The Chromosome Axis Controls Meiotic Events through a Hierarchical Assembly of HORMA Domain Proteins

Abstract

Summary Proteins of the HORMA domain family play central but poorly understood roles in chromosome organization and dynamics during meiosis. In C. elegans, four such proteins (HIM-3, HTP-1, HTP-2, and HTP-3) have distinct but overlapping functions. Through combined biochemical, structural, and in vivo analysis, we find that these proteins form hierarchical complexes through binding of their HORMA domains to cognate peptides within their partners’ C-terminal tails, analogous to the “safety belt” binding mechanism of Mad2. These interactions are critical for recruitment of HIM-3, HTP-1, and HTP-2 to chromosome axes. HTP-3, in addition to recruiting the other HORMA domain proteins to the axis, plays an independent role in sister chromatid cohesion and double-strand break formation. Finally, we find that mammalian HORMAD1 binds a peptide motif found both at its own C-terminus and that of HORMAD2, indicating that this mode of intermolecular association is a conserved feature of meiotic chromosome structure in eukaryotes.

Superresolution microscopy reveals the three-dimensional organization of meiotic chromosome axes in intact Caenorhabditis elegans tissue

“…We therefore refer to this pair of paralogs as HTP-1/2. In vitro and in vivo experiments have demonstrated that HTP-1/2 are recruited to the chromosome axes through their interaction with closure motifs within the long tail of HTP-3 and at the extreme C terminus of HIM-3 (16). Our aligned and averaged data confirm that HTP-1/2 localize to the chromosome axes at 57.3 ± 1.6 nm off-center (Fig.…”

“…This observation indicates that the electron-dense elements that flank the SC may not correspond to assemblies of axis proteins, although some axis proteins appear to localize at or near these structures (22). Although recent work has illuminated some structural details of axis proteins and their interactions (16,(22)(23)(24), the overall organization of the chromosome axis remains largely undetermined.…”

“…HTP-3 is the largest of four meiotic HORMA domain proteins expressed in C. elegans. It is recruited to the axis independently of HTP-1/2 and HIM-3 (5,14,16), which are recruited to the axis through binding of their HORMA domains to "closure motifs," short peptide sequences within the long tail of HTP-3 ( Fig. 2A).…”

“…Biochemical, structural, and genetic evidence has revealed that these HORMA domain proteins form a hierarchical complex. HTP-3 recruits HTP-1, HTP-2, and HIM-3 through interactions of their respective HORMA domains with cognate closure motifs in the C-terminal tail of HTP-3 (16). However, how HTP-3 is recruited to the axis and how these meiotic HORMA domain proteins interact with cohesins is still unclear.…”

“…This broad distribution in z may reflect flexibility of the C terminus of HTP-3, which spans almost 500 amino acids and has no predicted structured domains. Interestingly, HIM-3, which binds to motifs in the middle of the long tail of HTP-3 (16), is constrained to the central plane of the axes. Given the known interaction of HIM-3 and the C-terminal tail of HTP-3, we wondered whether the distinct localization patterns of HIM-3 and HTP-3 might be caused by the insertion of epitope tags and/or use of antibodies.…”