Architecture and function of the meiotic metazoan HORMAD chromatin binding region (CBR).

Abstract

During early meiotic prophase, the chromosome axis is formed by DNA-binding cohesin proteins, filamentous axis core proteins, and meiotic HORMA domain (HORMAD) proteins. HORMADs are master regulators of meiotic recombination, recruiting Spo11 and associated proteins to create DNA double-strand breaks (DSBs) and promoting their repair as interhomolog crossovers (COs). In earlier work, we discovered a central chromatin binding region (CBR) domain in the S. cerevisiae HORMAD protein Hop1 (SNU, unpublished), and further found that diverse eukaryotes including many plants and metazoans within the opisthokonta family also possess a central CBR with variable architecture. In fungi such as Vanderwaltozyma polyspora and Saccharomyces cerevisiae, Hop1 has an internal CBR domain containing a variant plant homeodomain (PHD) zinc finger domain and a DNA-binding winged helix-turn-helix domain (wHTH), plus a conserved C-terminal extension (HTH-C). Similarly, metazoan HORMAD CBR domains are predicted to contain a canonical PHD domain and a wHTH domain. Here we show two x-ray crystal structures of metazoan HORMAD CBRs from Schistosoma mansoni (blood fluke) and Patiria miniata (starfish). Both structures contain PHD and wHTH domains like the fungal Hop1 CBR, but do not contain a C-terminal extension (HTH-C). The PHD domain contains two beta strands with a conserved pocket that we propose binds a histone tail. We are currently pursuing both biochemical and cell-based approaches to determine the molecular basis for the HORMAD CBR-chromatin interaction and identify any specificity for histone tail modifications. This work will define how the HORMAD CBR modulates chromosome axis architecture and localization in diverse eukaryotes.