Investigating the functional domains of meiotic HORMA-domain protein HTP-1

Abstract

Proper chromosome segregation during meiosis is required to prevent the formation of aneuploid gametes, which result in sterility, miscarriages and birth defects. Formation of euploid gametes requires a series of events that occur during meiotic prophase, including: homologue pairing, synaptonemal complex assembly, and the formation of inter-homologue crossovers. All these events are promoted by a group of conserved HORMA (HOP-1, REV-1 and MAD2) domain proteins that associate with chromosomes at the onset of meiosis. C. elegans carries four of these proteins: HIM-3, HTP-1, HTP-2 and HTP-3, which form part of the proteinaceous axis of meiotic chromosomes. HTP-1 promotes pairing and crossover formation, and participates in two checkpoint mechanisms coordinating meiotic events, while HTP-2 alone cannot promote any of these events. HTP-1 and HTP-2 are 82% homologous at the amino acid level, hinting that essential residues for HTP-1 functions lie in the 62-amino-acid difference between the two proteins. Using recombinant proteins in which HTP-1 domains are substituted for the corresponding regions of HTP-2, I show that an 84-amino-acid region within the HORMA domain of HTP-1, named “HORMA C” is essential for HTP-1 function. Specifically, I show that ten residues wherein are essential for pairing, synapsis, correct timing of double strand break repair, chiasma formation and proper chromosome segregation. Moreover, the HORMA C region of HTP-1 functions with the support of the C-terminus, and the HORMA C region and C-terminus of HTP-1 are, together, sufficient to confer HTP-1 function to the HTP-2 protein sequence. This study provides insight into the relationship between sequence and function in HORMA domain proteins, and how proteins of this family have evolved specific roles supported by cis sequences.