Mutational analysis of Csc1/Vps4p: Involvement of endosome in regulation of antophagy in yeast

In the yeast Saccharomyces cerevisiae, autophagy, a bulk protein degradation in the vacuole, is induced in response to nutrient starvation. In a screen for mutations that result in induction of autophagy even in the presence of nutrients, we have isolated four mutants representing two csc complementation groups. These mutants induce autophagy of which activity is represented by activation of truncated alkaline phosphatase that is designed to be expressed in the cytosol. CSC1 was cloned by complementation of loss of viability phenotype of cscl-l mutant and shown to be identical to END13/VPS4/GRD13. Though cscl-l mutation is recessive, cells of Delta csc1 do not induce autophagy in rich media, suggesting that cscl-l allele is not a complete loss-of-function. Csc1p is a member of novel ATPase family named AAA protein including Sec18p/NSF, Cdc48p/p97, and Pas8p. Mutation site in cscl-l is found in the SRH region that is highly conserved among AAA proteins. Cells of cscl-l show sorting defect of CPY and the appearance of the class E compartment. These mutant phenotypes suggest the role of the protein that is involved in the traffic among the Golgi, endosome, and the vacuole in autophagy.