| Abstract: | We have characterized the biochemical nature of the Ku protein, the antigen recognized by autoantibodies from certain patients with scleroderma-polymyositis overlap syndrome. From extracts of HeLa cells labeled with 32P]orthophosphate, anti-Ku antibodies precipitated a high molecular weight nucleic acid identified as DNA because of sensitivity to DNase I and resistance to RNase. From extracts of cells labeled with 35S] methionine, these antibodies precipitated two polypeptides of 70,000 and 80,000 Da. These proteins were purified using immunoaffinity column chromatography. In immunoblots most sera containing anti-Ku antibodies recognized both Ku proteins but one serum bound only to the 70,000-Da subunit. When nucleosomal segments of chromatin were used as antigen, anti-Ku antibodies precipitated dinucleosomes and larger forms of chromatin but not mononucleosomes. Thus, the Ku antigen is a novel DNA-binding protein that is at least partially exposed on nucleosomal segments of chromatin. |
