Two different proteases are involved in the proteolysis of lamin during apoptosis.

To investigate the involvement of different proteases in the execution step of apoptosis and to determine their intracellular location, isolated rat thymocyte nuclei were incubated either in the presence of Ca2+ and Mg2+ or with cytosolic extract from Jurkat T lymphocytes treated with anti-Fas (APO-1, CD-95) antibody. Inhibitors of caspases, VADcmk and DEVDcho, were not effective in hindering Ca2+-induced apoptotic changes in isolated nuclei, but did prevent similar changes in nuclei treated with the cytosolic extract from apoptotic Jurkat cells. In contrast, the inhibitor of the Ca2+-regulated, nuclear scaffold-associated serine protease, AAPFcmk, was able to inhibit lamin B1 breakdown, as well as chromatin cleavage in nuclei incubated in the presence of Ca2+ and Mg2+, but only partially prevented the same changes induced with cytosolic extract. Our findings provide evidence for the involvement of at least two proteases in lamin cleavage. One belongs to the caspase family and to cleave lamins this enzyme must be translocated from the cytoplasm into the nucleus. The second protease has a nuclear location and is activated by Ca2+. Finally, neither of these two lamin-cleaving proteases is responsible for the cleavage of another nuclear target protein, poly(ADP-ribose)polymerase (PARP), during apoptosis.