HA95 is a protein of the chromatin and nuclear matrix regulating nuclear envelope dynamics

We report a role for HA95, a nuclear protein with high homology to the nuclear A-kinase anchoring protein AKAP95, in the regulation of nuclear envelope-chromatin interactions. Biochemical and photobleaching data indicate that HA95 is tightly associated with chromatin and the nuclear matrix/lamina network in interphase, and bound to chromatin at mitosis. HA95 resides in a complex together with lamin B receptor (LBR), lamina-associated polypeptide (LAP)2 and emerin, integral proteins of the inner nuclear membrane. Cross-linking experiments, however, illustrate a tight association of HA95 with LBR and LAP2 only. Intra-nuclear blocking of HA95 with anti-HA95 antibodies abolishes nuclear breakdown in a mitotic HeLa cell extract. The antibodies inhibit nuclear membrane breakdown and chromatin condensation - the latter independently of nuclear membranes. However, lamina disassembly is not affected, as judged by immunological analyses of A/C- and B-type lamins. In contrast, immunoblocking of HA95 bound to condensed chromosomes does not impair chromatin decondensation, nuclear membrane reassembly or lamina reformation. Our results argue for a role for HA95 in anchoring nuclear membranes and lamins to chromatin in interphase, and in releasing membranes from chromatin at mitosis. The data also suggest that HA95 is not involved in initial binding of membranes to chromatin upon nuclear reassembly. We propose that HA95 is a central platform at the chromatin/nuclear matrix interface implicated in regulating nuclear envelope-chromatin interactions during the cell cycle.