Ion-induced conformational and stability changes in Nereis sarcoplasmic calcium binding protein: Evidence that the APO state is a molten globule

Nereis sarcoplasmic Ca2+-binding protein (NSCP) is a calcium buffer protein that binds Ca2+ ions with high affinity but is also able to bind Mg2+ ions with high positive cooperativity, We investigated the conformational and s tability changes induced by the two metal ions. The thermal reversible unfo lding, monitored by circular dichroism spectroscopy, shows that the thermal stability is maximum at neutral pH and increases in the order apo < Mg2+ < Ca2+. The stability against chemical denaturation (urea, guanidinium chlor ide) studied by circular dichroism or intrinsic fluorescence was found to h ave a similar ion dependence. To explore in more detail the structural basi s of stability, we used the fluorescent probes to evaluate the hydrophobic surface exposure in the different ligation states. The apo-NSCP exhibits ac cessible hydrophobic surfaces, able to bind fluorescent probes, in clear co ntrast with denatured or Ca2+/Mg2+-bound states. Gel filtration experiments showed that, although the metal-bound NSCP has a hydrodynamic volume in ag reement with the molecular mass, the volume of the apo form is considerably larger. The present results demonstrate that the apo state has many proper ties in common with the molten globule, The possible factors of the metal-d ependent structural changes and stability are discussed. Proteins 2000; 40: 177-184, (C) 2000 Wiley-Liss, Inc.