The L-Type Calcium Channel C-Terminus is a Mobile Domain that Competes with Calmodulin Modulation of Calcium Current

Abstract

The L-type Ca channel (CaV1.2) distal carboxyl-terminus (CCt) has multiple functions. CCt inhibits L-type calcium current (ICa,L), and is a mobile element that translocates to the nucleus where it regulates CaV1.2 transcription. CCt interacts with CaV1.2 in a similar domain as calmodulin (CaM). The purpose of this study is test the hypothesis that CaM and CCt compete for functional interaction with CaV1.2. ICa,L and barium current (IBa,L) was recorded from HEK 293 cells transfected with CaV1.2 + CaVbeta2a. This background was compared to cells additionally transfected with CaM and/or CCt. The CaV1.2 expressed was deleted at position 1733 (numbering based on rabbit sequence), and CCt corresponded to amino acids 1821-2171. ICa,L and IBa,L was recorded in each cell, and we compared the increase of current, the shift of activation midpoint, and current kinetics of ICa,L versus IBa,L within a given cell. Maximal conductance ratio Ca/Ba is ∼0.4 for CaV1.2+CaVbeta2a expression. Addition of CaM co-expression does not alter Ca /Ba conductance. CCt co-expression significantly increases the relative Ca /Ba ratio 2-fold, and this effect is reversed by CCt+CaM co-expression. Examination of the peak I(V) curves suggests that midpoint of activation was not affected, and ICa,L density is not different in for all transfection conditions. We conclude that CCt attenuation of conductance occurs only with Ba, and is consistent with a Ca alleviation of CCt block. Thus, CaM and Ca functionally compete to limit CCt auto-inhibition of CaV1.2 current.