The IQ motif is crucial for Cav1.1 function

The Crystal Structure of Calmodulin Bound to the Cardiac Ryanodine Receptor (RyR2) at Residues Phe4246-Val4271 Reveals a Fifth Calcium Binding Site

Calmodulin (CaM) regulates the activity of a Ca2+ channel known as the cardiac ryanodine receptor (RyR2), which facilitates the release of Ca2+ from the sarcoplasmic reticulum during excitation-contraction coupling in cardiomyocytes. Mutations that disrupt this CaM-dependent channel inactivation result in cardiac arrhythmias. RyR2 contains three different CaM binding sites: CaMBD1 (residues 1940-1965), CaMBD2 (residues 3580-3611), and CaMBD3 (residues 4246-4275). Here, we report a crystal structure of Ca2+-bound CaM bound to RyR2 CaMBD3. The structure reveals Ca2+ bound to the four EF-hands of CaM as well as a fifth Ca2+ bound to CaM in the interdomain linker region involving Asp81 and Glu85. The CaM mutant E85A abolishes the binding of the fifth Ca2+ and weakens the binding of CaMBD3 to Ca2+-bound CaM. Thus, the binding of the fifth Ca2+ is important for stabilizing the complex in solution and is not a crystalline artifact. The CaMBD3 peptide in the complex adopts an α-helix (between Phe4246 and Val4271) that interacts with both lobes of CaM. Hydrophobic residues in the CaMBD3 helix (Leu4255 and Leu4259) form intermolecular contacts with the CaM N-lobe, and the CaMBD3 mutations (L4255A and L4259A) each weaken the binding of CaM to RyR2. Aromatic residues on the opposite side of the CaMBD3 helix (Phe4246 and Tyr4250) interact with the CaM C-lobe, but the mutants (F4246A and Y4250A) have no detectable effect on CaM binding in solution. We suggest that the binding of CaM to CaMBD3 and the binding of a fifth Ca2+ to CaM may contribute to the regulation of RyR2 channel function.

Calmodulin-Binding Proteins in Muscle: A Minireview on Nuclear Receptor Interacting Protein, Neurogranin, and Growth-Associated Protein 43

Calmodulin (CaM) is an important Ca2+-sensing protein with numerous downstream targets that are either CaM-dependant or CaM-regulated. In muscle, CaM-dependent proteins, which are critical regulators of dynamic Ca2+ handling and contractility, include calcineurin (CaN), CaM-dependant kinase II (CaMKII), ryanodine receptor (RyR), and dihydropyridine receptor (DHPR). CaM-regulated targets include genes associated with oxidative metabolism, muscle plasticity, and repair. Despite its importance in muscle, the regulation of CaM-particularly its availability to bind to and activate downstream targets-is an emerging area of research. In this minireview, we discuss recent studies revealing the importance of small IQ motif proteins that bind to CaM to either facilitate (nuclear receptor interacting protein; NRIP) its activation of downstream targets, or sequester (neurogranin, Ng; and growth-associated protein 43, GAP43) CaM away from their downstream targets. Specifically, we discuss recent studies that have begun uncovering the physiological roles of NRIP, Ng, and GAP43 in skeletal and cardiac muscle, thereby highlighting the importance of endogenously expressed CaM-binding proteins and their regulation of CaM in muscle.

STAC proteins: The missing link in skeletal muscle EC coupling and new regulators of calcium channel function

Excitation-contraction coupling is the signaling process by which action potentials control calcium release and consequently the force of muscle contraction. Until recently, three triad proteins were known to be essential for skeletal muscle EC coupling: the voltage-gated calcium channel CaV1.1 acting as voltage sensor, the SR calcium release channel RyR1 representing the only relevant calcium source, and the auxiliary CaV β1a subunit. Whether CaV1.1 and RyR1 are directly coupled or whether their interaction is mediated by another triad protein is still unknown. The recent identification of the adaptor protein STAC3 as fourth essential component of skeletal muscle EC coupling prompted vigorous research to reveal its role in this signaling process. Accumulating evidence supports its possible involvement in linking CaV1.1 and RyR1 in skeletal muscle EC coupling, but also indicates a second, much broader role of STAC proteins in the regulation of calcium/calmodulin-dependent feedback regulation of L-type calcium channels.

Duplex signaling by CaM and Stac3 enhances CaV1.1 function and provides insights into congenital myopathy

CaV1.1 is essential for skeletal muscle excitation-contraction coupling. Its functional expression is tuned by numerous regulatory proteins, yet underlying modulatory mechanisms remain ambiguous as CaV1.1 fails to function in heterologous systems. In this study, by dissecting channel trafficking versus gating, we evaluated the requirements for functional CaV1.1 in heterologous systems. Although coexpression of the auxiliary β subunit is sufficient for surface-membrane localization, this baseline trafficking is weak, and channels elicit a diminished open probability. The regulatory proteins calmodulin and stac3 independently enhance channel trafficking and gating via their interaction with the CaV1.1 carboxy terminus. Myopathic stac3 mutations weaken channel binding and diminish trafficking. Our findings demonstrate that multiple regulatory proteins orchestrate CaV1.1 function via duplex mechanisms. Our work also furnishes insights into the pathophysiology of stac3-associated congenital myopathy and reveals novel avenues for pharmacological intervention.

