Differentiation in Yeast: Mechanisms of Mating and Meiosis

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Eukaryotic cells fuse during fertilization to produce diploid zygotes that go on to differentiate into specialized cells during development. Somatic cell fusion also occurs during development to produce multinucleate tissues, such as myoblast fusion to form muscle. Diploid organisms may then undergo meiosis to produce haploid gametes for subsequent fertilization, completing the cycle of sexual reproduction. Cell fusion and meiosis must be tightly regulated. Inappropriate cell fusion, as occurs in some virally-infected cells and metastatic cancers, results in pathological syncytia. Despite advances in elucidating the process of cell fusion, much remains unknown. Meiotic defects may cause aneuploidy, or even sterility. Recent research revealed that meiotic regulation is surprisingly complex. Overlaid on waves of transcriptional regulation are complex layers of mRNA modification and translational regulation which are thought to provide exquisite temporal control. However, much remains to be learned about how meiosis is regulated. The long-term goals of the research in my lab are to understand the fundamental, conserved molecular mechanisms of cell fusion, nuclear fusion, and meiotic regulation using one of the most powerful model organisms, the budding yeast Saccharomyces cerevisiae. During yeast mating and somatic cell fusion, cells must signal that they are in contact and competent to fuse, remove the extracellular matrix separating them, and fuse the plasma membranes. Each step is only poorly understood. We have shown that in yeast, as in mouse myoblasts, Cdc42p is a key regulator of cell fusion. We will address how cell contact regulates Cdc42p and how Cdc42p in turn mediates cell fusion, focusing on membrane curvature and the cell wall integrity pathway. Using a novel genetic screen, we will pursue identification of the yeast membrane fusogen. After mating and cell fusion, many induced proteins may be hazardous and must be rapidly degraded. We discovered that Srl4p stabilizes mating-induced proteins by inhibiting a branch of the proteasomal degradation pathway. We will examine how Srl4p differentially regulates the turnover of proteins in mating and mitosis. After fertilization in many organisms, the pronuclei fuse. Nuclear fusion is a challenging biological problem – how do the two membranes fuse sequentially and in register? We identified Kar5p, a protein conserved in plants, animals, and fungi, as mediating nuclear envelope fusion; however, its precise role is unknown. We will test our hypothesis that Kar5p acts as a novel inner nuclear envelope fusogen. In meiosis, mRNA N6-adenine methylation has been revealed to be a critical regulator, although its functions are not yet understood. Kar4p is the yeast homologue of human METTL14, a core component of the methyl-transferase. Surprisingly, we found that Kar4p regulates both meiotic transcription and meiotic protein levels. We will examine how Kar4p regulates meiosis at multiple levels, testing potential roles in transcription, translational regulation, and mRNA modification. Our studies will aid the understanding of basic conserved mechanisms of cell fusion, nuclear membrane fusion, and meiosis.