A point mutation in domain 4-segment 6 of the skeletal muscle sodium channel produces an atypical inactivation state.

We compared wild-type rat skeletal muscle NaChs (micro1) and a mutant NaCh (Y1586K) that has a single amino acid substitution, lysine (K) for tyrosine (Y), at position 1586 in the S6 transmembrane segment of domain 4. In Y1586K, macroscopic current decay is faster, the V(1/2) of the activation curve is shifted in the depolarized direction, and the fast-inactivation curve is less steep compared with mu1. After an 8-ms depolarization pulse, Y1586K recovers from inactivation much more slowly than mu1. The recovery is double exponential, suggesting recovery from two inactivation states. Varying the depolarization protocols isolates entry into an additional, "atypical" inactivation state in Y1586K that is distinct from typical fast or slow inactivation. Substitution of positively charged arginine (R) at Y1586 produces an inactivation phenotype similar to that of Y1586K. Substitution by negatively charged aspartic acid (D) or uncharged alanine (A) at Y1586 produces an inactivation phenotype similar to mu1. Our results suggest that the positive charge of lysine (K) produces the atypical inactivation state in Y1586K. We propose that a conformational change during depolarization alters the relative position of the 1586K residue in the D4-S6 segment and that atypical inactivation in Y1586K occurs via an electrostatic interaction in or near the inner pore region.