Secretion mutants that block protein exit from the endoplasmic reticulum define genes involved in the formation, targeting and fusion of a small vesicle intermediate. SEC genes corresponding to the mutants defective in vesicle budding define the cytoplasmic machinery responsible for transport vesicle morphogenesis. A biochemical reaction that reproduces ER vesicle budding was reconstituted with gently-broken yeast cells and pure recombinant Sec proteins required in vivo for this budding event. The Sec proteins assemble on the ER membrane in the presence of GTP which activates a small GTPase, Sar1, initiating the formation of a coat protein complex called COPII.
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