Caspase-mediated cleavage of the chromosome-binding domain of lamina-associated polypeptide 2 alpha.

Lamina-associated polypeptide 2 alpha (LAP2 alpha) is a non-membrane-bound isoform of the LAP2 family involved in nuclear structure organization. Using various cell systems, including Jurkat, HL-60, and HeLa cells, and different death-inducing agents, such as anti-Fas antibody, topoisomerase inhibitors, and staurosporine, we found that LAP2 alpha was cleaved during apoptosis as rapidly as lamin B in a caspase-dependent manner yielding stable N- and C-terminal fragments of approximately 50 and 28 kDa, respectively. Based on fragment size and localization of immunoreactive epitopes, four potential cleavage sites were mapped between amino acids 403-485. These sites were located within a domain that has previously been described to be essential and sufficient for association of LAP2 alpha with chromosomes, suggesting that LAP2 alpha cleavage impairs its chromatin-binding properties. Immunofluorescence microscopy demonstrated that, unlike full length protein, apoptotic fragments did not colocalize with condensed chromatin, but remained in the nuclear compartment as long as a single nucleus was visible. Subfractionation analyses showed that the N-terminal LAP2 alpha fragment was extracted from intranuclear structures in detergent/salt buffers, whereas the C-terminal fragment remained associated with a residual framework devoid of chromatin. Our data suggest that early cleavage of LAP2 alpha) is important for chromatin reorganization during apoptosis.