Regulation of Cardiac Cav1.2 Channels by Calmodulin

Figures

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  Figure 2. Effect of CaM in cell-free patches of cardiomyocytes. (A) Concentration-dependent effect of CaM on activity of Cav1.2 channels recorded in inside-out patches of guinea-pig cardiomyocytes at fixed [Ca2+], as indicated by colors on the right side of the graph. Channel activity was normalized to that recorded in the preceding cell-attached mode. Date are represented as the mean ± S.E.M. (n = 6–11) with regression curves with an equation as Amax · ([CaM]/Kdf)nf / {1+([CaM]/Kdf)nf } · 1/{1+([CaM]/Kdi)ni }, where Amax is maximum channel activity, [CaM] is concentration of CaM, Kdf and Kdi are CaM’s dissociation constant for facilitation and inactivation, respectively, and nf and ni are the Hill’s coefficients for facilitation and inactivation, respectively. (B) Relationship between estimated maximum activity (Amax) and [Ca2+]. Data were fitted with Hill’s curve with EC50 ~1 μM (red bar). Reproduced from Han et al. [51] with permission.

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  Figure 1. Structures of calmodulin (CaM) and the Cav1.2 Ca2+ channel. (A,B) apoCaM ((A), PDB:1CFD) and Ca2+/CaM ((B), PDB:1UP5) shown in rainbow color from the N-terminus (blue) to C-terminus (red) with Ca2+ indicated by the yellow spheres. (C) Schematic structure of pore-forming α1C subunit of Cav1.2 channel. CaM binding sites are indicated in red and labeled as follows: NT, NSCaTE domain; I–II, a site in I–II loop; (A,C) regions of preIQ domain; and IQ, IQ domain. The C-terminal tail is divided into CT1, CT2, and CT3 regions.

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  Figure 3. Models for CDI and CDF. (A) CDI model of Kim et al. [97]. Ca2+ binding to CaM, which is tethered to the preIQ-IQ region of the channel, induces a conformation change that initiates interaction with the EF-hand region, resulting in further interaction with the I–II loop, leading to CDI. (B) CDI model of Yue’s group [98]. Ca2+ binding to CaM induces bridging between the N- and C-termini, leading to CDI. (C) ‘Two CaM hypothesis’ [43,51,99]. At resting conditions, a fraction of channels are bound by CaM and the rest are CaM-free (low activity). (A) moderate increase in [Ca2+] increases the fraction of CaM-bound channels (high activity), leading to CDF. A further increase in [Ca2+] induces binding of the second Ca2+/CaM to the putative inactivation site, resulting in CDI. CaM’s N-lobe (light blue) and C-lobe (pink) with Ca2+-free (white circles) and partially Ca2+-saturated (pink circles) and Ca2+-saturated (red) EF-hand domains are illustrated. Other abbreviations are the same as those in Figure 1.

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  Figure 4. Structure models of preIQ-IQ/Cav1.2 bound with CaM. (A) Crystallographic structure of two preIQ-IQ bound with four CaM. CaM N-lobes, tan; C-lobes, light blue; Ca2+, gold spheres. The preIQ domain (I and II) are blue to orange rainbow starting from the N termini (N); IQ domain of CaV I is red. The preIQ-IQ loop regions, IQ domain of CaV II and C-lobe of CaM IV are computer modeled (thin light gray lines). From [127] with permission. (B) Structure model of preIQ-IQ of Cav1.2 bound with two CaM. The ‘?’ signifies the potential that the second Ca2+/CaM anchors at the C region of preIQ to interact with other components of Cav1.2. Cavβ (green and blue) bound to I–II loop (red) are shown as a candidate of such components. Orientation of the C-terminal tail relative to the other intracellular components of Cav1.2 is not identified. From [107] with permission. For the second CaM, the C-lobe is bound to preIQ, and the N-lobe binds another region, possibly the Cavβ/I–II loop. (C,D). Structure model of preIQ-IQ of Cav1.2 bound with two CaM. The preIQ structure is assumed to bend or turn at 1610QANE1613 region (rabbit) between the A and C regions (C) to allow the second CaM to bridge these…