title: Enzymatic E-colicins bind to their target receptor BtuB by presentation of a small binding epitope on a coiled-coil scaffold.
abstract: Toxins and viruses often initiate their attacks by binding to specific proteins on the surfaces of target cells. Bacterial toxins (e.g. bacteriocins) and viruses (bacteriophages) targeting Gram-negative bacteria typically bind to outer membrane proteins. Bacterial E-colicins target Escherichia coli by binding to the outer membrane cobalamin transporter BtuB. Colicins are tripartite molecules possessing receptor-binding, translocation, and toxin domains connected by long coiled-coil alpha-helices. Surprisingly, the crystal structure of colicin E3 does not possess a recognizable globular fold in its receptor-binding domain. We hypothesized that the binding epitope of enzymatic E-colicins is a short loop connecting the two alpha-helices that comprise the coiled-coil region and that this flanking coiled-coil region serves to present the loop in a binding-capable conformation. To test this hypothesis, we designed and synthesized a 34-residue peptide (E-peptide-1) corresponding to residues Ala366-Arg399 of the helix-loop-helix region of colicin E3. Cysteines placed near the ends of the peptide (I372C and A393C) enabled crosslinking for reduction of conformational entropy and formation of a peptide structure that would present the loop epitope. A fluorescent analog was also made for characterization of binding by measurement of fluorescence polarization. Our analysis shows the following. (i). E-peptide-1 is predominantly random coil in aqueous solution, but disulfide bond formation increases its alpha-helical content in both aqueous buffer and solvents that promote helix formation. (ii). Fluorescein-labeled E-peptide-1 binds to purified BtuB in a calcium-dependent manner with a Kd of 43.6 +/- 4.9 nm or 2370 +/- 670 nm in the presence or absence of calcium, respectively. (iii). In the presence of calcium, cyanocobalamin (CN-Cbl) displaces E-peptide-1 with a nanomolar inhibition constant (Ki = 78.9 +/- 5.6 nm). We conclude that the BtuB binding sites for cobalamins and enzymatic E-colicins are overlapping but inequivalent and that the distal loop and (possibly) the short alpha-helical flanking regions are sufficient for high affinity binding.
authors.affiliation: Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, Virginia 22908-0736, USA.
authors.forename: Arun K
authors.initials: AK
authors.lastname: Mohanty
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Bishop
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Bishop
authors.forename: William C
authors.initials: WC
authors.lastname: Wimley
authors.forename: Michael C
authors.initials: MC
authors.lastname: Wiener
mesh_terms: D000595:Amino Acid Sequence; D016880:Anisotropy; D001425:Bacterial Outer Membrane Proteins; D002118:Calcium / Q000378:metabolism; D002942:Circular Dichroism; D003087:Colicins / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004305:Dose-Response Relationship, Drug; D019277:Entropy; D000939:Epitopes / Q000737:chemistry*; D029968:Escherichia coli Proteins / Q000737:chemistry*; D007700:Kinetics; D026901:Membrane Transport Proteins; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D018000:Receptors, Peptide / Q000737:chemistry*
chemical_list: D001425:Bacterial Outer Membrane Proteins; C044592:BtuB protein, E coli; D003087:Colicins; D000939:Epitopes; D029968:Escherichia coli Proteins; D026901:Membrane Transport Proteins; D010455:Peptides; D018000:Receptors, Peptide; D002118:Calcium