Purification and characterization of calcium-binding protein from chick chorioallantoic membrane

A procedure is described for the purification of the calcium-binding protein (CaBP) from the chorioallantoic membrane of the chick embryo. With this scheme, a 180- to 200-fold purification was achieved with a 40% yield. Characterization of the CaBP revealed that its properties differ from those of previously studied calcium-binding proteins. The CaBP has a molecular weight of 95,000 to 100,000 and appears to be composed of four subunits of identical molecular weight (22,000 to 25,000). The CaBP is a basic protein as indicated by its high electrophoretic mobility under acidic conditions and its relatively high isoelectric point of 8.06. The calcium-binding activity of the CaBP is sulfhydryl dependent and highly specific for calcium ions (10 high affinity sites, ka = 2.35 X 10(7) m-1; 100 to 120 low affinity sites, ka = 2.00 X 10(5) M-1). Amino acid analysis indicated that the CaBP contains 2 to 10 residues of a modified amino acid, gamma-carboxyglutamate (gamma-CGlu). The presence of gamma-CGlu residues suggested that vitamin K may be involved in the expression of the CaBP in the chorioallantoic membrane.