Got1p and Sft2p: membrane proteins involved in traffic to the Golgi complex

Traffic through the yeast Golgi complex depends on a member of the syntaxin family of SNARE proteins, Sed5p, present in early Golgi cisternae, Sft2p i s a non-essential tetra-spanning membrane protein, found mostly in the late Golgi, that can suppress some sed5 alleles, We screened for mutations that show synthetic lethality with sft2 and found one that affects a previously uncharacterized membrane protein, Got1p, as well as new alleles of sed5 an d vps3. Got1p is an evolutionarily conserved non-essential protein with a m embrane topology similar to that of Sft2p, Immunofluorescence and subcellul ar fractionation indicate that it is present in early Golgi cisternae, got1 mutants, but not sft2 mutants, show a defect in an in vitro assay for ER-G olgi transport at a step after vesicle tethering to Golgi membranes. In viv o, inactivation of both Got1p and Sft2p results in phenotypes ascribable to a defect in endosome-Golgi traffic, while their complete removal results i n an ER-Golgi transport defect. Thus the presence of either Got1p or Sft2p is required for vesicle fusion with the Golgi complex in vivo. We suggest t hat Got1p normally facilitates Sed5p-dependent fusion events, while Sft2p p erforms a related function in the late Golgi.