Inactivation of Ran1/Pat1 kinase bypasses the requirement for high-level expression of mei2 during fission yeast meiosis.

Ran1/Pat1 kinase and cAMP-dependent protein kinase (PKA) regulate sexual differentiation in Schizosaccharomyces pombe. A reduction in the activity of both enzymes is a prerequisite for meiosis. Together, PKA and Pat1 control the level of expression of the Mei2 RNA-binding protein. Pat1 further regulates the activity of Mei2 by phosphorylation. Phosphorylation inactivates Mei2 by interfering with its cellular localization and by causing degradation of the protein via the ubiquitin-proteasome pathway. The inhibitor of Pat1, Mei3, is found only in diploid cells undergoing meiosis. Expression of mei3 is sufficient to induce meiosis. Here, we examine the relationship between Pat1, PKA and Mei3. We demonstrate that Mei3 is an in vitro substrate for PKA. Using site-specific mutagenesis, the major PKA phosphorylation site is identified. In vivo assays indicate that phosphorylation of Mei3 by PKA does not significantly alter the ability of the inhibitor to regulate Pat1. Although it does not function as an inhibitor for PKA, ectopic expression of Mei3 causes cells containing high PKA levels to undergo meiosis. Expression of various mei3 alleles in cells containing unregulated PKA activity shows that the ability to undergo meiosis correlates with Pat1 activity. Notably, induced levels of mei2 are not a prerequisite for meiotic differentiation, as previously thought. The implications of this result to developmental regulation are discussed.