Abstract
Several members of the voltage-gated sodium channel family are regulated by calmodulin (CaM) and ionic calcium. The neuronal voltage-gated sodium channel Na(V)1.2 contains binding sites for both apo (calcium-depleted) and calcium-saturated CaM. We have determined equilibrium dissociation constants for rat Na(V)1.2 IQ motif [IQRAYRRYLLK] binding to apo CaM (~3nM) and (Ca(2+))(4)-CaM (~85nM), showing that apo CaM binding is favored by 30-fold. For both apo and (Ca(2+))(4)-CaM, NMR demonstrated that Na(V)1.2 IQ motif peptide (Na(V)1.2(IQp)) exclusively made contacts with C-domain residues of CaM (CaM(C)). To understand how calcium triggers conformational change at the CaM-IQ interface, we determined a solution structure (2M5E.pdb) of (Ca(2+))(2)-CaM(C) bound to Na(V)1.2(IQp). The polarity of (Ca(2+))(2)-CaM(C) relative to the IQ motif was opposite to that seen in apo CaM(C)-Na(v)1.2(IQp) (2KXW), revealing that CaM(C) recognizes nested, anti-parallel sites in Na(v)1.2(IQp). Reversal of CaM may require transient release from the IQ motif during calcium binding, and facilitate a re-orientation of CaM(N) allowing interactions with non-IQ Na(V)1.2 residues or auxiliary regulatory proteins interacting in the vicinity of the IQ motif.