Detergent-salt resistance of LAP2α in interphase nuclei and phosphorylation-dependent association with chromosomes early in nuclear assembly implies functions in nuclear structure dynamics

Lamina-associated polypeptide (LAP) 2 of the inner nuclear membrane (now LAP2 beta) and LAP2 alpha are related proteins produced by alternative splicing, and contain a common 187 amino acid N-terminal domain. We show here that, unlike LAP2 beta, LAP2 alpha behaved like a nuclear non-membrane protein in subcellular fractionation studies and was localized throughout the nuclear interior in interphase cells. It co-fractionated with LAP2 beta in nuclear lamina/matrix-enriched fractions upon extraction of nuclei with detergent, salt and nucleases, During metaphase LAP2 alpha dissociated from chromosomes and became concentrated around the spindle poles. Furthermore, LAP2 alpha was mitotically phosphorylated, and phosphorylation correlated with increased LAP2 alpha solubility upon extraction of cells in physiological buffers. LAP2 alpha relocated to distinct sites around chromosomes at early stages of nuclear reassembly and intermediarily co-localized with peripheral lamin B and intranuclear lamin A structures at telophase. During in vitro nuclear assembly LAP2 alpha was dephosphorylated and assembled into insoluble chromatin-associated structures, and recombinant LAP2 alpha was found to interact with chromosomes in vitro. Some LAP2 alpha may also associate with membranes prior to chromatin attachment. Altogether the data suggest a role of LAP2 alpha in post-mitotic nuclear assembly and in the dynamic structural organization of the nucleus.