Structural Comparisons of Immunoglobulin Folds of Human Navβ Subunits

Abstract

Mammalian voltage gated Na+ channels (Nav) consist of a pore forming α subunit together with one or more auxiliary β subunits that interact either non-covalently (β1 or β3) or covalently (β2 or β4). These β subunits contain an N-terminal extracellular domain that adopts an immunoglobulin fold, a single transmembrane helix, and a short cytoplasmic C-terminal tail. We previously solved the crystal structure of the Navβ4 extracellular domain, and the structure of the corresponding Navβ3 domain has also been reported. We now report the crystal structure of the Navβ2 extracellular domain at high resolution (1.4A). A direct comparison of all three available Navβ structures shows that each displays unique loop conformations, different lengths of β strands, and a different distribution of disulfide bonds. Although all three contain a central buried disulfide bond, both Navβ2 and Navβ3 contain an additional bridge, albeit at different locations. Both Navβ2 and Navβ4 contain an exposed cysteine at identical locations that appears crucial for interaction with the α subunit. However, the surface features around the crucial cysteine differ appreciably among both isoforms. These differences affect the surface properties, and likely underlie the differential effects of Navβ subunits on the gating and pharmacological properties of the α subunits. We also solved the structure of a glycosylated form of Navβ4, which shows that there is a minimal effect of glycosylation on the final structure. We provide a detailed structural comparison of different Navβ subunits in wild type and mutant forms.