Distinct Functions of the Unique C Terminus of LAP2α in Cell Proliferation and Nuclear Assembly

The non-membrane-bound lamina-associated polypeptide 2 isoform, LAP2α, forms nucleoskeletal structures with A-type lamins and interacts with chromosomes in a cell cycle-dependent manner. LAP2α contains a LEM (LAP2, emerin, and MAN1) domain in the constant N terminus that binds to chromosomal barrier-to-autointegration factor, and a C-terminal unique region that is essential for chromosome binding. Here we show that C-terminal LAP2α fragment efficiently bound to mitotic chromosomes and inhibited assembly of endogenous LAP2α, nuclear membranes, and lamins A/C in in vitro nuclear assembly assays. Full-length recombinant LAP2α, which bound to chromosomes, and N-terminal fragment, which did not bind, had no effect on assembly. This suggested an essential role for the LAP2α C terminus in chromosome association and for the N-terminal LEM domain in subsequent assembly stages. In vivo analysis upon transient expression of GFP-tagged LAP2α fragments confirmed that, unlike the N-terminal fragment, the C-terminal fragment was able to bind to chromosomes during mitosis, if expressed weakly. At higher expression levels, C-terminal LAP2α fragment and full-length protein led to cell cycle arrest in interphase and apoptosis, as shown by fluorescence-activated cell sorter analysis, time lapse microscopy, and BrdUrd incorporation assays. These data indicated distinct functions of LAP2α in cell cycle progression during interphase and in nuclear reassembly during mitosis.