Regulation of meiotic development in Saccharomyces cerevisiae by the meiosis-specific Ime2 protein kinase.

摘要

Meiosis is the evolutionarily conserved process by which sexually reproducing organisms generate haploid gametes. In the yeast, Saccharomyces cerevisiaie, meiosis is linked to the formation of spores. As in mitosis, meiosis is driven by the cyclin-dependent kinase (CDK), Cdk1. Additionally, Ime2, a meiosis-specific CDK-like kinase, superimposes meiosis-specific regulation on the cell cycle machinery that is shared between mitosis and meiosis. Here, I define the Ime2 phosphorylation consensus motif to be R-P-X-S/T-A, which is distinct from Cdk1 (S/T-P). The Ime2 consensus motif has predictive value in identifying novel substrates. Cdc20 is substrate targeting subunit of the Anaphase Promoting Complex (APC/C) that triggers chromosome segregation. I show that Ime2 inhibits Cdc20 through a single phospho-acceptor site that is adjacent to an APC/C binding motif. In addition to protein kinase signaling, the sporulation program is tightly controlled by a transcriptional cascade. The commitment point, after which the inducing signal is no longer necessary for completion of the program, is at the end of premeiotic prophase just before the cells execute the first meiotic division. This transition requires the induction of Middle Sporulation Genes (MSGs). The Sum1 repressor inhibits MSG expression while the Ndt80 transcription factor is the principal inducer of MSGs. NDT80 is itself controlled by a tightly regulated MSG promoter in a positive autoregulatory loop. Sum1 represses the NDT80 promoter in vegetative cells and early prophase. I show that Ime2 and Cdk1 negatively regulate Sum1. Phosphorylation of Sum1, specifically at the NDT80 promoter, derepresses NDT80. This allows the NDT80 autoregulatory loop to proceed, leading to MSG induction and commitment to meiosis. Genetic evidence indicates one mechanism used by Ime2 and Cdk1 to trigger NDT80 is inhibition of Rfm1/Hst1, a histone deacetylase complex known to bind Sum1. In this mechanism, Ime2 works with Cdk1 to exert meiosis-specific control over MSG expression and meiotic commitment. This work suggests a paradigm by which CDK and cell-type specific CDK-like kinases co-regulate commitment to a cellular differentiation program by regulating a transcriptional cascade.