific phosphoinositide. Transmembrane cargo proteins and SNAREs begin to gather at
the assembling coat. (2) Budding. The membrane-distal coat components (green) are added and polymerize into a mesh-like structure. Cargo
becomes concentrated and membrane curvature increases. (3) Scission. The neck between the vesicle and the donor compartment is severed
either by direct action of the coat or by accessory proteins. (4) Uncoating. The vesicle loses its coat due to various events including inactivation
of the small GTPase, phosphoinositide hydrolysis, and the action of uncoating enzymes. Cytosolic coat proteins are then recycled for additional
rounds of vesicle budding. (5) Tethering. The “naked” vesicle moves to the acceptor compartment, possibly guided by the cytoskeleton, and
becomes tethered to the acceptor compartment by the combination of a GTP bound Rab and a tethering factor. (6) Docking. The v- and
t-SNAREs assemble into a four-helix bundle. (7) This “trans-SNARE complex” promotes fusion of the vesicle and acceptor lipid bilayers. Cargo
is transferred to the acceptor compartment, and the SNAREs are recycled as shown in Figure 7B. The synapse is a major site of regulated secretion Synaptic vesicles 35 Frog Sartorius Neuromuscular Junction
36 Membrane tra c: Botulism and Tetanus
neurotoxins destroy SNARE proteins to block
neuronal exocytosis & neurotransmission Membrane tra c: “Botox” = Botulinum neurotoxin A presynapic cell’s cytoplasm
cytoplasm neurotransmi er presynaptic
membrane neurotransmi er
vesicle membrane cytoplasm presynaptic membrane
Open University EM Core Facility synaptic
cleft synaptic cleft
37 How are proteins kept in the ER?
1. Exclusion from transport vesicles to Golgi?
2. Retrieval from the cis-Golgi? ER retention signal was discovered by sequence gazing:
C-terminal KDEL in mammals
How was it experimentally veri ed?
And the KDEL receptor is in the cis-Golgi! What does this suggest
about the mechanism?
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