Discovering functions of the H2A.Z histone variant in meiosis

Abstract

Chromatin structure is regulated by post-translational modifications of histones as well as incorporation of histone variants. This regulation is important for chromosome dynamics during meiotic prophase, in which synapsis and recombination between homologs are crucial events to promote accurate chromosome segregation. Defects in these processes activate the so-called pachytene checkpoint or meiotic recombination checkpoint, which blocks meiotic cell cycle progression to avoid formation of aneuploid gametes. We are studying the possible meiotic function(s) of the histone variant H2A.Z, which replaces the canonical histone H2A at some positions in the genome, due to the action of the SWR1 complex. We found that, in S. cerevisiae, H2A.Z is incorporated into meiotic chromatin in a SWR1-dependent manner, except in the unsynapsed rDNA region. To explore the role of this histone variant, we have examined various meiotic events in the htz1, swr1 and htz1 swr1 mutants. The htz1 mutant displays delayed and inefficient meiotic progression and shows reduced spore viability. These meiotic defects are at least partially abolished by mutation of SWR1, indicating that they could be caused by instability of nucleosomes generated in absence of H2A.Z. Nevertheless, when the pachytene checkpoint is activated (as in a synapsisdefective zip1 mutant) the absence of H2A.Z generates a total block of meiosis, and this block is not rescued by the absence of SWR1. Several lines of evidence indicate that the tight arrest of the zip1 htz1 and zip1 swr1 mutants results from sustained activation of the meiotic recombination checkpoint resulting in persistent Swe1-dependent inhibitory phosphorylation of CDK, and point to a role for H2A.Z in the adaptation or recovery from the checkpoint-induced meiotic block. We are currently investigating the possible targets of H2A.Z regulating this process. On the other hand, we are also studying the function of H2A.Z during unperturbed meiosis. In vegetative cells, an interaction between H2A.Z and the SUN-domain Mps3 protein has been reported. Since Mps3 is required for the rapid prophase movements (RPMs) of meiotic chromosomes driven by the telomeres attached to the nuclear envelope, we have explored the role of H2A.Z in this process. Using a Zip1-GFP fusion to visualize prophase chromosomes, we found that, indeed, the velocity of RPMs is reduced in the htz1 and htz1 swr1, but not in swr1 mutant. This suggests that this particular H2A.Z function in promoting chromosome movements is independent of its incorporation into chromatin. We are currently analyzing the contribution of H2A.Z to the organization of meiotic nuclear architecture. In summary, our results reveal a variety of SWR1-dependent and independent H2A.Z functions orchestrating multiple aspects of chromosome dynamics during meiosis.