In synaptic terminals of retinal photoreceptors, Cav1.4 (L-type) Ca2+ channels mediate Ca2+ influx that promotes neurotransmitter release. Mutations in Cav1.4 are associated with multiple vision disorders including congenital stationary night blindness type 2(CSNB2). Cav1.4 undergoes weak Ca2+-dependent inactivation (CDI) – a negative feedback mechanism seen for other L-type channels (e.g., Cav1.2 and Cav1.3) mediated by calmodulin (CaM) binding to a consensus IQ domain in the proximal C-terminal domain (CT) of the pore-forming a1 subunit. The lack of CDI in Cav1.4 is due to a C-terminal automodulatory domain (CTM), located in the distal CT of Cav1.4. The CTM is thought to suppress CDI of Cav1.4 channels by competing with CaM binding to sites in the proximal CT. A CSNB2-causing mutation (K1591X) in Cav1.4 that deletes the CTM promotes CaM binding and CDI, but also causes channel activation at more negative potentials than full-length channels (Cav1.4FL).
We have identified a human-specific Cav1.4 splice variant that removes part of the CTM due to the deletion of exon 47 (Cav1.4Δex47). In electrophysiological recordings of transfected HEK 293T cells, we found that Cav1.4Δex47 channels undergo robust CDI and activates at more negative potentials, like K1591X. The presence of CDI and very negative activation thresholds in a naturally occurring variant of Cav1.4 are perplexing considering that these properties are expected to be maladaptive for visual signaling and result in night blindness in the case of K1591X. Here we show that Cav1.4Δex47 and K1591X exhibit fundamental differences in their regulation by CaM. In Cav1.4Δex47, CDI requires both the N-terminal (N lobe) and C-terminal (C lobe) lobes of CaM to bind Ca2+, whereas CDI in K1591X is driven mainly by Ca2+ binding to the C lobe. Moreover, the CaM N lobe causes a Ca2+-dependent enhancement of activation of Cav1.4Δex47 but not K1591X. We conclude that the residual CTM in Cav1.4Δex47 enables a form of CaM N lobe regulation of activation and CDI that is absent in K1591X. Interaction with the N lobe of CaM, which is more sensitive to global elevations in cytosolic Ca2+ than the C lobe, may allow Cav1.4Δex47 to be modulated by a wider range of synaptic Ca2+ concentrations than K1591X; this may distinguish the normal physiological function of Cav1.4Δex47 from the pathological consequences of K1591X.