Cloning and nucleotide sequence of cDNA for Ki antigen, a highly conserved nuclear protein detected with sera from patients with systemic lupus erythematosus.

Patients with systemic lupus erythematosus (SLE) produce autoantibodies against a variety of nuclear antigens including Ki antigen. Although anti-Ki autoantibodies were found in a significant number of SLE patients, the nature of Ki antigen is poorly characterized. By using anti-Ki serum as a probe we have cloned a bovine cDNA directing the synthesis in Escherichia coli of a polypeptide immunologically indistinguishable from the authentic Ki antigen. A homologous human cDNA was also cloned and its nucleotide sequence predicted the entire primary structure of a novel nuclear protein with a molecular weight of 29 508 and with highly hydrophilic and weakly acidic character. The gene is highly conserved not only in the coding region but also in the 3'-untranslated region. The bacterially produced Ki antigen would be valuable for diagnosis of SLE.