NPI1, AN ESSENTIAL YEAST GENE INVOLVED IN INDUCED DEGRADATION OF GAP1AND FUR4 PERMEASES, ENCODES THE RSP5 UBIQUITIN-PROTEIN LIGASE

When yeast cells growing on a poor nitrogen source are supplied with N H4+ ions, several nitrogen permeases including the general amino acid permease (Gap1p) are rapidly and completely inactivated. This report s hows that inactivation by NH4+ of the Gap1 permease is accompanied by its degradation. A functional NPI1 gene product is required for both i nactivation and degradation of Gap1p. Molecular analysis of the NPI1 g ene showed that it is identical to RSP5. The RSP5 product is a ubiquit in-protein ligase (E3 enzyme) whose physiological function was, howeve r, unknown. Its C-terminal region is very similar to that of other mem bers of the E6-AP-like family of ubiquitin-protein ligases. Its N-term inal region contains a single C-2 domain that may be a Ca2+-dependent phospholipid interaction motif, followed by several copies of a recent ly identified domain called WW(P). The Npi1/Rsp5 protein has a homolog ue both in humans and in mice, the latter being involved in brain deve lopment. Stress-induced degradation of the uracil permease (Fur4p), a process in which ubiquitin is probably involved, was also found to req uire a functional NPI1/RSP5 product. Chromosomal deletion of NPI1/RSP5 showed that this gene is essential for cell viability. In the viable npi1/rsp5 strain, expression of NPI1/RSP5 is reduced as a result of in sertion of a Ty1 element in its 5' region. Our results show that the N pi1/Rsp5 ubiquitin-protein ligase participates in induced degradation of at least two permeases, Gap1p and Fur4p, and probably also other pr oteins.