RETROGRADE TRAFFIC OUT OF THE YEAST VACUOLE TO THE TGN OCCURS VIA THEPREVACUOLAR ENDOSOMAL COMPARTMENT/

A large number of trafficking steps occur between the last compartment of the Golgi apparatus (TGN) and the vacuole of the yeast Saccharomyc es cerevisiae. To date, two intracellular routes from the TGN to the v acuole have been identified. Carboxypeptidase Y (CPY) travels through a prevacuolar/endosomal compartment (PVC), and subsequently on to the vacuole, while alkaline phosphatase (ALP) bypasses this compartment to reach the same organelle, Proteins resident to the TGN achieve their localization despite a continuous flux of traffic by continually being retrieved from the distal PVC by virtue of an aromatic amino acid-con taining sorting motif. In this study we report that a hybrid protein b ased on ALP and containing this retrieval motif reaches the PVC not by following the CPY sorting pathway, but instead by signal-dependent re trograde transport from the vacuole, an organelle previously thought o f as a terminal compartment. In addition, we show that a mutation in V AC7, a gene previously identified as being required for vacuolar inher itance, blocks this trafficking step. Finally we show that Vti1p, a v- SNARE required for the delivery of both CPY and ALP to the vacuole, us es retrograde transport out of the vacuole as part of its normal cellu lar itinerary.