SMC PROTEINS CONSTITUTE 2 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 a nd deletions through DNA recombination. DNA polymerase epsilon, DNA li gase III and a DNA structure-specific endonuclease co-purify with the five polypeptide complex, Here we describe the identification of two h itherto unknown subunits of RC-1. N-terminal amino acid sequences of t he 160 and 130 kDa polypeptides display up to 100% identity to protein s of the structural maintenance of chromosomes (SMC) subfamilies 1 and 2. SMC proteins are involved in mitotic chromosome segregation and co ndensation, as well as in certain DNA repair pathways in fission (rad1 8 gene) and budding (RHC18 gene) yeast. The assignment was substantiat ed by immuno-cross-reactivity of the RC-1 subunits with polyclonal ant ibodies specific for Xenopus laevis SMC proteins. These antibodies, an d polyclonal antibodies directed against the bovine 160 and 130 kDa po lypeptides, named BSMC1 and BSMC2 (bovine SMC), inhibited RC-1-mediate d DNA transfer, indicating that the SMC proteins are necessary compone nts of the reaction. Two independent assays revealed DNA reannealing a ctivity of RC-1, which resides in its BSMC subunits, thereby demonstra ting a novel function of these proteins. To our knowledge, this is the first evidence for the association of mammalian SMC proteins with a m ultiprotein complex harboring, among others, DNA recombination, DNA li gase and DNA polymerase activities.