A presumed human nuclear autoantigen that translocates to plasma membrane blebs during apoptosis

The structure and subcellular localization of a number of molecules change during apoptosis. These molecules are recognized by the immune system, leading to the development of autoimmunity when apoptotic cells fail to be effectively cleared by phagocytosis. We searched for such molecules by analyzing sera from 12 individuals who suffered from autoimmune diseases and from 3 patients with amyotrophic lateral sclerosis. One serum sample, designated 681, detected an antigen that fulfilled the above criteria. In Western blotting of lysates of human Jurkat T cells, the 681 antigen appeared as a distinct signal with a molecular mass of 60 kDa in normal cells, and 2 additional signals with faster mobilities were detected in apoptotic cells. The results of subcellular fractionation and immunofluorescence experiments revealed this antigen to be strictly localized in the nucleus of normal cells, but to be translocated to a region near the plasma membrane, to membrane blebs in particular, after the induction of apoptosis. Under conditions in which membrane blebbing was inhibited in apoptotic cells, the antigen still moved away from the nucleus, but its accumulation at the periplasmic region was completely abolished. The apparent partial cleavage and intracellular redistribution of the 681 antigen in apoptotic cells mimics changes previously reported for the nuclear autoantigen La, but the 681 antigen was clearly distinct from La. These results suggest that cleavage-dependent exit from the nucleus during apoptosis is a phenomenon common to nuclear autoantigens.