SMC proteins constitute two subunits of the mammalian recombination complex RC-1

Recombination protein complex RC-1, purified from calf thymus nuclear extracts, catalyzes cell-free DNA strand transfer and repair of gaps and deletions through DNA recombination. DNA polymerase E, DNA ligase III and a DNA structure-specific endonuclease co-purify with the five polypeptide complex. Here we describe the identification of two hitherto unknown subunits of RC-1. N-terminal amino acid sequences of the 160 and 130 kDa polypeptides display up to 100% identity to proteins of the structural maintenance of chromosomes (SMC) subfamilies 1 and 2. SMC proteins are involved in mitotic chromosome segregation and condensation, as well as in certain DNA repair pathways in fission (rad18 gene) and budding (RHC18 gene) yeast. The assignment was substantiated by immuno-cross-reactivity of the RC-1 subunits with polyclonal antibodies specific for Xenopus laevis SMC proteins. These antibodies, and polyclonal antibodies directed against the bovine 160 and 130 kDa polypeptides, named BSMC1 and BSMC2 (bovine SMC), inhibited RC-1-mediated DNA transfer, indicating that the SMC proteins are necessary components of the reaction. Two independent assays revealed DNA reannealing activity of RC-1, which resides in its BSMC subunits, thereby demonstrating a novel function of these proteins. To our knowledge, this is the first evidence for the association of mammalian SMC proteins with a multiprotein complex harboring, among others, DNA recombination, DNA ligase and DNA polymerase activities.