MOP2 (SLA2) AFFECTS THE ABUNDANCE OF THE PLASMA-MEMBRANE H-ATPASE OF SACCHAROMYCES-CEREVISIAE()

The abundance of yeast plasma membrane H+-ATPase on the cell surface i s tightly regulated. Modifier of pma1 (mop) mutants were isolated as e nhancers of the mutant phenotypes of pma1 mutants, mop2 mutations redu ce the abundance and activity of Pma1 protein on the plasma membrane w ithout affecting the abundance of other prominent plasma membrane prot eins. The MOP2 gene encodes a 108-kDa protein that has previously been identified both as a gene affecting the yeast cytoskeleton (SLA2) (Ho ltzman, D.A., Yang, S., and Drubin, D. G. (1993) J. Cell Biol. 122, 63 5-644) and as a gene affecting endocytosis (END4) (Raths, S., Roher, J ., Crausaz, F., and Riezman, H. (1993) J. Cell Biol. 120, 55-65). In s ome strains, MOP2 (SLA2) is essential for cell viability; in others, a deletion mutant is temperature sensitive for growth. mop2 mutations d o not reduce the transcription of PMA1 nor do they lead to the accumul ation of Pma1 protein in any intracellular compartment. An epitope-tag ged MOP2 protein behaves as a plasma membrane-associated protein whose abundance is proportional to its level of gene expression. Over-expre ssion of MOP2 relieved the toxicity caused by the over-expression of P MA1 from a high copy plasmid; conversely, the growth of mop2 strains w as inhibited by the presence of a single extra copy of PMA1. We conclu de that MOP2 (SLA2) encodes a plasma membrane-associated protein that is required for the accumulation and/or maintenance of plasma membrane H+-ATPase on the cell surface.