The mammalian Voltage-gated sodium (Na_v) channel is composed of a single α subunit (~ 260 kDa), a multi-pass membrane protein that renders ion selectivity and two or more Na_vβ subunits (25‒40 kDa), that are Type I single-pass membrane proteins and regulate Na_vα subunit function. These subunits are assembled on the plasma membrane of electrically-excitable cells as an intrinsic membrane protein complex and help to initiate and propagate the action potential. The four major mammalian Na_vβ-subunit isoforms, Na_vβ1‒4 proteins possess an N-terminal extracellular Immunoglobulin (Ig) domain (ECD), a single transmembrane α-helix, and an intracellular C-terminal region (ICD).

This thesis is mainly focused on the structural biology aspects of the human Na_vβ3 subunit. It reports the atomic structure of the Na_vβ3-Ig domain as determined by X-ray crystallography. Interestingly, the Na_vβ3-Ig domain is observed as a trimer in the crystal structure. The homo-trimer assembly interface lies at the N-terminus and is constrained by a disulphide bond not normally present in Ig domains. The Na_vβ3 subunit Ig domain is known to be glycosylated and contains four potential N-linked glycosylation sites. However, the X-ray crystallography was conducted on deglycosylated protein. Using computational modelling, it is shown that glycan addition would not interfere with Na_vβ3-Ig domain trimerization. Independent evidence gathered using Analytical Ultracentrifugation (crosslinked, glycosylated Na_vβ3-Ig domain, in vitro), Proximity Ligation Assay (full-length Na_vβ3, in vivo), Atomic Force Microscopy (isolated full-length Na_vβ3, in vitro) and Photo-activated Localisation Microscopic experiments (full-length Na_vβ3, in situ) support the view that the Na_vβ3 subunit can form trimers when expressed in cells. The biological significance of Na_vβ3 subunit trimerization is discussed.

Strategies to express and purify the Na_vβ1/β2/β4-Ig domains were made. Wild type Na_vβ2- and Na_vβ4-Ig domains exist as monomers and dimers, simultaneously in solution, although crystals that diffracted to the necessary resolution were not produced.