THE SACCHAROMYCES-CEREVISIAE APS1 GENE ENCODES A HOMOLOG OF THE SMALL-SUBUNIT OF THE MAMMALIAN CLATHRIN AP-1 COMPLEX - EVIDENCE FOR FUNCTIONAL INTERACTION WITH CLATHRIN AT THE GOLGI-COMPLEX

Clathrin-associated protein (AP) complexes have been implicated in the assembly of clathrin coats and the selectivity of clathrin-mediated p rotein transport processes. We have identified a yeast gene, APS1, enc oding a homolog of the small (referred to herein as sigma) subunits of the mammalian AP-1 complex. Sequence comparisons have shown that Aps1 p is more similar to the sigma subunit of the Golgi-localized mammalia n AP-1 complex than Aps2p, which is more related to the plasma membran e AP-2 sigma subunit. Like their mammalian counterparts, Aps1p and Aps 2p are components of distinct, large (>200 kDa) complexes and asignifi cant portion of the Aps proteins co-fractionate with clathrin-coated v esicles during gel filtration chromatography, Unexpectedly, even thoug h the evolutionary conservation of AP small subunits is substantial (5 0% identity between mammalian and yeast proteins), disruptions of APS1 (aps1 Delta) and APS2 (aps2 Delta), individually or in combination, e licit no detectable mutant phenotypes. These data indicate that the Ap s proteins are not absolutely required for clathrin-mediated selective protein transport in cells expressing wild typeclathrin. However, aps 1 Delta accentuated the slow growth and alpha-factor pheromone maturat ion defect of cells carrying a temperature-sensitive allele of clathri n heavy chain (Chc) (chc1-ts). In contrast, aps1 Delta did not influen ce the effects of chc1-ts on vacuolar protein sorting or receptor-medi ated endocytosis. The aps2 Delta mutation resulted in a sight effect o n chc1-ts cell growth but had no additional effects. The growth defect of cells completely lacking Chc was compounded by aps1 Delta but not aps2 Delta. These results comprise evidence that Aps1p is involved in a subset of clathrin functions at the Golgi apparatus. The effect of a ps1 Delta on cells devoid of clathrin function suggests that Aps1p als o participates in clathrin-independent processes.