Control of ion conduction in L-type Ca2+ channels by the concerted action of S5-6 regions

Voltage-gated L-type Ca2+ channels from cardiac (alpha(1C)) and skeletal (alpha(1S)) muscle differ from one another in ion selectivity and permeation properties, including unitary conductance. In 110 mM Ba2+, unitary conductance of alpha(1S) is approximately half that of alpha(1C). As a step toward understanding the mechanism of rapid ion flux through these highly selective ion channels, we used chimeras constructed between alpha1C and alpha(1S) to identify structural features responsible for the difference in conductance. Combined replacement of the four pore-lining P-loops in alpha(1C) with P-loops from alpha(1S) reduced unitary conductance to a value intermediate between those of the two parent channels. Combined replacement of four larger regions that include sequences flanking the P-loops (S5 and S6 segments along with the P-loop-containing linker between these segments (S5-6)) conferred alpha(1S)-like conductance on alpha(1C). Likewise, substitution of the four S5-6 regions of alpha(1C) into alpha(1S) conferred alpha(1C)-like conductance on alpha(1S). These results indicate that, comparing alpha(1C) with alpha(1S), the differences in structure that are responsible for the difference in ion conduction are housed within the S5-6 regions. Moreover, the pattern of unitary conductance values obtained for chimeras in which a single P-loop or single S5-6 region was replaced suggest a concerted action of pore-lining regions in the control of ion conduction.