title: Gas phase characterization of the noncovalent quaternary structure of cholera toxin and the cholera toxin B subunit pentamer.
abstract: Cholera toxin (CTx) is an AB5 cytotonic protein that has medical relevance in cholera and as a novel mucosal adjuvant. Here, we report an analysis of the noncovalent homopentameric complex of CTx B chain (CTx B5) using electrospray ionization triple quadrupole mass spectrometry and tandem mass spectrometry and the analysis of the noncovalent hexameric holotoxin usingelectrospray ionization time-of-flight mass spectrometry over a range of pH values that correlate with those encountered by this toxin after cellular uptake. We show that noncovalent interactions within the toxin assemblies were maintained under both acidic and neutral conditions in the gas phase. However, unlike the related Escherichia coli Shiga-like toxin B5 pentamer (SLTx B), the CTx B5 pentamer was stable at low pH, indicating that additional interactions must be present within the latter. Structural comparison of the CTx B monomer interface reveals an additional alpha-helix that is absent in the SLTx B monomer. In silico energy calculations support interactions between this helix and the adjacent monomer. These data provide insight into the apparent stabilization of CTx B relative to SLTx B.
authors.affiliation: Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, United Kingdom. j.p.williams@warwick.ac.uk
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Williams
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Smith
authors.forename: Brian N
authors.initials: BN
authors.lastname: Green
authors.forename: Brian D
authors.initials: BD
authors.lastname: Marsden
authors.forename: Keith R
authors.initials: KR
authors.lastname: Jennings
authors.forename: Lynne M
authors.initials: LM
authors.lastname: Roberts
authors.forename: James H
authors.initials: JH
authors.lastname: Scrivens
mesh_terms: D002772:Cholera Toxin / Q000737:chemistry*; D008958:Models, Molecular; D044367:Phase Transition; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
chemical_list: D021122:Protein Subunits; D002772:Cholera Toxin