title: Colicin crystal structures: pathways and mechanisms for colicin insertion into membranes.
abstract: The X-ray structures of the channel-forming colicins Ia and N, and endoribonucleolytic colicin E3, as well as of the channel domains of colicins A and E1, and spectroscopic and calorimetric data for intact colicin E1, are discussed in the context of the mechanisms and pathways by which colicins are imported into cells. The extensive helical coiled-coil in the R domain and internal hydrophobic hairpin in the C domain are important features relevant to colicin import and channel formation. The concept of outer membrane translocation mediated by two receptors, one mainly used for initial binding and second for translocation, such as BtuB and TolC, respectively, is discussed. Helix elongation and conformational flexibility are prerequisites for import of soluble toxin-like proteins into membranes. Helix elongation contradicts suggestions that the colicin import involves a molten globule intermediate. The nature of the open-channel structure is discussed.
authors.affiliation: Department of Biological Sciences, Purdue University, West Lafayette, IN 47907-1392, USA.
authors.forename: Stanislav D
authors.initials: SD
authors.lastname: Zakharov
authors.forename: William A
authors.initials: WA
authors.lastname: Cramer
mesh_terms: D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000378:metabolism; D002152:Calorimetry, Differential Scanning; D002942:Circular Dichroism; D003087:Colicins / Q000737:chemistry* / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism; D007425:Intracellular Membranes / Q000737:chemistry / Q000378:metabolism; D007473:Ion Channels / Q000737:chemistry; D008051:Lipid Bilayers / Q000737:chemistry; D008564:Membrane Potentials; D026901:Membrane Transport Proteins; D008958:Models, Molecular; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D018000:Receptors, Peptide / Q000737:chemistry / Q000378:metabolism; D017550:Spectroscopy, Fourier Transform Infrared; D014961:X-Ray Diffraction
chemical_list: D001425:Bacterial Outer Membrane Proteins; C044592:BtuB protein, E coli; D003087:Colicins; D029968:Escherichia coli Proteins; D007473:Ion Channels; D008051:Lipid Bilayers; D026901:Membrane Transport Proteins; D018000:Receptors, Peptide