Ion-binding properties of recombinant S100beta and two derivatives with either an inactivated Ca2+ site II or a...

S100beta contains one unusual and one canonical Ca2+-binding motif. In this study, we measured Ca2+-binding and ensuing conformational changes of recombinant S100beta (rS100beta) and of two mutant forms in which either the canonical loop was inactivated (NoEF) or the unusual one replaced by a canonical one (Caloops). Caloops binds two Ca2+ per monomer with a 3-fold higher affinity than rS100beta; the affinity of NoEF was too low for accurate direct determination. All three proteins bind 3-4 Zn2+ per monomer. Tyrosine 17 fluorescence spectra showed a decrease of intensity upon binding of Ca2+ to the three proteins and an increase upon binding of Zn2+ to rS100beta and NoEF but not in Caloops. The fluorescence change as a function of the Ca2+ concentration yielded half-maximal changes ([Ca2+]0.5) at 1.7, 11.3 and 0.55 mM free Ca2+ for rS100beta NoEF and Caloops, respectively. Our data demonstrate that in S100beta alterations in one site can affect the Ca2+ binding properties of the other site.