Neurogranin Alters the Structure and Calcium Binding Properties of Calmodulin.

Neurogranin (Ng) is a member of the IQ motif class of calmodulin (CaM)-binding proteins, and interactions with CaM are its only known biological function. In this report we demonstrate that the binding affinity of Ng for CaM is weakened by Ca2+ but to a lesser extent (2-3-fold) than that previously suggested from qualitative observations. We also show that Ng induced a >10-fold decrease in the affinity of Ca2+ binding to the C-terminal domain of CaM with an associated increase in the Ca2+ dissociation rate. We also discovered a modest, but potentially important, increase in the cooperativity in Ca2+ binding to the C-lobe of CaM in the presence of Ng, thus sharpening the threshold for the C-domain to become Ca2+-saturated. Domain mapping using synthetic peptides indicated that the IQ motif of Ng is a poor mimetic of the intact protein and that the acidic sequence just N-terminal to the IQ motif plays an important role in reproducing Ng-mediated decreases in the Ca2+ binding affinity of CaM. Using NMR, full-length Ng was shown to make contacts largely with residues in the C-domain of CaM, although contacts were also detected in residues in the N-terminal domain. Together, our results can be consolidated into a model where Ng contacts residues in the N- and C-lobes of both apo- and Ca2+-bound CaM and that although Ca2+ binding weakens Ng interactions with CaM, the most dramatic biochemical effect is the impact of Ng on Ca2+ binding to the C-terminal lobe of CaM. [ABSTRACT FROM AUTHOR]