A new look at sodium channel β subunits.

Voltage-gated sodium (Nav) channels are intrinsic plasma membrane proteins that initiate the action potential in electrically excitable cells. They are a major focus of research in neurobiology, structural biology, membrane biology and pharmacology. Mutations in Nav channels are implicated in a wide variety of inherited pathologies, including cardiac conduction diseases, myotonic conditions, epilepsy and chronic pain syndromes. Drugs active against Nav channels are used as local anaesthetics, anti-arrhythmics, analgesics and anti-convulsants. The Nav channels are composed of a pore-forming α subunit and associated β subunits. The β subunits are members of the immunoglobulin (Ig) domain family of cell-adhesion molecules. They modulate multiple aspects of Nav channel behaviour and play critical roles in controlling neuronal excitability. The recently published atomic resolution structures of the human β3 and β4 subunit Ig domains open a new chapter in the study of these molecules. In particular, the discovery that β3 subunits form trimers suggests that Nav channel oligomerization may contribute to the functional properties of some β subunits

Crystal structure and molecular imaging of the Nav channel β3 subunit indicates a trimeric assembly.

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

The Voltage-Dependent Sodium Channel Family.

In neurones and other electrically excitable tissues, voltage-dependent sodium (Nav) channels play an essential role in initiating and propagating the action potential. High-resolution structures of sodium channels have revealed new details concerning these macromolecules that provide insights into their ion-specificity and the conformational changes they undergo during the action potential. Nav channels typically exist in vivo as multicomponent macromolecular assemblies, containing auxiliary proteins that modulate channel gating and trafficking. The properties of some of these auxiliary proteins raise the possibility that Nav channels may exist as functionally coupled complexes. The close similarity between different Nav channel subtypes has frustrated attempts to develop isoform-specific inhibitors. However, the combination of new structural insights, together with antibody-based reagents and site-directed mutagenesis of protein-based toxin inhibitors, raises the possibility of higher target specificities than previously possible. Such reagents may form the basis for a new generation of Nav channel drugs

Mechanisms of noncovalent β subunit regulation of NaV channel gating

Voltage-gated Na(+) (NaV) channels comprise a macromolecular complex whose components tailor channel function. Key components are the non-covalently bound β1 and β3 subunits that regulate channel gating, expression, and pharmacology. Here, we probe the molecular basis of this regulation by applying voltage clamp fluorometry to measure how the β subunits affect the conformational dynamics of the cardiac NaV channel (NaV1.5) voltage-sensing domains (VSDs). The pore-forming NaV1.5 α subunit contains four domains (DI-DIV), each with a VSD. Our results show that β1 regulates NaV1.5 by modulating the DIV-VSD, whereas β3 alters channel kinetics mainly through DIII-VSD interaction. Introduction of a quenching tryptophan into the extracellular region of the β3 transmembrane segment inverted the DIII-VSD fluorescence. Additionally, a fluorophore tethered to β3 at the same position produced voltage-dependent fluorescence dynamics strongly resembling those of the DIII-VSD. Together, these results provide compelling evidence that β3 binds proximally to the DIII-VSD. Molecular-level differences in β1 and β3 interaction with the α subunit lead to distinct activation and inactivation recovery kinetics, significantly affecting NaV channel regulation of cell excitability

The C-Terminal Domain of the MutL Homolog from Neisseria gonorrhoeae Forms an Inverted Homodimer

The mismatch repair (MMR) pathway serves to maintain the integrity of the genome by removing mispaired bases from the newly synthesized strand. In E. coli, MutS, MutL and MutH coordinate to discriminate the daughter strand through a mechanism involving lack of methylation on the new strand. This facilitates the creation of a nick by MutH in the daughter strand to initiate mismatch repair. Many bacteria and eukaryotes, including humans, do not possess a homolog of MutH. Although the exact strategy for strand discrimination in these organisms is yet to be ascertained, the required nicking endonuclease activity is resident in the C-terminal domain of MutL. This activity is dependent on the integrity of a conserved metal binding motif. Unlike their eukaryotic counterparts, MutL in bacteria like Neisseria exist in the form of a homodimer. Even though this homodimer would possess two active sites, it still acts a nicking endonuclease. Here, we present the crystal structure of the C-terminal domain (CTD) of the MutL homolog of Neisseria gonorrhoeae (NgoL) determined to a resolution of 2.4 Å. The structure shows that the metal binding motif exists in a helical configuration and that four of the six conserved motifs in the MutL family, including the metal binding site, localize together to form a composite active site. NgoL-CTD exists in the form of an elongated inverted homodimer stabilized by a hydrophobic interface rich in leucines. The inverted arrangement places the two composite active sites in each subunit on opposite lateral sides of the homodimer. Such an arrangement raises the possibility that one of the active sites is occluded due to interaction of NgoL with other protein factors involved in MMR. The presentation of only one active site to substrate DNA will ensure that nicking of only one strand occurs to prevent inadvertent and deleterious double stranded cleavage

The crystal structure of human Navβ3-Ig domain and its implications

The mammalian Voltage-gated sodium (Nav) channel is composed of a single α subunit (~ 260 kDa), a multi-pass membrane protein that renders ion selectivity and two or more Navβ subunits (25‒40 kDa), that are Type I single-pass membrane proteins and regulate Navα 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 Navβ-subunit isoforms, Navβ1‒4 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 Navβ3 subunit. It reports the atomic structure of the Navβ3-Ig domain as determined by X-ray crystallography. Interestingly, the Navβ3-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 Navβ3 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 Navβ3-Ig domain trimerization. Independent evidence gathered using Analytical Ultracentrifugation (crosslinked, glycosylated Navβ3-Ig domain, *in vitro*), Proximity Ligation Assay (full-length Navβ3, *in vivo*), Atomic Force Microscopy (isolated full-length Navβ3, *in vitro*) and Photo-activated Localisation Microscopic experiments (full-length Navβ3, *in situ*) support the view that the Navβ3 subunit can form trimers when expressed in cells. The biological significance of Navβ3 subunit trimerization is discussed. Strategies to express and purify the Navβ1/β2/β4-Ig domains were made. Wild type Navβ2- and Navβ4-Ig domains exist as monomers and dimers, simultaneously in solution, although crystals that diffracted to the necessary resolution were not produced.Cambridge Nehru Trust (partial) Scholarship St. John's College Bursar