Human placental calreticulin: purification, characterization and association with other proteins

Calreticulin is an intracellular protein known to be involved in calcium binding, but is also known to appear as an autoantigen in certain autoimmune diseases. The cDNA sequence is known but the protein has not yet been well characterized at the amino acid level. Owing to the possible involvement of this protein in autoimmune disease and with the aim of making monoclonal antibodies for use in assay development and immunohistochemistry, we have purified calreticulin using human placental material. Amino acid analysis of the purified protein confirmed the cDNA-derived composition, and only one discrepancy between the cDNA-predicted sequence and the amino acid sequence was found by peptide mapping and microsequencing. The protein contains one disulfide bridge and has one free SH group and the protein is neither glycosylated nor phosphorylated. Affinity chromatography of a placental protein extract on a column with immobilized calreticulin showed the existence of at least six proteins interacting with calreticulin. Using the purified calreticulin in Western blots, two out of eight patients with autoimmune disease diagnosed as having anti DNA antibodies in their serum were found also to contain autoantibodies to calreticulin in their serum.