Crystal Structure and Molecular Imaging of the Na(v) Channel β3 Subunit Indicates a Trimeric Assembly

The vertebrate sodium (Na(v)) channel is composed of an ion-conducting α subunit and associated β subunits. Here, we report the crystal structure of the human β3 subunit immunoglobulin (Ig) domain, a functionally important component of Na(v) channels in neurons and cardiomyocytes. Surprisingly, we found that the β3 subunit Ig domain assembles as a trimer in the crystal asymmetric unit. Analytical ultracentrifugation confirmed the presence of Ig domain monomers, dimers, and trimers in free solution, and atomic force microscopy imaging also detected full-length β3 subunit monomers, dimers, and trimers. Mutation of a cysteine residue critical for maintaining the trimer interface destabilized both dimers and trimers. Using fluorescence photoactivated localization microscopy, we detected full-length β3 subunit trimers on the plasma membrane of transfected HEK293 cells. We further show that β3 subunits can bind to more than one site on the Na(v) 1.5 α subunit and induce the formation of α subunit oligomers, including trimers. Our results suggest a new and unexpected role for the β3 subunits in Na(v) channel cross-linking and provide new structural insights into some pathological Na(v) channel mutations.