Molecular mimicry in development: identification of ste11+ as a substrate and mei3+ as a pseudosubstrate inhibitor of ran1+ kinase.

ran1+ (pat1+) kinase inhibits exit from the mitotic cell cycle and entry into meiosis. Inactivation of ran1+ by mei3+ is sufficient to precipitate the entire meiotic developmental program. Here, we show that the ste11+ transcription factor is a substrate for ran1+ in vitro and that this reaction is directly inhibited by mei3+. Sequence comparison reveals that ste11+ contains two domains homologous to each other and to a domain of mei3+. Mutagenesis studies reveal that the regions of homology contain substrate specificity determinants. These results identify sequences critical for phosphorylation of ste11+ by ran1+ and suggest that mei3+ employs a pseudosubstrate mechanism for its inhibitory function.