Three Ca2+-binding proteins from porcine liver and intestine differ immunologically and physicochemically and are distinct in Ca2+ affinities

Abstract

Intestinal brush-border-derived membrane vesicles contain, after demembranation in the presence of Ca2+, a subset of polypeptides that are specifically solubilized by the addition of Ca2+ chelators. As described previously, this fractionation scheme leads to the enrichment of two major proteins (I and II), one of which has been shown to be identical to the cellular p36K target of Rous sarcoma virus-encoded tyrosine-specific protein kinase (Gerke, V., and Weber, K., (1984) EMBO J. 3, 227-233). We have applied a similar protocol to membrane vesicles from porcine liver and purified a third Ca2+-binding protein (III). All three proteins had wide tissue distributions, and were absent from brain, red blood cells, and cardiac and skeletal muscle. Relative amounts varied between tissues, with protein I low in liver and protein III very low in intestine. Despite their similar extractability the three proteins (I, II, and III) are clearly distinct as far as immunological, biochemical, and physicochemical properties are concerned. They also show characteristic differences in their affinities for Ca2+ ions. The association constants of Ca2+ binding for proteins I and III have been estimated by means of indirect methods to be 10(4) M-1 (protein I) and 10(6) M-1 (protein III), while the direct Hummel-Dreyer method reveals Ca2+ binding to protein II, characterized by an association constant of 0.4 X 10(5) M-1 in the absence and 0.2 X 10(5) M-1 in the presence of 2 mM MgCl2. Conformational changes upon binding Ca2+ are described for protein II using circular dichroism, fluorescence emission, and UV difference spectra. These alterations could be attributed to an increased exposure of tyrosine and tryptophan residues to a more aqueous environment, and led to increased hydrophobicity of protein II that would explain the observed Ca2+-dependent interaction with hydrophobic matrices like phenyl-Sepharose.