A LATE GOLGI SORTING FUNCTION FOR SACCHAROMYCES-CEREVISIAE APM1P, BUTNOT FOR APM2P, A 2ND YEAST CLATHRIN AP MEDIUM CHAIN-RELATED PROTEIN

Mammalian clathrin-associated protein (AP) complexes, AP-1 (trans-Golg i network) and AP-2 (plasma membrane), are composed of two large subun its of 91-107 kDa, one medium chain (mu) of 47-50 kDa and one small ch ain (sigma) of 17-19 kDa. Two veast genes, APM1 and APM2, have been id entified that encode proteins related to AP mu chains. APM1, whose seq uence was reported previously, codes for a protein of 54 kDa that has greatest similarity to the mammalian 47-kDa mu 1 chain of AP-1. APM2 e ncodes an AP medium chain-related protein of 605 amino acids (predicte d molecular weight of 70 kDa) that is only 30-33% identical to the oth er family members. In yeast containing a normal clathrin heavy chain g ene (CHC1), disruptions of the APM genes, singly or in combination, ha d no detectable phenotypic consequences. However, deletion of APM1 gre atly enhanced the temperature-sensitive growth phenotype and the a-fac tor processing defect displayed by cells carrying a temperature-sensit ive allele of the clathrin heavy chain gene. In contrast, deletion of APM2 caused no synthetic phenotypes with clathrin mutants. Biochemical analysis indicated that Apm1p and Apm2p are components of distinct hi gh molecular weight complexes. Apm1p, Apm2p, and clathrin cofractionat ed in a discrete reside population, and the association of Apm1p with the vesicles was disrupted in CHC1 deletion strains. These results sug gest that Apm1p is a component of an AP-1-like complex that participat es with clathrin in sorting at the trans-Golgi in yeast. We propose th at,Apm2p represents a new class of AP-medium chain-related proteins th at may be involved in a nonclathrin-mediated vesicular transport proce ss in eukaryotic cells.