A carboxy-terminal α-helical segment in the rat skeletal muscle voltage-dependent Na + channel is responsible for its interaction with the amino-terminus

Abstract

Cytoplasmic segments of the adult rat skeletal muscle sodium channel α-subunit (rSkM1) comprise a major portion (∼40%) of the total protein and are involved in channel functions both general, such as inactivation, and isoform-specific, for example, protein kinase A modulation. Far ultraviolet circular dichroism measurements of synthetic peptides and overexpressed fusion proteins containing individual channel cytoplasmic segments suggest that cytoplasmic domains of rSkM1 contain ordered secondary structures even in the absence of adjoining transmembrane segments. Intrinsic fluorescence experiments with a nested set of carboxy-terminal deletion proteins confirm a specific interaction between the channel’s amino- and carboxy-termini and identify residues 1716–1737 in the carboxy-terminus as the region that binds to the amino-terminus. Circular dichroism measurements suggest that this same region is organized as an α-helix and that electrostatic forces may contribute to this association. The interaction of the amino- and carboxy-termini is not accompanied by secondary structure changes detectable by circular dichroism spectroscopy, but a decrease in intrinsic fluorescence indicates that this association is accompanied by a change in the environment of Trp 1617.