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

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 alpha1C subunit is coexpressed with the auxiliary beta1, beta3, or beta4, but not beta2, subunit. These four beta 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 beta1 and beta2 subunits, we have mapped essential sequences for L-type Ca2+ channel facilitation. A first sequence, located in the second conserved domain of all beta subunits, is responsible for the promotion of current facilitation by the beta subunit. A second sequence of 16 amino acids, located on the N-terminal tail of the beta2 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 beta2 subunit suggests the existence of an additional inhibitory sequence in the beta2 subunit.