Neuronal excitability is modulated in part by the Ca
2+-dependent activity of CaV channels localized at the synaptic membrane. CaM binding to CaV serves to increase channel activity at low cytosolic Ca
2+ levels under basal conditions ([Ca
2+]
i = 100 nM). Conversely, CaM decreases CaV channel activity at higher cytosolic Ca
2+ levels (([Ca
2+]
i = 1.0 μM) caused by neuronal stimulation. Thus, CaM acts as both an accelerator and a brake to control CaV channel opening
[16]. Ca
2+ influx through CaV channels causes elevated intracellular Ca
2+ levels that in turn promote a rapid negative feedback channel inactivation (called Ca
2+-dependent inactivation or CDI
[16]), mediated by CaM (
Figure 4). Rapid CDI requires CaM to be pre-associated with CaV under basal conditions
[29][33]. The channel has been suggested to be pre-associated with apoCaM under basal conditions (
Figure 4A)
[35], and apoCaM binding to CaV may increase Ca
2+ currents (I
Ca) and channel open probability (Po)
[56], whereas I
Ca is dramatically decreased at elevated Ca
2+ levels, because Ca
2+-bound CaM inactivates the channel
[15][16]. As a result, apoCaM binding to CaV in which the CaM C-lobe is bound to the IQ motif (red box in
Figure 4) and CaM N-lobe is bound to the channel EF-hand (orange box in
Figure 4) is believed to stabilize the channel in the open state at low Ca
2+ levels under basal conditions (
Figure 4B). At elevated Ca
2+ levels (caused by neuronal stimulation), Ca
2+-saturated CaM has been suggested to bind to the full-length CaV at two different sites: The N-lobe binds to the NSCaTE domain
[15][57] and the CaM C-lobe binds to the IQ motif
[44], which is hypothesized to stabilize the channel in the inactive state (
Figure 4C). Atomic-level structures are known for Ca
2+/CaM bound to IQ
[44] and NSCaTE
[57] domains. However, structures are not yet known for apoCaM and Ca
2+/CaM each bound to the entire C-terminal cytosolic domain of CaV comprised of the channel EF-hand and IQ-motif (called CT1 domain,
Figure 4C). Future studies are needed to elucidate the structural interaction of apoCaM and Ca
2+/CaM each bound to the full-length channel to further test the model in
Figure 4.
Figure 4. Conventional Model of CDI from CaV regulated by CaM and CaBP1. (A) Under resting conditions ([Ca2+]i = 100 nM), CaV (dark blue) is in the closed channel state, which has been suggested to be pre-associated with Ca2+-free forms of CaM (cyan) or CaBP1 (yellow). (B) Membrane depolarization causes channel opening, which causes Ca2+ influx. Initially at low cytosolic Ca2+ levels (([Ca2+]i < 300 nM), CaV is bound to Ca2+-free forms of CaM or CaBP1, which stabilize the active open state. (C) After sufficient Ca2+ influx, the cytosolic Ca2+ level increases to above 1 micromolar, which causes Ca2+ binding to CaM and the Ca2+-bound CaM promotes channel inactivation (CDI). Alternatively, CaV binding to CaBP1 (yellow) displaces CaM and prevents CDI (bottom panel). (D) The binding of Ca2+-bound CaBP1 to CaV promotes channel opening at elevated Ca2+ levels (called CDF). Bound Ca2+ are indicated by red circles.