Investigating the functional domains of horma-domain protein HTP-1 during C. elegans meiosis

Abstract

Proper chromosome segregation during meiosis requires homologue recognition, synapsis and recombination during meiotic prophase, followed by two-step release of sister chromatid cohesion (SCC) during the meiotic divisions. All these events are promoted by a conserved family of HORMA-domain proteins, which associate with the chromosome axes at the start of meiotic prophase. In C. elegans, HORMA-domain protein HTP-1 promotes homologue pairing, participates in an early prophase checkpoint that monitors synaptonemal complex assembly, promotes the formation of inter-homologue crossover events during meiotic recombination, and, along with its paralogue HTP-2, prevents the premature release of SCC during the first meiotic division. However, the molecular mechanisms by which HTP-1 exerts its functions remain unknown. HTP-1 consists of a HORMA domain (~220 amino acids) that is structurally similar to the spindle assembly checkpoint protein Mad2 and from two additional N- and C-terminal domains extending from the HORMA domain whose role has previously remained uncharacterised. Here, I show that the HORMA-domain of HTP-1 is required for HTP-1 loading to axial elements by engaging into a physical interaction with HTP-3 and that this interaction is disrupted by a point mutation at M127. I also show that the HTP-1 N-terminal region (first 30 amino acids) is essential to prevent the precocious release of SCC during the first meiotic division by acting as a platform for recruitment of the PP1 phosphatase interactor LAB-1 to axial elements, locally antagonizing H3 T3 phosphorylation, a chromatin mark that recruits Aurora B. Finally, I assign a role for the C-terminus of HTP-1 in supporting the protein's functions in homologue pairing and synapsis checkpoints as well as in promoting initiation of recombination by acting as a regulator of the master kinase CHK-2. All in all, this study uncovers unexpected roles for the meiotic HORMADs as local regulators of histone modifications and of kinase activity.