Switching off calcium-dependent inactivation in l -type calcium channels by an autoinhibitory domain

Abstract

The retinal L-type Ca 2؉ channel Cav1.4 is distinguished from all other members of the high voltage-activated (HVA) Ca 2؉ channel family by lacking Ca 2؉ -calmodulin-dependent inactivation. In synaptic terminals of photoreceptors and bipolar cells, this feature is essential to translate graded membrane depolarizations into sustained Ca 2؉ influx and tonic glutamate release. The sequences conferring Ca 2؉ -dependent inactivation (CDI) are conserved throughout the HVA calcium channel family, raising the question of how Cav1.4 manages to switch off CDI. Here, we identify an autoinhibitory domain in the distal C terminus of Cav1.4 that serves to abolish CDI. We show that this domain (ICDI, inhibitor of CDI) uncouples the molecular machinery conferring CDI from the inactivation gate by binding to the EF hand motif in the proximal C terminus. Deletion of ICDI completely restores Ca 2؉ -calmodulinmediated CDI in Cav1.4. CDI can be switched off again in the truncated Cav1.4 channel by coexpression of ICDI, indicating that ICDI works as an autonomous unit. Furthermore, we show that in the Cav1.2 L-type Ca 2؉ -channel replacement of the distal C terminus by the corresponding sequence of Cav1.4 is sufficient to block CDI. This finding suggests that autoinhibition of CDI can be introduced principally into other Ca 2؉ channel types. Our data provide a previously undescribed perspective on the regulation of HVA calcium channels by Ca 2؉ .calmodulin ͉ Cav1.4 ͉ retina ͉ Ca channels

A channelopathy mechanism revealed by direct calmodulin activation of TrpV4

“…Our model is akin to the classical activation mechanism whereby an autoinhibitory interaction is disrupted by the binding of diffusible Ca 2+ -CaM to activate many enzymes (17,18) and some channels (19,20). This model is similar to that proposed by Strotmann et al (10), where the CBD itself binds an N-terminal region (S117-S134) in the absence of CaM.…”

Complex Regulation of Voltage-dependent Activation and Inactivation Properties of Retinal Voltage-gated Cav1.4 L-type Ca2+ Channels by Ca2+-binding Protein 4 (CaBP4)

“…In the present study, we explored this issue by examining functional effects of CaBP4 on heterologously expressed Cav1. 4. We show that CaBP4 dramatically increases Cav1.4 channel availability.…”

“…Using FRET experiments, we demonstrate that CaBP4 interacts with the IQ motif of Cav1. 4 and that it interferes with the binding of the ICDI domain. Based on these findings, we suggest that CaBP4 increases Cav1.4 channel availability by relieving the inhibitory effects of the ICDI domain on voltage-dependent Cav1.4 channel gating.…”

“…These graded signals are coupled with high fidelity to tonic glutamate release via permanent Ca 2ϩ influx through presynaptic Cav1.4 L-type Ca 2ϩ channels. Cav1.4 channels are tailored for their function as long-lasting presynaptic Ca 2ϩ entry pathways by a lack of Ca 2ϩ -dependent inactivation (CDI) 3 and very slow voltage-dependent inactivation (1)(2)(3)(4)(5)(6). In Cav1.4 channels, CDI is specifically abolished by a recently discovered inhibitory domain (ICDI: inhibitor of CDI) in the C-terminal tail of the Cav1.4 channel, which binds to upstream C-terminal regulatory motifs (4,5).…”

Structural Insights into Activation of the Retinal L-type Ca2+ Channel (Cav1.4) by Ca2+-binding Protein 4 (CaBP4)

“…The C-terminal regulatory region of Cav1.4 binds to CaBP4 (4), which enables channel activation at high cytosolic Ca 2ϩ levels and hyperpolarized voltages (1). A distinctive characteristic of Cav1.4 is the lack of Ca 2ϩ -dependent inactivation (CDI), because calmodulin (CaM) does not bind to its C-terminal region (5). Instead, Cav1.4 contains a stretch of residues in the C-terminal tail called the inhibitor of Ca 2ϩ -dependent inactivation (ICDI) that prevents binding of CaM to the IQ motif.…”