Critical roles of the S3 segment and S3-S4 linker of repeat I in activation of L-type calcium channels.

Each of the four repeats (or motifs) of voltage-gated ion channels is thought to contain six transmembrane segments (S1-S6). Mutational analyses indicate that S4 functions as a voltage sensor and that the S5, S6, and S5-S6 linker contribute to formation of the ion pore. However, little information exists regarding the functional role(s) of the amino-terminal portion (S1-S3-S4 linker) of the repeats. Here we report that the amino acid composition of the S3 segment of repeat I and the linker connecting S3 and S4 segments of repeat I is critical for the difference in activation kinetics between cardiac and skeletal muscle L-type calcium channels. Mutant dihydropyridine receptors that have the skeletal muscle dihydropyridine receptor sequence in this region activated relatively slowly with the time constant of current activation (tau act) > 5 ms, whereas mutants that have the cardiac counterpart there activated relatively rapidly with tau act < 5 ms. Comparison of these two mutant groups indicates that a total of 11 conservative and 10 nonconservative amino acid changes from skeletal muscle to cardiac dihydropyridine receptor sequence are sufficient to convert activation from slow to fast. These data demonstrate a functional role for this region of voltage-gated ion channels.