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Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, Bronx, New York 10461
A novel genetic selection was used to identify genes regulating traffic in the yeast endosomal system. We took advantage of a temperature-sensitive mutant in PMA1, encoding the plasma membrane ATPase, in which newly synthesized Pma1 is mislocalized to the vacuole via the endosome. Diversion of mutant Pma1 from vacuolar delivery and rerouting to the
plasma membrane is a major mechanism of suppression
of pma1ts. 16 independent suppressor of pma1 (sop)
mutants were isolated. Identification of the corresponding genes reveals eight that are identical with VPS
genes required for delivery of newly synthesized vacuolar proteins. A second group of SOP genes participates in vacuolar delivery of mutant Pma1 but is not
essential for delivery of the vacuolar protease carboxypeptidase Y. Because the biosynthetic pathway to the
vacuole intersects with the endocytic pathway, internalization of a bulk membrane endocytic marker FM 4-64 was assayed in the sop mutants. By this means, defective endosome-to-vacuole trafficking was revealed in a
subset of sop mutants. Another subset of sop mutants
displays perturbed trafficking between endosome and
Golgi: impaired pro-
factor processing in these strains
was found to be due to defective recycling of the trans-Golgi protease Kex2. One of these strains defective in
Kex2 trafficking carries a mutation in SOP2, encoding a
homologue of mammalian synaptojanin (implicated in
synaptic vesicle endocytosis and recycling). Thus, cell
surface delivery of mutant Pma1 can occur as a consequence of disturbances at several different sites in the
endosomal system.
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