SMCkleisin

SMCkleisin refers to the kleisin family of proteins that form the central connecting subunit of Structural Maintenance of Chromosomes (SMC) complexes. Kleisins bridge the ATPase head domains of paired SMC proteins to close a tripartite ring that entraps DNA, a configuration essential for chromosome cohesion, condensation, and DNA repair. Different SMC complexes contain distinct kleisin paralogs (for example, cohesin and condensin kleisins), but they share a conserved role in linking SMC heterodimers and recruiting accessory factors.

Structurally, kleisins have an extended central region that binds regulatory subunits and flexible N- and C-terminal domains that associate with SMC head motifs. Their interactions enable dynamic opening and closing of the SMC ring and allow regulated loading and unloading of DNA. In eukaryotes, kleisin cleavage by separase triggers sister chromatid separation at anaphase, and post-translational modifications such as phosphorylation and SUMOylation modulate kleisin function and complex stability.

Kleisins are conserved across bacteria, archaea, and eukaryotes, though sequence and regulatory complexity increase in higher organisms. Mutations affecting kleisins or their interacting partners are linked to genome instability and developmental syndromes through impaired chromosome segregation or altered gene regulation. Structural biology (cryo-EM, X-ray crystallography), genetics, and biochemical assays continue to refine understanding of how kleisins coordinate SMC-driven processes like loop extrusion, cohesion, and chromatin organization.