LAMIN-BINDING FRAGMENT OF LAP2 INHIBITS INCREASE IN NUCLEAR VOLUME DURING THE CELL-CYCLE AND PROGRESSION INTO S-PHASE

Lamina-associated polypeptide 2 (LAP2) is an integral membrane protein of the inner nuclear membrane that binds to both lamin B and chromati n and has a putative role in nuclear envelope (NE) organization. We fo und that microinjection of a recombinant polypeptide comprising the nu cleoplasmic domain of rat LAP2 (residues 1-398) into metaphase HeLa ce lls does not affect the reassembly of transport-competent nuclei conta ining NEs and lamina, but strongly inhibits nuclear volume increase. T his effect appears to be specifically due to lamin binding, because it also is caused by microinjection of the minimal lamin-binding region of LAP2 (residues 298-373) but not by the ckromatin-binding domain (re sidues 1-88). Injection of the lamin-binding region of rat LAP2 into e arly G1 phase HeLa cells also strongly affects nuclear growth; it almo st completely prevents the threefold nuclear volume increase that norm ally occurs during the ensuing 10 h. Moreover, injection of the fragme nt during early G1 phase strongly inhibits entry of cells into S phase , whereas injection during S phase has no apparent effect on ongoing D NA replication. Since the lamin-binding fragment of LAP2 most likely a cts by inhibiting dynamics of the nuclear lamina, our results suggest that a normal function of LAP2 involves regulation of nuclear lamina g rowth. These data also suggest that lamina dynamics are required for g rowth of the NE and for nuclear volume increase during the cell cycle, and that progression into S phase is dependent on the acquisition of a certain nuclear volume.