Transmembrane cargo proteins and snares begin to

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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 presynaptic Ca2+ membrane synaptic cleft postsynaptic cell’s cytoplasm neurotransmi er presynaptic vesicles 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? Two models 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? 39 38...
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