The coat protein complex II (COPII) generates transport vesicles that mediate protein export from the endoplasmic reticulum (ER). The first step of COPII vesicle formation involves conversion of Sar1p-GDP to Sar1p-GTP by guanine-nucleotide-exchange factor (GEF) Sec12p. In Saccharomyces cerevisiae, Sed4p is a structural homolog of Sec12p, but no GEF activity toward Sar1p has been found. Although the role of Sed4p in COPII vesicle formation is implied by the genetic interaction with SAR1, the molecular basis by which Sed4p contributes to this process is unclear. This study showed that the cytoplasmic domain of Sed4p preferentially binds the nucleotide-free form of Sar1p and that Sed4p binding stimulates both the intrinsic and Sec23p GTPase-activating protein (GAP)-accelerated GTPase activity of Sar1p. This stimulation of Sec23p GAP activity by Sed4p leads to accelerated dissociation of coat proteins from membranes. However, Sed4p binding to Sar1p occurs only when cargo is not associated with Sar1p. On the basis of these findings, Sed4p appears to accelerate the dissociation of the Sec23/24p coat from the membrane, but the effect is limited to Sar1p molecules that do not capture cargo protein. We speculate that this restricted coat disassembly may contribute to the concentration of specific cargo molecules into the COPII vesicles.
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