Characterization of two scleroderma autoimmune antigens that copurify with human ribonuclease P

Human RNase P has been purified more than 2000-fold from HeLa cells. In addition to the RNA component, H1 RNA, polypeptides of molecular masses 14, 20, 25, 30, 38, and 40 kDa copurify with the enzyme activity. Sera from two different patients with the autoimmune disease scleroderma were used to immunodeplete human RNase P activity. These same sera cross-reacted on immunoblots with two of the copurifying polypeptides, p30 and p38, whereas an autoimmune serum that does not immunodeplete RNase P activity did not react with these proteins. Peptide fragments derived from purified p30 and p38 facilitated the molecular cloning and sequencing of cDNAs coding for these two polypeptides, which are now designated as Rpp30 and Rpp38, respectively. RPP38 cDNA encodes a polypeptide that may be identical to a previously identified antigen of ≈40 kDa, which is immunoprecipitated by Th and To autoimmune antisera, and that has been implicated as a protein subunit of human RNase P by virtue of its ability to bind to H1 RNA in vitro. The second autoimmune antigen, Rpp30, as such, has not been described previously.