Characterization of the histone H1-binding protein, NASP, as a cell cycle-regulated somatic protein

Nuclear autoantigenic sperm protein (NASP), initially described as a highly autoimmunogenic testis and sperm-specific protein, is a histone-binding protein that is a homologue of the N1/N2 gene expressed in oocytes of Xenopus laevis. Here, we report a somatic form of NASP (sNASP) present in all mitotic cells examined, including mouse embryonic cells and several mouse and human tissue culture cell lines. Affinity chromatography and histone isolation demonstrate that NASP from myeloma cells is complexed only with H1, linker histones. Somatic NASP is a shorter version of testicular NASP (tNASP) with two deletions in the coding region arising from alternative splicing and differs from tNASP in its 5' untranslated regions. We examined the relationship between NASP mRNA expression and the cell cycle and report that in cultures of synchronized mouse 3T3 cells and HeLa cells sNASP mRNA levels increase during S-phase and decline in G(2), concomitant with histone mRNA levels. NASP protein levels remain stable in these cells but become undetectable in confluent cultures of nondividing CV-1 cells and in nonmitotic cells in various body tissues. Expression of sNASP mRNA is regulated during the cell cycle and, consistent with a role as a histone transport protein, NASP mRNA expression parallels histone mRNA expression.