Definition of epitopes for monoclonal antibodies developed against purified sodium channel protein: implications for channel structure.

To test sodium channel structural models, we defined the epitopes for nineteen independently cloned monoclonal antibodies previously generated against purified, detergent-solubilized, adult rat skeletal muscle sodium channel protein using channel proteolysis, synthetic peptides, and fusion proteins. All identified epitopes were continuous and unique to the skeletal muscle subtype alpha-subunit. Of the nineteen independent clones, seventeen had epitopes located either in the origin of the amino-terminus or in the interdomain 2-3 region while only two antibodies had epitopes located in the mid-portion of the interdomain 1-2 region. No immunogenic regions were identified on the alpha-subunit's extracellular regions, interdomain 3-4 segment, or carboxyl-terminus or on channel beta-subunits. While immune tolerance may explain the lack of immunogenicity of extracellular regions, the lack of immunogenicity of most of the channel's cytoplasmic mass may be due to segment inaccessibility from organization of these regions as globular domains, to insertion of parts of these regions into the membrane phase, or to interaction with other protein elements. The definition of monoclonal antibody epitopes allows us to reinterpret previously reported monoclonal antibody competition studies, providing independent support for our model of sodium channel cytoplasmic domain structure. In addition, these data suggest additional testable hypotheses concerning the interactions of the sodium channel amino- and carboxyl-termini with each other as well as with other protein elements.