DETERGENT-SALT RESISTANCE OF LAP2-ALPHA 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 (n ow LAP2 beta) and LAP2 alpha are related proteins produced by alternat ive splicing, and contain a common 187 amino acid N-terminal domain. W e show here that, unlike LAP2 beta, LAP2 alpha behaved like a nuclear non-membrane protein in subcellular fractionation studies and was loca lized throughout the nuclear interior in interphase cells. It co-fract ionated with LAP2 beta in nuclear lamina/matrix-enriched fractions upo n extraction of nuclei with detergent, salt and nucleases, During meta phase LAP2 alpha dissociated from chromosomes and became concentrated around the spindle poles. Furthermore, LAP2 alpha was mitotically phos phorylated, and phosphorylation correlated with increased LAP2 alpha s olubility upon extraction of cells in physiological buffers. LAP2 alph a relocated to distinct sites around chromosomes at early stages of nu clear reassembly and intermediarily co-localized with peripheral lamin B and intranuclear lamin A structures at telophase. During in vitro n uclear assembly LAP2 alpha was dephosphorylated and assembled into ins oluble 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 th e data suggest a role of LAP2 alpha in post-mitotic nuclear assembly a nd in the dynamic structural organization of the nucleus.