Expression of novel NP95 (nuclear protein, 95 kDa), which contains a leucine zipper motif, a zinc finger motif, a putative cyclin A/E-cyclin-dependent protein kinase 2 phosphorylation site, and retinoblastoma protein-binding motifs, is associated with S-phase progression of mouse cells. It is suppressed during G1 and G2/M phases in normal thymocytes but expressed at a constantly high level irrespective of cell stage in mouse T cell lymphoma cells. NP95 was shown previously to be expressed strongly only in proliferative tissues and cells. In this immunohistochemical study, we demonstrate that NP95 is localized in S-phase nuclei as dot-like foci. Double immunostaining of NP95 and proliferating cell nuclear antigen (PCNA) showed that NP95 was co-localized with PCNA. Construction of three-dimensional images indicated that NP95 was localized with PCNA in replication sites in a somewhat distinct temporal manner. During meiosis, NP95 was present not only in proliferating spermatogonia but also in meiotic spermatocytes and differentiating spermatids which were not proliferating. The possible role of NP95 in mitotic and meiotic cells is discussed.
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