Calmodulin promotes a Ca²⁺‐dependent conformational change in the C‐terminal regulatory domain of Ca_V1.2

Abstract

Calmodulin (CaM) binds to the membrane-proximal cytosolic C-terminal domain of Ca V 1.2 (residues 1520-1669, CT(1520-1669)) and causes Ca 2+induced conformational changes that promote Ca 2+ -dependent channel inactivation (CDI). We report biophysical studies that probe the structural interaction between CT(1520-1669) and CaM. The recombinantly expressed CT (1520-1669) is insoluble, but can be solubilized in the presence of Ca 2+saturated CaM (Ca 4 /CaM), but not in the presence of Ca 2+ -free CaM (apoCaM). We show that half-calcified CaM (Ca 2 /CaM 12 ) forms a complex with CT(1520-1669) that is less soluble than CT(1520-1669) bound to Ca 4 / CaM. The NMR spectrum of CT(1520-1669) reveals spectral differences caused by the binding of Ca 2 /CaM 12 versus Ca 4 /CaM, suggesting that the binding of Ca 2+ to the CaM N-lobe may induce a conformational change in CT(1520-1669).

Regulation of Cardiac Cav1.2 Channels by Calmodulin

“…CDF might be produced by burying the Ile of the IQ motif and preventing its interaction with the EF hand [ 62 ]. Ames et al [ 15 ] and Yadav et al [ 99 ] proposed a similar model of CDI in which the C-lobe of apoCaM, or half-calcified CaM (Ca 2+ /C-lobe and apoN-lobe), interacts with the IQ domain while the N-lobe is free, allowing IQ to also interact with the EF-hand domain in the pCT. This conformation prevents interactions between the EF-hand domain and the inactivation gate in the III-IV linker of Cav1.2 (instead of I-II linker, see above), maintaining the channel in an activatable state.…”

“…Ca 2+ binding to CaM induces bridging between the N- and C-termini, leading to CDI. ( C ) ‘Two CaM hypothesis’ [ 43 , 51 , 99 ]. At resting conditions, a fraction of channels are bound by CaM and the rest are CaM-free (low activity).…”