Crystallographic insights into sodium-channel modulation by the β4 subunit

Area of Science:

• Neuroscience
• Molecular Biology
• Structural Biology

Background:

• Voltage-gated sodium channels (Nav) are critical for cellular excitability.
• Nav channel function is modulated by associated β-subunits, and mutations in these subunits can lead to disorders.
• The precise mechanisms by which β-subunits influence Nav channel function, particularly ligand interactions, are not fully understood.

Purpose of the Study:

• To investigate the role of β-subunits in modulating ligand interactions with Nav channels.
• To elucidate the structural basis for β4-subunit's influence on toxin binding to Nav1.2.
• To understand how β-subunit mutations contribute to neurological disorders like epilepsy.

Main Methods:

• Crystal structure determination of the extracellular β4 domain.
• Site-directed mutagenesis of specific residues, including (58)Cys and a β1 epilepsy-associated mutation.
• Assessment of toxin binding to Nav1.2 in the presence and absence of wild-type and mutant β4 subunits.

Main Results:

• The β4 subunit significantly alters toxin binding to the Nav1.2 channel.
• An exposed residue, (58)Cys, within the β4 extracellular domain was identified as crucial for this modulation; its mutation abolished the effect.
• A conserved docking site, stabilized by a buried cysteine bridge in β4, was proposed. Disrupting this bridge via a β1 epilepsy mutation altered the (58)Cys loop positioning and abolished β4's modulatory effect on Nav1.2.

Conclusions:

• The β4 subunit directly influences Nav channel pharmacology through specific structural features, notably the (58)Cys residue and a conserved docking site.
• These findings provide a mechanistic explanation for tissue-specific Nav channel pharmacology and β-subunit-related disorders.
• The study offers insights for developing therapeutic molecules targeting aberrant β-subunit behavior in neurological conditions.