Red1p, a MEK1-dependent phosphoprotein that physically interacts with Hop1p during meiosis in yeast

The aptonemal complex (SC) is a proteinaceous structure formed between pair s of homologous chromosomes during prophase I of meiosis, The proper assemb ly of axial elements (AEs), lateral components of the SC, during meiosis in the yeast, Saccharomyces cerevisiae, is essential for wild-type levels of recombination and for the accurate segregation of chromosomes at the first meiotic division. Genetic experiments have indicated that the stoichiometry between two meiosis-specific components of AEs in S. cerevisiae, HOP1 and RED1, is critical for proper assembly and function of the SC. A third meios is specific gene, MEK1, which encodes a putative serine/threonine protein k inase, is also important for proper AE function, suggesting that AE formati on is regulated by phosphorylation, In this paper, we demonstrate that Mek1 p is a functional kinase in vitro and that catalytic activity is an essenti al part of the meiotic function of Mek1 in vivo. Immunoblot analysis reveal ed that Red1p is a MEK1-dependent phosphoprotein, Co-immunoprecipitation ex periments demonstrated that the interaction between Hop1p and Red1p is enha nced by the presence of MEK1. Thus, MEK1-dependent phosphorylation of Red1p facilitates the formation of Hop1p/Red1p hetero-oligomers, thereby enablin g the formation of functional AEs.