PURIFICATION AND PRIMARY STRUCTURE OF CAPL, AN S-100-RELATED CALCIUM-BINDING PROTEIN ISOLATED FROM BOVINE RETINA

Calcium protein placental homolog (Capl) is an S-100-related calcium-b inding protein selectively expressed in cell lines that have been indu ced to grow or differentiate. In addition, the expression of Capl corr elates with the induction of the metastatic phenotype in tumor cell li nes and the transformation of normal cells by activated oncogenes or c hemical carcinogens. Although not previously associated with the nervo us system, in this study, Capl was purified from bovine neural retina by a combination of phenyl-Sepharose and organomercurial chromatograph y. The complete amino acid sequence of bovine Capl was established pri marily by Edman degradation of peptides generated by cleavage of methi onyl, lysyl, glutamyl, and aspartyl bonds. NH2-terminal methionyl and aspartyl peptides were analyzed by tandem mass spectrometry, which pro vided the sequence of the first 8 residues and identified the NH2-term inal blocking group as an acetyl moiety. The molecular mass of the int act protein determined by electrospray mass spectrometry (M(r) = 11,71 6.75 +/- 0.42) and the calculated molecular mass deduced from the amin o acid composition (M(r) = 11,718) were in agreement, thus supporting the accuracy of the sequence assignment. Capl isolated from the retina was shown to be indistinguishable by mass and immunochemical. propert ies from its counterpart in the bovine aorta, which previously was the only source of purified Capl, Northern analysis using cloned Capl cDN A revealed that Capl mRNA is present not only in the retina but the ch oroid as well. Further support for choroidal localization came from im munohistochemical experiments using specific anti-Capl antibodies. The physiological significance of Capl in ocular tissues and the aorta is discussed.