Protein Kinase Activity Associated with the IME2 Gene Product, a Meiotic Inducer in the Yeast Saccharomyces Cerevisiae

The IME2 gene product (Ime2) is required for entry into meiosis and sporulation in S. cerevisiae. It has been predicted to be composed of two domains, an amino-terminal domain with homology to protein kinases and a carboxy-terminal acidic domain. The Ime2 was identified in extracts of meiotic cells carrying multi- but not low-copy IME2 in immunoblot analysis using an Ime2-specific antibody. Immune complexes were found to phosphorylate Ime2 and several exogenous proteins. Low-copy plasmids expressing truncated Ime2 proteins that lack part of or the entire carboxy-terminal domain enabled cells to undergo sporulation even under a certain repressive nutritional condition. These cells contained increased levels of protein kinase activity compared with control cells. These results suggest that the amino-terminal domain has a protein kinase activity and that the acidic tail is not essential for either the kinase activity or sporulation but serves in a negative role. An Ime2-beta-galactosidase fusion was shown by immunofluorescence microscopy to be localized predominantly to the nucleus, suggesting a nuclear function of Ime2.