Nereis sarcoplasmic Ca2+-binding protein has a highly unstructured apo state which is switched to the native state upon binding of the first Ca2+ ion

Abstract NSCP, a sarcoplasmic Ca 2+ /Mg 2+ -binding protein from Nereis diversicolor , shows an allosteric change during Ca 2+ binding and a high positive cooperativity for Mg 2+ binding. Here we report the results of CD and NMR experiments aiming to characterize the apo state and the Ca 2+ -induced conformational changes in this protein. Circular dichroism spectra of the apo form are indicative of a reduced helical structure. In contrast, NMR spectra show no element of regular secondary or tertiary structure. Addition of one Ca 2+ determines large spectral changes bringing the molecule in a conformation which is very close to the native three Ca 2+ state. Addition of the second and third Ca 2+ shifts this equilibrium progressively towards the liganded conformation but affects only minimally the spectrum of the liganded species.