title: Folded state of the integral membrane colicin E1 immunity protein in solvents of mixed polarity.
abstract: The colicin E1 immunity protein (ImmE1), a 13.2-kDa hydrophobic integral membrane protein localized in the Escherichia coli cytoplasmic membrane, protects the cell from the lethal, channel-forming activity of the bacteriocin, colicin E1. Utilizing its solubility in organic solvents, ImmE1 was purified by 1-butanol extraction of isolated membranes, followed by gel filtration and ion-exchange chromatography in a chloroform/methanol/H(2)O (4:4:1) solvent system. Circular dichroism analysis indicated that the alpha-helical content of ImmE1 is approximately 80% in 1-butanol or 2,2,2-trifluoroethanol, consistent with a previous membrane-folding model with three extended hydrophobic transmembrane helical domains, H1-H3. Each of these extended hydrophobic domains contains a centrally located single Cys residue that could be used as a probe of protein structure. The presence of tertiary structure of purified ImmE1 in a solvent of mixed polarity, chloroform/methanol/H(2)O (4:4:1) was demonstrated by (i) the constraints on Tyr residues shown by the amplitude of near-UV circular dichroism spectra in the wavelength interval, 270-285 nm; (ii) the correlation between the near-UV Tyr CD spectrum of single and double Cys-to-X mutants of the Imm protein and their in vivo activity; (iii) the upfield shift of methyl groups in a 1D NMR spectrum, a 2D- HSQC NMR spectrum of ImmE1 in the mixed polarity solvent mixture, and a broadening and disappearance of the indole (1)H proton resonance from Trp94 in H3 by a spin label attached to Cys16 in the H2 hydrophobic domain; (iv) near-UV circular dichroism spectra with a prominent ellipticity band centered at 290 nm from a single Trp inserted into the extended hydrophobic domains. It was concluded that the colicin E1 immunity protein adopts a folded conformation in chloroform/methanol/H(2)O (4:4:1) that is stabilized by helix-helix interactions. Analysis of the probable membrane folding topology indicated that several Tyr residues in the bilayer region of the three transmembrane helices could contribute to the near-UV CD spectrum through helix-helix interactions.
authors.affiliation: Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, USA.
authors.forename: R M
authors.initials: RM
authors.lastname: Taylor
authors.forename: S D
authors.initials: SD
authors.lastname: Zakharov
authors.forename: J
authors.initials: J
authors.lastname: Bernard Heymann
authors.forename: M E
authors.initials: ME
authors.lastname: Girvin
authors.forename: W A
authors.initials: WA
authors.lastname: Cramer
mesh_terms: D000595:Amino Acid Sequence; D002462:Cell Membrane / Q000737:chemistry; D002725:Chloroform; D002942:Circular Dichroism; D003087:Colicins / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics / Q000494:pharmacology; D003545:Cysteine / Q000235:genetics; D004926:Escherichia coli / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D008565:Membrane Proteins / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D000432:Methanol; D008969:Molecular Sequence Data; D008970:Molecular Weight; D019906:Nuclear Magnetic Resonance, Biomolecular; D017510:Protein Folding; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000302:isolation & purification / Q000494:pharmacology; D017386:Sequence Homology, Amino Acid; D012995:Solubility; D012997:Solvents; D014867:Water
chemical_list: D003087:Colicins; D008565:Membrane Proteins; D011994:Recombinant Proteins; D012997:Solvents; D014867:Water; D002725:Chloroform; D003545:Cysteine; D000432:Methanol