Promotion and inhibition of L-type Ca2+ channel facilitation by distinct domains of the β subunit

Abstract

Ca2+ current potentiation by conditioning depolarization is a general mechanism by which excitable cells can control the level of Ca2+ entry during repetitive depolarizations. Several types of Ca2+ channels are sensitive to conditioning depolarization, however, using clearly distinguishable mechanisms. In the case of L-type Ca2+ channels, prepulse- induced current facilitation can only be recorded when the pore-forming α(1c) subunit is coexpressed with the auxiliary β1, β3, or β4, but not β2, subunit. These four β subunits are composed of two conserved domains surrounded by central, N-terminal, and C-terminal variable regions. Using different deleted and chimeric forms of the β1 and β2 subunits, we have mapped essential sequences for L-type Ca2+ channel facilitation. A first sequence, located in the second conserved domain of all β subunits, is responsible for the promotion of current facilitation by the β subunit. A second sequence of 16 amino acids, located on the N-terminal tail of the β2 subunit, induces a transferable block of L-type current facilitation. Site- specific mutations reveal the essential inhibitory role played by three positive charges on this segment. The lack of prepulse-induced current facilitation recorded with some truncated forms of the β2 subunit suggests the existence of an additional inhibitory sequence in the β2 subunit.