TRAFFIC INTO THE PREVACUOLAR ENDOSOMAL COMPARTMENT OF SACCHAROMYCES-CEREVISIAE - A VPS45-DEPENDENT INTRACELLULAR ROUTE AND A VPS45-INDEPENDENT, ENDOCYTIC ROUTE/

The vps (vacuolar protein sorting) mutants have been used to dissect a nd charaterize the vacuolar biogenesis pathway in the yeast Saccharomy ces cerevisiae. The vps mutants were isolated through their loss of ab ility to correctly sort the vacuolar hydrolase CPY, which travels from Golgi membranes to the vacuole through a prevacuolar compartment, Ove r 50 VPS genes have been divided into 6 classes according to vacuolar morphology, Mutations in any one of the class E VPS genes, such as VPS 27, lead to an exaggerated form of the prevacuolar compartment. This c lass E compartment contains endocytosed proteins as well as proteins e n route to the vacuole, and is thus taken to represent an intersection point between the endocytic and biosynthetic pathways. Mutations in t he class D gene VPS45 can be used to define a second transport interme diate along the vacuolar biogenesis pathway, Golgi-derived transport v esicles carrying vacuolar membrane proteins on their way to the vacuol e, Here we demonstrate that the Sec1p-like protein Vps45p is required for the fusion of Golgi-derived vesicles with the prevacuolar compartm ent indicating that VPS45 functions before VPS27 in the vacuolar bioge nesis pathway, In addition, we show that VPS45 function is not require d for the delivery of endocytosed proteins to the prevacuolar compartm ent from the plasma membrane suggesting that the function of Vps45p is restricted to a single vesicular pathway.