Roles of LAP2 proteins in nuclear assembly and DNA replication:: Truncated LAP2β proteins alter lamina assembly, envelope formation, nuclear size, and DNA replication efficiency in Xenopus laevis extracts

Humans express three major splicing isoforms of LAP2, a lamin- and chromatin-binding nuclear protein. LAP2 beta and gamma are integral membrane proteins, whereas alpha is intranuclear, When truncated recombinant human LAP2 beta proteins were added to cell-free Xenopus laevis nuclear assembly reactions at high concentrations, a domain common to all LAP2 isoforms (residues 1-187) inhibited membrane binding to chromatin, whereas the chromatin- and lamin-binding region (residues 1-408) inhibited chromatin expansion. At lower concentrations of the common domain, membranes attached to chromatin: with a unique scalloped morphology, but these nuclei neither accumulated lamins nor replicated. At lower concentrations of the chromatin- and lamin-binding region, nuclear envelopes and lamins assembled, but nuclei failed to enlarge and replicated on average 2.5-fold better than controls. This enhancement was not due to rereplication, as shown by density substitution experiments, suggesting the hypothesis that LAP2 beta is a downstream effector of lamina assembly in promoting replication competence. Overall, our findings suggest that LAP2 proteins mediate membrane-chromatin attachment and lamina assembly, and may promote replication by influencing chromatin structure.