Binary architecture of the Nav1.2-β2 signaling complex

To investigate the mechanisms by which beta-subunits influence Nav channel function, we solved the crystal structure of the beta 2 extracellular domain at 1.35A. We combined these data with known bacterial Nav channel structural insights and novel functional studies to determine the interactions of specific residues in beta 2 with Nav1.2. We identified a flexible loop formed by (72)Cys and (75)Cys, a unique feature among the four beta-subunit isoforms. Moreover, we found that (55)Cys helps to determine the influence of beta 2 on Nav1.2 toxin susceptibility. Further mutagenesis combined with the use of spider toxins reveals that (55)Cys forms a disulfide bond with 91 Cys in the Nav1.2 domain II pore loop, thereby suggesting a 1:1 stoichiometry. Our results also provide clues as to which disulfide bonds are formed between adjacent Nav1.2 (CyS)-Cy-912/918 residues. The concepts emerging from this work will help to form a model reflecting the beta-subunit location in a Nav channel complex.