Isolation and characterization of a novel 68,000-Mr Ca2+-binding protein of lymphocyte plasma membrane

A 68 000-Mr protein is a major component of a Nonidet P-40-insoluble fraction of lymphocyte plasma membrane prepared from human B lymphoblastoid cells ( BRI 8) and pig mesenteric lymph nodes. The association of the protein with the detergent-insoluble complex depends on free Ca2+ concentrations of greater than 10 microM. The human and pig 68 000-Mr proteins were purified and appear to be homologous on the basis of amino acid composition and peptide mapping. The protein is monomeric, has pI 5.8 and a single high-affinity Ca2+-binding site (KD 1.2 microM). The results are discussed in terms of the possible role of the 68 000-Mr protein as an intracellular Ca2+ receptor in lymphocytes.