title: Toward elucidating the membrane topology of helix two of the colicin E1 channel domain.
abstract: The membrane-bound closed state of the colicin E1 channel domain was investigated by site-directed fluorescence labeling using a bimane fluorophore attached to each single cysteine residue within helix 2 of each mutant protein. The fluorescence properties of the bimane fluorophore were measured for the membrane-associated form of the closed channel and included fluorescence emission maximum, fluorescence anisotropy, apparent polarity, surface accessibility, and membrane bilayer penetration depth. The fluorescence data show that helix 2 is an amphipathic alpha-helix that is situated parallel to the membrane surface, but it is less deeply embedded within the bilayer interfacial region than is helix 1 in the closed channel. A least squares fit of the various data sets to a harmonic wave function indicated that the periodicity and angular frequency for helix 2 in the membrane-bound state are typical for an amphipathic alpha-helix (3.8 +/- 0.1 residues per turn and 94 +/- 4 degrees, respectively) that is located at an interfacial region of a membrane bilayer. Dual quencher analysis also revealed that helix 2 is peripherally membrane associated, with one face of the helix dipping into the interfacial region of the lipid bilayer and the other face projecting outwardly into the aqueous solvent. Finally, our data show that helices 1 and 2 remain independent helices upon membrane association with a short connector link (Tyr(363)-Gly(364)) and that short amphipathic alpha-helices participate in the formation of a lipid-dependent, toroidal pore for this colicin.
authors.affiliation: Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
authors.forename: Dawn
authors.initials: D
authors.lastname: White
authors.forename: Abdiwahab A
authors.initials: AA
authors.lastname: Musse
authors.forename: Jie
authors.initials: J
authors.lastname: Wang
authors.forename: Erwin
authors.initials: E
authors.lastname: London
authors.forename: A Rod
authors.initials: AR
authors.lastname: Merrill
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000737:chemistry; D003087:Colicins / Q000737:chemistry*; D003545:Cysteine / Q000378:metabolism; D004926:Escherichia coli / Q000737:chemistry / Q000378:metabolism; D005454:Fluorescence Polarization; D005456:Fluorescent Dyes; D007473:Ion Channels / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008051:Lipid Bilayers / Q000737:chemistry; D008055:Lipids / Q000737:chemistry; D008565:Membrane Proteins / Q000737:chemistry* / Q000302:isolation & purification; D008566:Membranes / Q000378:metabolism; D008958:Models, Molecular; D015233:Models, Statistical; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D050505:Mutant Proteins / Q000737:chemistry; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012680:Sensitivity and Specificity; D012997:Solvents / Q000737:chemistry; D013050:Spectrometry, Fluorescence
chemical_list: D019086:Bridged Bicyclo Compounds, Heterocyclic; D003087:Colicins; D005456:Fluorescent Dyes; D007473:Ion Channels; D008051:Lipid Bilayers; D008055:Lipids; D008565:Membrane Proteins; D050505:Mutant Proteins; D012997:Solvents; C021149:bimanes; D003545:Cysteine