Genetic interaction between YPT6 and YPT1 in Saccharomyces cerevisiae.

Ypt6p, the yeast homologue of human RAB6, is required for protein trafficking at elevated temperatures. Biochemical data provide evidence that Ypt6p plays a role in an early step(s) of the secretory pathway: from ER to Golgi, or from cis to medial Golgi, or both. Here we show that overexpression of YPT1 suppresses the growth and secretion defects of a ypt6 temperature-sensitive (ts) strain. SLY1-20, encoding a dominant mutant allele that suppresses the lethal effect of YPT1, also suppresses the growth defect of a ypt6 ts strain. Conversely, SSD1, isolated as a suppressor of ypt6 ts, can suppress the growth defect of a ypt1 ts allele. These data suggest that Ypt6p has some redundant function with Ypt1p. However, overexpression of Ypt6p is toxic to a ypt1 ts strain, although it does not affect the growth of wild-type cells, suggesting that Ypt6p may sequester proteins shared with Ypt1p. This genetic evidence confirms the conclusion that Ypt6p is involved in an early step of the secretory pathway.