Disruption of the IS6-AID Linker Affects Voltage-gated Calcium Channel Inactivation and Facilitation

Two processes dominate voltage-gated calcium channel (Ca(V)) inactivation: voltage-dependent inactivation (VDI) and calcium-dependent inactivation (CDI). The Ca(V)β/Ca(V)α(1)-I-II loop and Ca(2+)/calmodulin (CaM)/Ca(V)α(1)–C-terminal tail complexes have been shown to modulate each, respectively. Nevertheless, how each complex couples to the pore and whether each affects inactivation independently have remained unresolved. Here, we demonstrate that the IS6–α-interaction domain (AID) linker provides a rigid connection between the pore and Ca(V)β/I-II loop complex by showing that IS6-AID linker polyglycine mutations accelerate Ca(V)1.2 (L-type) and Ca(V)2.1 (P/Q-type) VDI. Remarkably, mutations that either break the rigid IS6-AID linker connection or disrupt Ca(V)β/I-II association sharply decelerate CDI and reduce a second Ca(2+)/CaM/Ca(V)α(1)–C-terminal–mediated process known as calcium-dependent facilitation. Collectively, the data strongly suggest that components traditionally associated solely with VDI, Ca(V)β and the IS6-AID linker, are essential for calcium-dependent modulation, and that both Ca(V)β-dependent and CaM-dependent components couple to the pore by a common mechanism requiring Ca(V)β and an intact IS6-AID linker.