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Abstract:
Voltage-gated calcium (Ca-v) channels, which are regulated by membrane potential, cytosolic Ca2+, phosphorylation, and membrane phospholipids, govern Ca2+ entry into excitable cells. Ca-v channels contain a pore-forming alpha 1 subunit, an auxiliary alpha 2 delta subunit, and a regulatory beta subunit, each encoded by several genes in mammals. In addition to a domain that interacts with the alpha 1 subunit, beta 2e and beta 2a also interact with the cytoplasmic face of the plasma membrane through an electrostatic interaction for beta 2e and posttranslational acylation for beta 2a. We found that an increase in cytosolic Ca2+ promoted the release of beta 2e from the membrane without requiring substantial depletion of the anionic phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) from the plasma membrane. Experiments with liposomes indicated that Ca2+ disrupted the interaction of the beta 2e amino-terminal peptide with membranes containing PIP2. Ca2+ binding to calmodulin (CaM) leads to CaM-mediated inactivation of Ca-v currents. Although Ca(v)2.2 coexpressed with beta 2a required Ca2+-dependent activation of CaM for Ca2+-mediated reduction in channel activity, Ca(v)2.2 coexpressed with beta 2e exhibited Ca2+-dependent inactivation of the channel even in the presence of Ca2+-insensitive CaM. Inducible depletion of PIP2 reduced Ca(v)2.2 currents, and in cells coexpressing beta 2e, but not a form that lacks the polybasic region, increased intracellular Ca2+ further reduced Ca(v)2.2 currents. Many hormone-or neurotransmitter-activated receptors stimulate PIP2 hydrolysis and increase cytosolic Ca2+; thus, our findings suggest that beta 2e may integrate such receptor-mediated signals to limit Ca-v activity.
