SMC2, A SACCHAROMYCES-CEREVISIAE GENE ESSENTIAL FOR CHROMOSOME SEGREGATION AND CONDENSATION, DEFINES A SUBGROUP WITHIN THE SMC FAMILY

We characterized the SMC2 (structural maintenance of chromosomes) gene that encodes a new Saccharomyces cerevisiae member of the growing fam ily of SMC proteins. This family of evolutionary conserved proteins wa s introduced with identification of SMC1, a gene essential for chromos ome segregation in budding yeast. The analysis of the putative structu re of the Smc2 protein (Smc2p) suggests that it defines a distinct sub group within the SMC family. This subgroup includes the ScII, XCAPE, a nd cut14 proteins characterized concurrently. Smc2p is a nuclear, 135- kD protein that is essential for vegetative growth. The temperature-se nsitive mutation, smc2-6, confers a defect in chromosome segregation a nd causes partial chromosome decondensation in cells arrested in mitos is. The Smc2p molecules are able to form complexes in vivo both with S mc1p and with themselves, suggesting that they can assemble into a mul timeric structure. In this study we present the first evidence that tw o proteins belonging to two different subgroups within the SMC family carry nonredundant biological functions. Based on genetic, biochemical , and evolutionary data we propose that the SMC family is a group of p rokaryotic and eukaryotic chromosomal proteins that are likely to be o ne of the key components in establishing the ordered structure of chro mosomes.