Molecular Mechanism Of Interaction Between Meiosis Cohesin And HORMA Protein In Caenorhabditis Elegans

Meiosis is a type of specialized cell division.After single round of DNA is plication,homologous chromosomes and sister chromatids are separated sequentially during meiosis to produce haploid gametes.Caenorhabditis elegans has become an important model organism for studying the mechanisms underlying the precise regulation of meiotic chromosome segregation.The structure and dynamic process of meiotic chromosomes are conserved across species.A typical feature of meiotic chromosomes is the formation of the synaptonemal complex which is a complex protein structure consisting of a central region and two lateral elements(chromosomal axes)on both sides.The chromosome axis in C.elegans is composed of HORMA domain protein and cohesin complex.When the cell enters meiosis,the chromosomal axis is assembled,sister chromatids are connected together by cohesion complex on the axis,forming a linear array of chromatin rings loops with their bases attached to the axis.In Caenorhabditis elegans,there are four meiotic HORMA domain proteins(HIM-3,HTP-1,HTP-2,and HTP-3),which have related but not identical functions.Among them,HTP-3 is the core component for chromosome axis assembly,and is required for the axis localization of HIM-3,HTP-1 and HTP-2.Cohesin is a multi-subunit complex that mediates sister chromatids cohesion and is required for synaptonemal complexs formation.There are three different types of synaptonemal complexs cohesin complexes Caenorhabditis elegans,utilizing three different kleisin subunits REC-8,COH-3 and COH-4,which are functionally highly redundant.Both the HORMA complex and cohesin play a central role in meiosis,including homologous pairing,recombination,and precise chromosome separation.Previous studies have shown that there is a functional connection between the HORMA complex and cohesin.The efficient loading of REC-8 on chromosome depends on HORMA domain protein HTP-3;During DNA double-strand break repair,HIM-3 and HTP-1 promote recombination between homologous chromosomes and the formation of genetic crossovers by suppressing DNA repair between sister chromatids;HTP-1 and HTP-2 ensure the stepwise separation of homologous chromosomes and sister chromatids during meiosis by maintaining REC-8-cohesin on the chromosomes during the meiosis I.Although the HORMA domain protein and the cohesin complex have extensive connections in the assembly and functional regulation of the chromosomal axis,how the two protein complexes coordinate with each other to form a functional chromosomal axis is still unclear.In this study,the interactions between the two protein complexes was analyzed systematically by in vivo and in vitro methods.Plasmids expressing HORMA domain proteins and cohesin complex proteins with different tags were constructed,and the interactions and co-localization patterns between HORMA domain proteins and cohesin complex proteins were examined by immunoprecipitation,Western blotting and immunofluorescence microscopy.We detected extensive interactions between multiple HORMA domain proteins and cohesin complex proteins,but these interactions are weak interactions,suggesting that the multivalent weak interactions between the two complexes may play an important role in the assembly of chromosome axes.In Caenorhabditis elegans,HTP-3 plays a central role in axis assembly and is responsible for recruiting other HORMA proteins to the chromosome axis.Interestingly,we found that the absence of HTP-1 or HIM-3 weakens the chromosome binding of HTP-3,suggesting that multiple components of the HORMA complex may promote axis assembly through multivalent interactions.Furthermore,we explored the functions of different domains of HTP-3 in vivo and in vitro.We found that the deletion of the amino-terminal HORMA domain of HTP-3 in htp-3(del N)mutants destroyed the binding of the protein to the chromosome axis and led to polycomplex formation,indicating that the amino-terminal domain of HTP-3 plays a key role in axis assembly.However,no strong interaction between HTP-3 amino terminal domain and the other axial components through in vitro analysis,suggesting that there might be a synergistic effect between HTP-3 amino terminal domain and its carboxyl terminal or other HORMA proteins in the process of axis assembly,but the specific mechanism remains to be studied.In summary,we analyzed the interaction between HORMA proteins and cohesin complex in vitro and in vivo.We believe that there is a wide range of weak interactions between HORMA proteins and cohesin complex,and the multivalent weak interactions between these complexes may be an important mechanism of meiotic axis assembly.These findings provide a new perspective to understand the mechanisms underlying meiotic chromosome dynamics and genomic integrity maintenance during meiosis.