Disappearance of a structural chromatin protein A24 in mitosis: implications for molecular basis of chromatin condensation.

A chromatin protein, A24, a conjugate of histone H2A and evolutionally conserved ubiquitin, was virtually the only structural polypeptide that was present in interphase but missing in mitosis of a Chinese hamster cell line (DON). Because a 10% increase in the H2A/DNA ratio observed in interphase-mitosis transition explained the stoichiometric conversion of A24 to H2A, it appears that ubiquitin bound to H2A of nucleosomal surfaces in interphase is released at mitosis whereas the total H2A remains as a structural component of nucleosomes. Regardless of protein synthesis, ubiquitin was again bound to H2A when cells entered the G1 phase. Based on the electrostatic nature of the COOH-terminal region of H2A, where ubiquitin binds, and the mitosis-specific rise of covalently linked phosphates in histones H1 and H3, we propose that an ionic interaction between the positively charged H2A COOH-terminal regions on fibers and negatively charged phosphates linked to serine or threonine of H1 and H3 molecules on adjacent fibers could generate an assembly of chromatin fibers in mitosis.