STAC proteins associate to the IQ domain of CaV1.2 and inhibit calcium-dependent inactivation

Association of the adaptor protein STAC3 to the calcium channel was recently identified as essential for skeletal muscle contraction. While STAC3 is expressed exclusively in skeletal muscle, the other two isoforms, STAC1 and STAC2, are found in the brain, but their function has not yet been investigated. Recently we identified the C1 domain of STAC as crucial for its association with calcium channels. However, the corresponding binding domain remained unknown. Here, we demonstrate that STAC proteins associate with the C terminus of Ca_V1.2 at the IQ domain, and that this interaction results in the inhibition of calcium-dependent inactivation. Taken together, our data identify a functionally important STAC interaction domain and suggest that STAC proteins modulate calcium entry through Ca_V1 channels.

The adaptor proteins STAC1, STAC2, and STAC3 represent a newly identified family of regulators of voltage-gated calcium channel (Ca_V) trafficking and function. The skeletal muscle isoform STAC3 is essential for excitation–contraction coupling and its mutation causes severe muscle disease. Recently, two distinct molecular domains in STAC3 were identified, necessary for its functional interaction with Ca_V1.1: the C1 domain, which recruits STAC proteins to the calcium channel complex in skeletal muscle triads, and the SH3-1 domain, involved in excitation–contraction coupling. These interaction sites are conserved in the three STAC proteins. However, the molecular domain in Ca_V1 channels interacting with the STAC C1 domain and the possible role of this interaction in neuronal Ca_V1 channels remained unknown. Using Ca_V1.2/2.1 chimeras expressed in dysgenic (Ca_V1.1^−/−) myotubes, we identified the amino acids 1,641–1,668 in the C terminus of Ca_V1.2 as necessary for association of STAC proteins. This sequence contains the IQ domain and alanine mutagenesis revealed that the amino acids important for STAC association overlap with those making contacts with the C-lobe of calcium-calmodulin (Ca/CaM) and mediating calcium-dependent inactivation of Ca_V1.2. Indeed, patch-clamp analysis demonstrated that coexpression of either one of the three STAC proteins with Ca_V1.2 opposed calcium-dependent inactivation, although to different degrees, and that substitution of the Ca_V1.2 IQ domain with that of Ca_V2.1, which does not interact with STAC, abolished this effect. These results suggest that STAC proteins associate with the Ca_V1.2 C terminus at the IQ domain and thus inhibit calcium-dependent feedback regulation of Ca_V1.2 currents.

Evidence for Altered Ca2+ Handling in Growth Associated Protein 43-Knockout Skeletal Muscle

Neuronal growth-associated protein 43 (GAP43) has crucial roles in the nervous system, and during development, regeneration after injury, and learning and memory. GAP43 is expressed in mouse skeletal muscle fibers and satellite cells, with suggested its involvement in intracellular Ca²⁺ handling. However, the physiological role of GAP43 in muscle remains unknown. Using a GAP43-knockout (GAP43^-/-) mouse, we have defined the role of GAP43 in skeletal muscle. GAP43^-/- mice showed low survival beyond weaning, reduced adult body weight, decreased muscle strength, and changed myofiber ultrastructure, with no significant differences in the expression of markers of satellite cell and myotube progression through the myogenic program. Thus, GAP43 expression is involved in timing of muscle maturation in-vivo. Intracellular Ca²⁺ measurements in-vitro in myotubes revealed GAP43 involvement in Ca²⁺ handling. In the absence of GAP43 expression, the spontaneous Ca²⁺ variations had greater amplitudes and higher frequency. In GAP43^-/^- myotubes, also the intracellular Ca²⁺ variations induced by the activation of dihydropyridine and ryanodine Ca²⁺ channels, resulted modified. These evidences suggested dysregulation of Ca²⁺ homeostasis. The emerging hypothesis indicates that GAP43 interacts with calmodulin to indirectly modulate the activities of dihydropyridine and ryanodine Ca²⁺ channels. This thus influences intracellular Ca²⁺ dynamics and its related intracellular patterns, from functional excitation-contraction coupling, to cell metabolism, and gene expression.

Calmodulin regulation (calmodulation) of voltage-gated calcium channels

Calmodulin regulation (calmodulation) of the family of voltage-gated CaV1-2 channels comprises a prominent prototype for ion channel regulation, remarkable for its powerful Ca(2+) sensing capabilities, deep in elegant mechanistic lessons, and rich in biological and therapeutic implications. This field thereby resides squarely at the epicenter of Ca(2+) signaling biology, ion channel biophysics, and therapeutic advance. This review summarizes the historical development of ideas in this field, the scope and richly patterned organization of Ca(2+) feedback behaviors encompassed by this system, and the long-standing challenges and recent developments in discerning a molecular basis for calmodulation. We conclude by highlighting the considerable synergy between mechanism, biological insight, and promising therapeutics.