Retrograde transport of the mannosyltransferase Och1p to the early Golgi requires a component of the COG transport complex.

The yeast COG complex has been proposed to function as a vesicle-tethering complex on an early Golgi compartment, but its role is not fully understood. COG complex mutants exhibit a dramatic reduction in Golgi-specific glycosylation and other defects. Here we show that a strain carrying a COG3 temperature-sensitive allele, cog3-202, clearly exhibited the glycosylation defect while exhibiting nearly normal secretion kinetics. Two Golgi mannosyltransferases, Och1p and Mnn1p, were mislocalized in cog3-202 cells. In cog3-202 cells Och1-HA was found in lighter density membranes than in wild type cells. In sed5(ts) and sft1(ts) strains, Och1p rapidly accumulated in vesicle-like structures consistent with the delivery of Och1p back to the cis-Golgi on retrograde vesicles via a Sed5p/Sft1p-containing SNARE complex. In contrast to cog3-202 cells, the membranes in sed5(ts) cells that contained Och1p were denser than in wild type. Together these results indicate that Och1p does not accumulate in retrograde vesicles in the cog3-202 mutant and are consistent with the COG complex playing a role in sorting of Och1p into retrograde vesicles. In wild type cells Och1p has been shown previously to cycle between the cis-Golgi and minimally as far as the late Golgi. We find that Och1p does not cycle via endosomes during its normal itinerary suggesting that Och1p engages in intra-Golgi cycling only. However, Och1p does use a post-Golgi pathway for degradation because a portion of Och1p was degraded in the vacuole. Most surprisingly, Och1p can use either the carboxypeptidase Y or AP-3 pathways to reach the vacuole for degradation.