Structural characterisation of the prokaryotic sodium channel C-terminal domain

Miller, Wayne (2015) Structural characterisation of the prokaryotic sodium channel C-terminal domain. PhD thesis, Birkbeck, University of London.

Abstract

Since the discovery of the first prokaryotic voltage gated sodium channel (Nav) in 2001, prokaryotic Navs have been a high priority target for structural study. Prokaryotic Navs are of interest as a model system due to their homology to eukaryotic Navs, which are high value drug development targets for their roles in pain perception and neural function. While prokaryotic Navs have function and pharmacology distinct from their eukaryotic homologues, understanding their structure holds implications for drug development and for understanding diseases stemming from neuronal dysfunction. However, Navs have historically been challenging targets for structural study, resisting attempts at crystallisation until recently. In this study, expression, purification, and characterisation of a chimera of the NavBh channel and the ligand gating RCK domain from the prokaryotic potassium channel MthK has been performed. It was hypothesised that the addition of the RCK domain would improve the channel’s crystallisation potential, and create a ligand gated Nav for functional characterisation. Electrophysiological studies demonstrated that the RCK domain was capable of gating NavBh, however the chimera had reduced solubility, indicating that this chimeric fusion was not an ideal target for structural study due to low purification yields. Following this, and in light of recent studies that suggested the structure of the prokaryotic Nav C-terminus had a role in channel function, structural analysis of the C-terminus of a prokaryotic Nav homologue cloned from Bacillus alcalophilus has been performed. Synchrotron radiation circular dichroism analysis of serial C-terminal truncations demonstrated the structure of the NsvBa C-terminus consists of a helical region connected to the channel pore by a disordered neck region, despite conflicting bioinformatics predictions. This offers further support for the hypothesis that in functional Navs, the C-terminus consists of a disordered neck region connecting a coiled-coil to the base of the pore, which acts as a spring to assist in channel gating and inactivation.

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