Expression of novel NP95 (nuclear protein, 95 kDa), which contains a leucin
e 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 suppre
ssed during G1 and G2/M phases in normal thymocytes but expressed at a cons
tantly 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 N
P95 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 distin
ct temporal manner. During meiosis, NP95 was present not only in proliferat
ing spermatogonia but also in meiotic spermatocytes and differentiating spe
rmatids which were not proliferating. The possible role of NP95 in mitotic
and meiotic cells is discussed.