Ultra-slow inactivation in mu 1 Na+ channels is produced by a structural rearrangement of the outer vestibule

While studying the adult rat skeletal muscle Na+ channel outer vestibule, w e found that certain mutations of the lysine residue in the domain III P re gion at amino acid position 1237 of the alpha subunit, which is essential f or the Na+ selectivity of the channel, produced substantial changes in the inactivation process. When skeletal muscle alpha subunits (mu 1) with K1237 mutated to either serine (K12378) or glutamic acid (K1237E) were expressed in Xenopus oocytes and depolarized for several minutes, the channels enter ed a state of inactivation from which recovery was very slow, i.e., the tim e constants of entry into and exit from this state were in the order of sim ilar to 100 s. We refer to this process as "ultra-slow inactivation." By co ntrast, wild-type channels and channels with the charge-preserving mutation K1237R largely recovered within similar to 60 s, with only 20-30% of the c urrent showing ultra-slow recovery. Coexpression of the rat brain beta 1 su bunit along with the K1237E alpha subunit tended to accelerate the faster c omponents of recovery from inactivation, as has been reported previously of native channels, but had no effect on the mutation-induced ultra-slow inac tivation. This implied that ultra-slow inactivation was a distinct process different from normal inactivation. Binding to the pore of a partially bloc king peptide reduced the number of channels entering the ultra-slow inactiv ation state, possibly by interference with a structural rearrangement of th e outer vestibule. Thus, ultra-slow inactivation, favored by charge-alterin g mutations at site 1237 in mu 1 Na+ channels, may be analogous to C-type i nactivation in Shaker K+ channels.