ISOLATION AND CHARACTERIZATION OF SYS GENES FROM YEAST, MULTICOPY SUPPRESSORS OF THE FUNCTIONAL LOSS OF THE TRANSPORT GTPASE YPT6P

In Saccharomyces cerevisiae, the YPT6 gene encodes the homologue of th e mammalian Rab6 protein found in the Golgi apparatus, Deletion of YPT 6 in yeast produces a phenotype showing temperature-sensitive growth a nd partial missorting of the vacuolar enzyme, carboxypeptidase Y, To i dentify proteins that might: (1) interact with Ypt6p; or (2) act in th e same pathway, we have isolated four multicopy suppressors, named SYS 1, SYS2, SYS3 and SYS5, that can complement the temperature-sensitive growth phenotype of the ypt6 null mutant, On high expression, these ge nes are also able to partially suppress the missorting of carboxypepti dase Y. SYS2 on a multicopy plasmid suppresses in addition the tempera ture-sensitive phenotype of sec7-1, a mutant defective in transport be tween and from the Golgi compartment, Gene disruption of SYS1 and SYS2 did not result in significant growth defects, However, deletion of SY S1 and/or SYS2 in the ypt6 null mutant enhances defects in vacuolar pr otein sorting and in cell growth, Whereas protein secretion was not si gnificantly affected in these mutants, the processing of alpha-factor precursor by the Kex2 protease was inhibited, suggesting a function of YPT6 and its null mutant suppressors in transport between the late Go lgi and a prevacuolar, endosome-like compartment.