Chimeric study of sodium channels from rat skeletal and cardiac muscle

Two isoforms of voltage-dependent Na channels, cloned from rat skeletal muscle, were expressed in Xenopus oocytes. The currents of rSkM1 and rSkM2 differ functionally in 4 properties: (i) tetrodotoxin (TTX) sensitivity, (ii) mu-conotoxin (mu-CTX) sensitivity, (iii) amplitude of single channel currents, and (iv) rate of inactivation. rSkM1 is sensitive to both TTX and mu-CTX. rSkM2 is resistant to both toxins. Currents of rSkM1 have a higher single channel conductance and a slower rate of inactivation than those of rSkM2. We constructed (i) chimeras by interchanging domain 1 (D1) between the two isoforms, (ii) block mutations of 22 amino acids in length that interchanged parts of the loop between transmembrane segments S5 and S6 in both D1 and D4, and (iii) point mutations in the SS2 region of this loop in D1. The TTX sensitivity could be switched between the two isoforms by the exchange of a single amino acid, tyrosine-401 in rSkM1 and cysteine-374 in rSkM2 in SS2 of D1. By contrast most chimeras and point mutants had an intermediate sensitivity to mu-CTX when compared with the wild-type channels. The point mutant rSkM1 (Y401C) had an intermediate single-channel conductance between those of the wild-type isoforms, whereas rSkM2 (C374Y) had a slightly lower conductance than rSkM2. The rate of inactivation was found to be determined by multiple regions of the protein, since chimeras in which D1 was swapped had intermediate rates of inactivation compared with the wild-type isoforms.