Chemical-Biological Generator of Step Increases in Calmodulin Reveals Dual Modulation of L-Type Ca2+ Channels

Abstract

Voltage-gated L-type Ca2+ channels are prominent Ca2+ entry portals into many cells. Proper functioning of these channels requires Ca2+-dependent inhibition (CDI) of channel opening by calmodulin (CaM). CDI requires that Ca2+-free CaM (apoCaM) first bind to channels (preassociate), even before Ca2+ elevation. Subsequent Ca2+ binding to this 'resident' CaM then induces CDI; channels lacking preassociated apoCaM cannot undergo CDI. Here, using a novel generator of step increases in apoCaM, we find that apoCaM binding has another powerful effect. As shown below, apoCaM elevation not only heightens CDI as expected, but strongly boosts peak Ca2+ current (confirmed to increase PO in single-channel assays). This latter effect could result from the binding of a second CaM to channels, atop the CaM required for CDI. However, plotting normalized peak current as a function of CDI strength invariably resolves a single relationship, over multiple experiments and conditions. This invariance furnishes compelling evidence that the binding of one and the same apoCaM imparts increases in both CDI and opening. This newly recognized apoCaM effect to increase PO opens new dimensions through which Ca2+ homeostasis can be tuned.