Meiotic checkpoint control of chromosome dynamics

Abstract

Meiotic cells possess surveillance mechanisms or checkpoints that monitor critical meiotic events, such as recombination and chromosome synapsis. Defects in these processes, such as those resulting from the absence of the S. cerevisiae Zip1 protein, activate a meiosis-specific checkpoint network resulting in a delay or arrest of meiotic progression. We are studying this meiotic checkpoint at different levels. Pch2 is an evolutionarily conserved AAA+ ATPase initially discovered as a checkpoint protein required for the zip1-induced meiotic arrest in budding yeast. Pch2 impacts multiple aspects of meiotic chromosome metabolism, including negative regulation of Hop1 chromosomal abundance. It has been suggested that Pch2 promotes the turnover of the Hop1 protein; thus, the pch2 single mutant exhibits more continuous Hop1 localization along synapsed chromosomes. Interestingly, in the zip1 mutant, when the checkpoint is induced, Pch2 is only detectable at the rDNA region (nucleolus). A special chromatin environment determined by the chromosomal distribution of Dot1-dependent histone H3K79 methylation and by the action of the Sir2 histone deacetylase contributes to Pch2’s nucleolar confinement. To gain insight into the molecular mechanisms underlying the function of Pch2 in the synapsis checkpoint triggered by the absence of Zip1, we carried out a genetic screen for high-copy suppressors of meiotic progression in zip1 pch2 and, unexpectedly, we identified HOP1. Analysis of Mek1 activation revealed that Pch2 is required for zip1- induced Mek1 autophosphorylation, but not for the Mec1/Tel1 priming phosphorylation. HOP1 overexpression restored full Mek1 activation in zip1 pch2, thus explaining the reestablishment of the meiotic block. Surprisingly, we observed that, in contrast with the pch2 single mutant, Hop1 localization on the axes was no more abundant in zip1 pch2 compared with zip1. Likewise, although Mec1/Tel1-dependent phosphorylation of Hop1 at T318 is increased in the pch2 single mutant, it is strongly reduced in zip1 pch2 compared to zip1. Unlike wild-type HOP1, overexpression of a hop1-T318A mutant does not restore checkpoint function in zip1 pch2. Taken together, these observations imply that Pch2 activity is required for proper accumulation of phosphorylated Hop1-T318 on unsynapsed chromosome axes and argue that Pch2 regulates Hop1 in different and even opposite manners in a wild type (checkpoint off) versus a zip1 (checkpoint on) situation.