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

We report the isolation and characterization of a murine gene encoding a conserved mammalian nucleolar protein. The protein, called Tsgll8, has a predicted 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 Tsgll8 protein is predominantly expressed in proliferating somatic cells and in male germ cells. Indirect immunofluorescence microscopy analysis using an affinity-purified anti Tsgll8 serum shows colocalization of Tsg118 and a known nucleolar protein, fibrillarin, to the dense fibrillar component of the nucleolus. The nucleolar localization of the Tsgll8 protein appears to be temporally restricted to the interphase stages of the somatic cell cycle and to the meiotic phase of spermatogenesis. We find that the Tsgll8 protein localizes to the nucleolus in both proliferating and serum-starved cells. Interestingly, as the nucleolar signal disappears in mitotic cells, the Tsgll8 protein instead becomes associated with the surface of the condensed chromosomes.