Exploring the C(2)M Cohesin Complex: Structure, Dynamics, and Ability to Facilitate Assembly of the Synaptonemal Complex

There are two meiotic cohesin pathways that regulate synaptonemal complex (SC) assembly in Drosophila . We previously proposed that C(2)M, which is required for SC assembly, is the only meiosis-specific component of a complex that includes Stromalin (SA), Nipped-B, SMC1 and SMC3. This model also predicts that specific residues within the C-terminus and N-terminus of C(2)M should interact with SMC1 and SMC3 to form a ring structure that may regulate the ability of C(2)M to facilitate SC assembly. Through mutant analysis, our results show several residues known to interact with SMC1 or SMC3 are critical for SC formation, suggesting that C(2)M may require a ring structure to perform meiosis-specific functions such as the formation of SC. We also show that SA colocalizes with and depends on C(2)M. However, the dynamics of C(2)M differ from SA and the SMCs in a way that suggests C(2)M regulates the dynamics and chromosome loading of the other cohesin proteins SMC1 and SA. Consistent with this conclusion, our results suggest that C(2)M can promote chromosome localization of the other cohesin components, and can induce SC assembly when ectopically expressed in germline mitotic cells.