Effects of Cation Binding on the Conformation of Calsequestrin and the High Affinity Calcium-binding Protein of Sarcoplasmic Reticulum

Abstract

Abstract The circular dichroism spectra of calsequestrin and the high affinity calcium-binding protein from rabbit skeletal muscle sarcoplasmic reticulum have been recorded in the presence and absence of Ca2+ and other cations. Calsequestrin Forms I and II show decreases in negativity upon Ca2+ binding from -12 to -6 x 103 and -20 to -10 x 103 deg cm2 dmole-1, respectively, at 200 nm, and increases from -5 to -6 x 103 and -7 to -10 x 103 deg cm2 dmole-1 at 220 nm. Above 250 nm there is little optical activity in the absence of cations, but both forms of calsequestrin show a complex signal with maximum of about 90 deg cm2 dmole-1 between 270 and 285 nm in their presence. The changes in circular dichroism are interpreted as reflecting a large conformational change in the protein upon cation binding. Absorption difference spectra in the 320 to 240 nm range confirm that calcium binding induces perturbation in the absorption bands of tryosine, tryptophan, and phenylalanine, probably as a result of greater folding. The circular dichroism and absorption spectra of the high affinity calcium-binding protein do not change upon binding calcium. This protein, even in the absence of calcium, has intense circular dichroism activity in the aromatic region, with a maximum of 250 deg cm2 dmole-1 at about 280 nm.