Different conformational switches underlie the calmodulin-dependent modulation of calcium pumps and channels

We have used fluorescence spectroscopy to investigate the structure of calmodulin (CaM) bound with CaM-binding sequences of either the plasma membrane Ca-ATPase or the skeletal muscle ryanodine receptor (RyR1) calcium release channel. Following derivatization with N-(1-pyrene)maleimide at engineered sites (T34C and T110C) within the N- and C-domains of CaM, contact interactions between these opposing domains of CaM resulted in excimer fluorescence that permits us to monitor conformational states of bound CaM. Complementary measurements take advantage of the unique conserved Trp within CaM-binding sequences that functions as a hydrophobic anchor in CaM binding and permits measurements of both a local and global peptide structure. We find that CaM binds with high affinity in a collapsed structure to the CaM-binding sequences of both the Ca-ATPase and RyR1, resulting in excimer formation that is indicative of contact interactions between the N- and the C-domains of CaM in complex with these CaM-binding peptides. There is a 4-fold larger amount of excimer formation for CaM bound to the CaM-binding sequence of the Ca-ATPase in comparison to RyR1, indicating a closer structural coupling between CaM domains in this complex. Prior to CaM association, the CaM-binding sequences of the Ca-ATPase and RyR1 are conformationally disordered. Upon CaM association, the CaM-binding sequence of the Ca-ATPase assumes a highly ordered structure. In comparison, the CaM-binding sequence of RyR1 remains conformationally disordered irrespective of CaM binding. These results suggest an important role for interdomain contact interactions between the opposing domains of CaM in stabilizing the structure of the peptide complex. The substantially different structural responses associated with CaM binding to Ca-ATPase and RyR1 indicates a plasticity in their respective binding mechanisms that accomplishes different physical mechanisms of allosteric regulation, involving either the dissociation of a C-terminal regulatory domain necessary for pump activation or the modulation of intersubunit interactions to diminish RyR1 channel activity.