HUMAN PLACENTAL CALRETICULIN - PURIFICATION, CHARACTERIZATION AND ASSOCIATION WITH OTHER PROTEINS

Calreticulin is an intracellular protein known to be involved in calci um binding, but is also known to appear as an autoantigen in certain a utoimmune diseases. The cDNA sequence is known but the protein has not yet been well characterized at the amino acid level. Owing to the pos sible involvement of this protein in autoimmune disease and with the a im of making monoclonal antibodies for use in assay development and im munohistochemistry, we have purified calreticulin using human placenta l material. Amino acid analysis of the purified protein confirmed the cDNA-derived composition, and only one discrepancy between the cDNA-pr edicted sequence and the amino acid sequence was found by peptide mapp ing and microsequencing. The protein contains one disulfide bridge and has one free SH group and the protein is neither glycosylated nor pho sphorylated. Affinity chromatography of a placental protein extract on a column with immobilized calreticulin showed the existence of at lea st six proteins interacting with calreticulin. Using the purified calr eticulin in Western blots, two out of eight patients with autoimmune d isease diagnosed as having anti DNA antibodies in their serum were fou nd also to contain autoantibodies to calreticulin in their serum.