Calcium binding induces interaction between the N- and C-terminal domains of yeast calmodulin and modulates its overall conformation

Calmodulin from the yeast Saccharomyces cerevisiae binds 3 mol of Ca2+ coop eratively. We report here lines of evidence supporting the intramolecular i nteraction between the N- and C-terminal domains which modulates the Ca2+ b inding properties of yeast calmodulin, First, the sum of the Ca2+ binding c urves of the N-terminal and the C-terminal half-molecule did not yield the Ca2+ binding curve of yeast calmodulin. Second, the mean residue CD of yeas t calmodulin at 222 nm (-Delta(epsilon 222)) decreased with increases in th e concentration of Ca2+, whereas those of each half-molecule increased. Fin ally, the C2 proton of His107 in the C-terninal domain of yeast calmodulin showed three resonance peaks with increases in the concentration of Ca2+, e ach corresponding to the ape, the intermediate, and the Ca2+-saturated stat e. The intermediate peak could not be observed in the C-terminal half-molec ule of yeast calmodulin. Computer simulation considering the macroscopic Ca 2+ binding constants assigned this intermediate to a species consisting of the apo C-terminal domain and the N-terminal domain with at least one of th e two sites occupied by Ca2+. Peptide segments spanning the defective fourt h Ca2+ binding site may be involved in the interdomain interaction and the yeast-specific function of calmodulin.