Novel meiosis-specific isoform of mammalian SMC1

Structural maintenance of chromosomes (SMC) proteins fulfill pivotal roles in chromosome dynamics. In yeast, the SMC1-SMC3 heterodimer is required for meiotic sister chromatid cohesion and DNA recombination. Little is known, however, about mammalian SMC proteins in meiotic cells. We have identified a novel SMC protein (SMC1 beta), which-except for a unique, basic, DNA binding C-terminal motif-is highly homologous to SMC1 (which may now be called SMC1 alpha) and is not present in the yeast genome. SMC1 beta is specifically expressed in testes and coimmunoprecipitates with SMC3 from testis nuclear extracts, but not from a variety of somatic cells. This establishes for mammalian cells the concept of cell-type- and tissue-specific SMC protein isoforms. Analysis of testis sections and chromosome spreads of various stages of meiosis revealed localization of SMC1 beta along the axial elements of synaptonemal complexes in prophase 1. Most SNIC1 beta dissociates from the chromosome arms in late-pachytene-diplotene cells. However, SMC1 beta, but not SMC1 alpha, remains chromatin associated at the centromeres up to metaphase II. Thus, SMC1 beta and not SMC1 alpha is likely involved in maintaining cohesion between sister centromeres until anaphase II.