Crystal Structure and Molecular Imaging of the Nav Channel {beta}3 Subunit Indicates a Trimeric Assembly [Neurobiology]

February 24th, 2014 by Namadurai, S., Balasuriya, D., Ra&jnodot;appa, R., Wiemhofer, M., Stott, K., Klingauf, J., Edwardson, J. M., Chirgadze, D. Y., Jackson, A. P.

The vertebrate sodium (Nav) 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 Nav 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 Nav 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 Nav channel cross-linking and provide new structural insights into some pathological Nav channel mutations.