Isolation of a novel gene from Schizosaccharomyces pombe: stm1(+) encodinga seven-transmembrane loop protein that may couple with the heterotrimericG alpha 2 protein, Gpa2

A putative seven transmembrane protein gene, stm1(+), which is required for proper recognition of nitrogen starvation signals, was isolated as a multi copy suppressor of a ras1 synthetic lethal mutant in Schizosaccharomyces po mbe. Under nitrogen-deficient conditions, transcription of the stm1 gene wa s induced; deletion of stm1 was associated with early entry into G, arrest. Under nutritionally sufficient conditions, overexpression of Stm1 inhibite d vegetative cell growth, resulted in decreased intracellular cAMP levels, increased the expression of the meiosis-specific genes ste11, mei2, and mam 2, and facilitated sexual development in homothallic cells. However inhibit ion of vegetative cell growth and reduction of cAMP levels were not observe d in a deletion mutant of the heterotrimeric G protein G alpha2 gene, gpa2, that is responsible for regulating intracellular cAMP levels, a key factor in determining the sexual development in S. pombe. Stm1 protein was shown to interact with Gpa2 through its C-terminal transmembrane domains 5-7. Mut ation at Lys(199) in the C-terminal domain (stm1(K199A)) abolished the Stm1 overexpression effect on lowering cAMP levels. Induction of ste11, a meios is-specific gene transcription factor, by Stm1 overexpression was enhanced in gpa2-deleted cells but was absent in a deletion mutant of sty1, a key pr otein kinase that links mitotic control with environmental signals and indu ces stress-responsive genes. Moreover, deletion of both stm1 and ras1 cause d delayed entry into G, arrest in S. pombe when the cells were grown in a n itrogen-deficient medium. Thus we consider that the stm1 gene can function through Gpa2-dependent and/or -independent pathways and may play a role in providing the prerequisite state for entering the pheromone-dependent diffe rentiation cycle in which heterotrimeric Gal protein, Gpa1, and Ras1 play m ajor roles. Stm1 could function as a sentinel molecule sensing the nutritio nal state of the cells, stopping the proliferative cell cycle, and preparin g the cell to enter meiosis under nutritionally deficient conditions.