C-terminal modulator controls Ca2+-dependent gating of Cav1.4 L-type Ca2+ channels

Tonic neurotransmitter release at sensory cell ribbon synapses is mediated by calcium (Ca 2+ ) influx through L-type voltage-gated Ca 2+ channels. This tonic release requires the channels to inactivate slower than in other tissues. Ca v 1.4 L-type voltage-gated Ca 2+ channels (LTCCs) are found at high densities in photoreceptor terminals, and α1 subunit mutations cause human congenital stationary night blindness type-2 (CSNB2). Ca v 1.4 voltage-dependent inactivation is slow and Ca 2+ -dependent inactivation (CDI) is absent. We show that removal of the last 55 or 122 (C 122 ) C-terminal amino acid residues of the human α1 subunit restores calmodulin-dependent CDI and shifts voltage of half-maximal activation to more negative potentials. The C terminus must therefore form part of a mechanism that prevents calmodulin-dependent CDI of Ca v 1.4 and controls voltage-dependent activation. Fluorescence resonance energy transfer experiments in living cells revealed binding of C 122 to C-terminal motifs mediating CDI in other Ca 2+ channels. The absence of this modulatory mechanism in the CSNB2 truncation mutant K1591X underlines its importance for normal retinal function in humans.