Characterization of a novel nucleolar protein that transiently associates with the condensed chromosome in mitotic cells

We-report the isolation and characterization of a murine gene encoding a co nserved mammalian nucleolar protein. The protein, called Tsg118, has a pred icted molecular mass of 59.4 kDa and a high content of basic amino acids. A homologous human gene was localized to chromosome 16p12.3. The Tsg118 prot ein is predominantly expressed in proliferating somatic cells and in male g erm cells. Indirect immunofluorescence microscopy analysis using an affinit y-purified anti-Tsg118 serum shows colocalization of Tsg118 and a known nuc leolar protein, fibrillarin, to the dense fibrillar component of the nucleo lus. The nucleolar localization of the Tsg118 protein appears to be tempora lty restricted to the interphase stages of the somatic cell cycle and to th e meiotic phase of spermatogenesis. We find that the Tsg118 protein localiz es to the nucleolus in both proliferating and serum-starved cells. Interest ingly, as the nucleolar signal disappears in mitotic cells, the TFsg118 pro tein instead becomes associated with the surface of the condensed chromosom es.