B cell activation leads to upregulated expression of the murine Sik-similar protein gene

B cells stimulated by the combination of CD40L plus anti-Ig were screened for upregulated gene expression in an unbiased fashion through differential display. An inducible transcript was obtained that corresponds to a sequence previously isolated from pre-B cells and termed murine Sik-similar protein (mSSP). The mSSP gene is predominantly expressed in lymphoid tissues, including spleen and thymus, as well as testis, with lesser amounts in kidney and ovary. Among lymphocyte cell lines, mSSP expression varies widely. The mSSP protein is localized to the nucleus in NIH3T3 cells, and its expression in BAL-17 B cells varies with cell cycle progression. Expression of mSSP increased significantly within the first hours of B cell treatment with either CD40L, anti-IgM, or phorbol myristate acetate (PMA) with or without ionomycin. The predicted amino acid sequence of mSSP bears some homology to proteins involved in ribosomal RNA synthesis and processing. mSSP is a previously identified pre-B cell gene now shown for the first time to be an activation-responsive transcript in mature, primary B cells.