SEC2P MEDIATES NUCLEOTIDE EXCHANGE ON SEC4P AND IS INVOLVED IN POLARIZED DELIVERY OF POST-GOLGI VESICLES

The small GTPase Sec4p is required for vesicular transport at the post -Golgi stage of yeast secretion. Here we present evidence that mutatio ns in SEC2, itself an essential gene that acts at the same stage of th e secretory pathway, cause Sec4p to mislocalize as a result of a rando m rather than a polarized accumulation of vesicles. Sec2p and Sec4p in teract directly, with the nucleotide-free conformation of Sec4p being the preferred state for interaction with Sec2p. Sec2p functions as an exchange protein, catalyzing the dissociation of GDP from Sec4 and pro moting the binding of GTP. We propose that Sec2p functions to couple t he activation of Sec4p to the polarized delivery of vesicles to the si te of exocytosis.