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 anal yses indicate that S4 functions as a voltage sensor and that the S5, S 6, and S5-S6 linker contribute to formation of the ion pore. However, little information exists regarding the functional role(s) of the amin o-terminal portion (S1-S3-S4 linker) of the repeats. Here we report th at the amino acid composition of the S3 segment of repeat I and the li nker connecting S3 and S4 segments of repeat I is critical for the dif ference in activation kinetics between cardiac and skeletal muscle L-t ype calcium channels. Mutant dihydropyridine receptors that have the s keletal muscle dihydropyridine receptor sequence in this region activa ted relatively slowly with the time constant of current activation (ta u(act)) > 5 ms, whereas mutants that have the cardiac counterpart ther e activated relatively rapidly with tau(act) < 5 ms. Comparison of the se two mutant groups indicates that a total of 11 conservative and 10 nonconservative amino acid changes from skeletal muscle to cardiac dih ydropyridine receptor sequence are sufficient to convert activation fr om slow to fast. These data demonstrate a functional role for this reg ion of voltage-gated ion channels.