Suppressors of ssy1 and ptr3 null mutations define novel amino acid sensor-independent genes in Saccharomyces cerevisiae

Ssy1p and Ptr3p are components of the yeast plasma membrane SPS amino acid sensor. In response to extracellular amino acids this sensor initiates meta bolic signals that ultimately regulate the functional expression of several amino acid-metabolizing enzymes and amino acid permeases (AAPs). hs a resu lt of diminished leucine uptake capabilities, ssy1 Delta leu2 and ptr3 Delt a leu2 mutant strains are unable to grow on synthetic complete medium (SC). Genes affecting the functional expression of AAPs were identified by selec ting spontaneous suppressing mutations in amino acid sensor-independent (AS I) genes that restore growth on SC. The suppressors define 11 recessive (as i) complementation groups and 5 dominant (ASI) linkage groups. Strains with mutations in genes assigned to these 16 groups fall into two phenotypic cl asses. Mutations in the class I genes (ASI1, ASI2, ASI3, TUP1, SSN6, ASI13) derepress the transcription of AAP genes. ASI1, ASI2, and ASI3 encode nove l membrane proteins, and Asi1p and Asi3p are homologous proteins that have conserved ubiquitin ligase-like RING domains at their extreme C termini. Se veral of the class II genes (DOA4, UBA1, BRO1, BUL1, RSP5, VPS20, VPS36) en code proteins implicated in controlling aspects of post-Golgi endosomal-vac uolar protein sorting. The results from genetic and phenotypic analysis ind icate that SPS sensor-initiated signals function positively to facilitate a mino acid uptake and that two independent ubiquitin-mediated processes nega tively modulate amino acid uptake.