Splicing of an automodulatory domain in Ca _v1.4 Ca 2+ channels confers distinct regulation by calmodulin

摘要

Ca _(v)1.4 Ca ~(2+) channels provide maintained Ca ~(2+) entry to support sustained neurotransmitter release, but a retinal splice variant exhibits calmodulin-dependent inactivation. Williams et al. show that the N lobe of calmodulin is involved in this process as well as Ca ~(2+)-dependent enhancement of channel activation. Ca ~(2+) influx through Ca _(v)1.4 L-type Ca ~(2+) channels supports the sustained release of glutamate from photoreceptor synaptic terminals in darkness, a process that is critical for vision. Consistent with this role, Ca _(v)1.4 exhibits weak Ca ~(2+)-dependent inactivation (CDI)—a negative feedback regulation mediated by Ca ~(2+)-bound calmodulin (CaM). CaM binds to a conserved IQ domain in the proximal C-terminal domain of Ca _(v) channels, but in Ca _(v)1.4, a C-terminal modulatory domain (CTM) disrupts interactions with CaM. Exon 47 encodes a portion of the CTM and is deleted in a Ca _(v)1.4 splice variant (Ca _(v)1.4Δex47) that is highly expressed in the human retina. Ca _(v)1.4Δex47 exhibits CDI and enhanced voltage-dependent activation, similar to that caused by a mutation that is associated with congenital stationary night blindness type 2, in which the CTM is deleted (K1591X). The presence of CDI and very negative activation thresholds in a naturally occurring variant of Ca _(v)1.4 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 Ca _(v)1.4Δex47 and K1591X exhibit fundamental differences in their regulation by CaM. In Ca _(v)1.4Δex47, CDI requires both the N-terminal (N lobe) and C-terminal (C lobe) lobes of CaM to bind Ca ~(2+), whereas CDI in K1591X is driven mainly by Ca ~(2+) binding to the C lobe. Moreover, the CaM N lobe causes a Ca ~(2+)-dependent enhancement of activation of Ca _(v)1.4Δex47 but not K1591X. We conclude that the residual CTM in Ca _(v)1.4Δex47 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 Ca ~(2+) than the C lobe, may allow Ca _(v)1.4Δex47 to be modulated by a wider range of synaptic Ca ~(2+) concentrations than K1591X; this may distinguish the normal physiological function of Ca _(v)1.4Δex47 from the pathological consequences of K1591X.