Amino acid analysis and cell cycle dependent phosphorylation of an H1-like, butyrate-enhanced protein (BEP; H1(0); IP25) from Chinese hamster cells.

A fraction enriched in the butyrate-enhanced protein (BEP) has been isolated from Chinese hamster (line CHO) cells by perchloric acid extraction and Bio-Rex 70 chromatography. Amino acid analyses indicate that the composition of BEP resembles that of CHO H1; however, BEP contains 11% less alanine than H1, and, in contrast to H1, BEP contains methionine. Treatment of BEP with cyanogen bromide results in the cleavage of a small fragment of approximately 20 amino acids so that the large fragment seen in sodium dodecyl sulfate--acrylamide gels has a molecular weight of approximately 20 000. Radiolabeling and electrophoresis indicate that BEP is phosphorylated in a cell cycle dependent fashion. In G1-arrested cells, little or no phosphate is incorporated into BEP. As cells progress through interphase, BEP becomes phosphorylated so that 12--35% of the BEP molecules are phosphorylated at one to two sites by late interphase. During mitosis, all BEP molecules become phosphorylated at approximately four sites per molecule (BEPM). Electrophoresis and the analysis of cell populations by electron microscopy indicate that the appearance of BEPM is temporally correlated with the mitotic phosphorylation of histone H1 (H1M) and with chromosomal condensation during prophase, metaphase, and anaphase. During exit from mitosis, BEPM undergoes dephosphorylation. The dephosphorylation of BEPM is temporally correlated with dephosphorylation of H1M and with the unraveling of fully condensed chromosomes near the anaphase--telophase transition. These data suggest that (1) BEP is a specialized histone of the H1 class and (2) BEP is the species equivalent of calf lung histone H1(0) [Panyim, S., & Chalkley, R. (1969) Biochem. Biophys. Res. Commun. 37, 1042], rat H1(0) [Medvedev, Zh. A., Medvedeva, M. N., & Huschtscha, L. I. (1977) Gerontology (Basel) 23, 334], and IP25, a protein enhanced in differentiated Friend erythroleukemia cells [Keppel, F., Allet, B., & Eisen, H. (1977) Proc. Natl. Acad. Sci. U.S.A. 74, 653]. The data also indicate that putative HMG1 and HMG2 proteins do not undergo the extensive cell cycle dependent phosphorylations measured for histone H1 and BEP.