Voltage-dependent calcium (CaV) channels
Movement of calcium ions from the extracellular space into the cell is prevented by the cellular membrane. The passage of calcium in a selective manner down the electrochemical gradient occurs by way of a family of proteins, namely voltage-dependent calcium (CaV) channels. These channels couple electrical excitation of the cell to varied physiological processes including muscle contraction and secretion of hormones and neurotransmitters.
The consequences of calcium channel activity in many tissues make them key drug targets. Hence, calcium channel blockers are widely used in the treatment of hypertension, angina, coronary spasm, and arrhythmias. Another class of calcium channel drugs is used against epilepsy and chronic pain.
CaV channels open upon depolarization of the plasma membrane, allowing inward flow of calcium ions from the extracellular milieu into the cell. The channels are multi-protein assemblies comprising four distinct subunits. The soluble beta subunit binds tightly to the membrane pore-forming subunit, alpha1. A region of the alpha1 subunit serves as beta’s primary anchor site. The beta protein acts by both directing channels to the cellular membrane and by modulating gating. We have examined the structure-function relationships of the beta subunit, include the three-dimensional structure determination of the beta protein by X-ray crystallography at atomic resolution. Importantly, we also solved the structure of beta in complex with its alpha1 subunit binding site. These results were based on a detailed experimental infrastructure involving recombinant expression of the relevant proteins, their purification and biochemical and biophysical characterization, using a variety of methods such as circular dichroism and fluorescence spectroscopies.
Our research on the structure, function and regulation of CaV channels is ongoing and remains a major focus of the lab.
Calcium sensors are proteins that are exquisitely sensitive to calcium concentration, generally ligating calcium ions, and often undergoing a conformation shift as a result. They can be independent proteins or modular domains in the context of a large protein.
Examples of proteins that we have studied in this context include the paradigm of this group, calmodulin. In addition, we have investigated the structure and function of the calcium sensor from NCX, a membrane protein that extrudes calcium from cells, using the sodium gradient. Another example is the DOC2B protein which is the calcium sensor for synaptic vescicle release in neurons.