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J. Biol. Chem., Vol. 276, Issue 43, 40190-40201, October 26, 2001

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Isolation of a Novel Gene from Schizosaccharomyces pombe: stm1⁺ Encoding a Seven-transmembrane Loop Protein That May Couple with the Heterotrimeric G2 Protein, Gpa2^*

Kyung-Sook Chung, Misun Won, Sang-Bong Lee, Young-Joo Jang, Kwang-Lae Hoe, Dong-Uk Kim, Ji-Won Lee, Kyu-Won Kim, and Hyang-Sook Yoo^§

From the Cell Cycle and Signal Transduction Research Unit, Korea Research Institute of Bioscience and Biotechnology (KRIBB), P. O. Box 115 Yusong, Taejon 305-606, and  Department of Molecular Biology, Pusan National University, Pusan 600-738, Korea

A putative seven transmembrane protein gene, stm1⁺, which is required for proper recognition of nitrogen starvation signals, was isolated as a multicopy suppressor of a ras1 synthetic lethal mutant in Schizosaccharomyces pombe. Under nitrogen-deficient conditions, transcription of the stm1 gene was induced; deletion of stm1 was associated with early entry into G₁ arrest. Under nutritionally sufficient conditions, overexpression of Stm1 inhibited vegetative cell growth, resulted in decreased intracellular cAMP levels, increased the expression of the meiosis-specific genes ste11, mei2, and mam2, and facilitated sexual development in homothallic cells. However inhibition of vegetative cell growth and reduction of cAMP levels were not observed in a deletion mutant of the heterotrimeric G protein G2 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. Mutation at Lys¹⁹⁹ in the C-terminal domain (stm1^K199A) abolished the Stm1 overexpression effect on lowering cAMP levels. Induction of ste11, a meiosis-specific gene transcription factor, by Stm1 overexpression was enhanced in gpa2-deleted cells but was absent in a deletion mutant of sty1, a key protein kinase that links mitotic control with environmental signals and induces stress-responsive genes. Moreover, deletion of both stm1 and ras1 caused delayed entry into G₁ arrest in S. pombe when the cells were grown in a nitrogen-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 differentiation cycle in which heterotrimeric G1 protein, Gpa1, and Ras1 play major roles. Stm1 could function as a sentinel molecule sensing the nutritional state of the cells, stopping the proliferative cell cycle, and preparing the cell to enter meiosis under nutritionally deficient conditions.

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