THE INNER NUCLEAR-MEMBRANE PROTEIN LAP1 FORMS A NATIVE COMPLEX WITH B-TYPE LAMINS AND PARTITIONS WITH SPINDLE-ASSOCIATED MITOTIC VESICLES

We have examined the in situ organization and nearest neighbours of th e 'lamina-associated polypeptide-1' (LAP1), a type II membrane protein and a major constituent of the mammalian nuclear envelope. We show he re that, during interphase, LAP1 forms multimeric assemblies which are suspended in the inner nuclear membrane and are specifically associat ed with B-type lamins. The LAP1-lamin B complex is distinct from analo gous complexes formed by the 'lamina-associated polypeptide-2' (LAP2), another inner nuclear membrane protein, and includes a protein kinase . Upon nuclear envelope breakdown, LAP1 partitions with mitotic vesicl es which carry nuclear lamin B. The LAP1 vesicles can be distinguished from fragments of the nuclear envelope containing LAP2 and exhibit a striking co-alignment with spindle microtubules. These observations su ggest that the inner nuclear membrane comprises discrete territories w hich accommodate specific integral membrane proteins and are different ially disassembled during mitosis.