title: Oligomeric structure of the carnitine transporter CaiT from Escherichia coli.
abstract: The carnitine transporter CaiT from Escherichia coli belongs to the betaine, choline, and carnitine transporter family of secondary transporters. It acts as an L-carnitine/gamma-butyrobetaine exchanger and is predicted to span the membrane 12 times. Unlike the other members of this transporter family, it does not require an ion gradient and does not respond to osmotic stress (Jung, H., Buchholz, M., Clausen, J., Nietschke, M., Revermann, A., Schmid, R., and Jung, K. (2002) J. Biol. Chem. 277, 39251-39258). The structure and oligomeric state of the protein was examined in detergent and in lipid bilayers. Blue native gel electrophoresis indicated that CaiT was a trimer in detergent solution. This result was further supported by gel filtration and cross-linking studies. Electron microscopy and single particle analysis of the protein showed a triangular structure of three masses or two parallel elongated densities. Reconstitution of CaiT into lipid bilayers yielded two-dimensional crystals that indicated that CaiT was a trimer in the membrane, similar to its homologue BetP. The implications of the trimeric structure on the function of CaiT are discussed.
authors.affiliation: Max Planck Institute of Biophysics, Max-von-Laue Strasse 3, D-60438 Frankfurt am Main, Germany.
authors.forename: Kutti Ragunath
authors.initials: KR
authors.lastname: Vinothkumar
authors.forename: Stefan
authors.initials: S
authors.lastname: Raunser
authors.forename: Heinrich
authors.initials: H
authors.lastname: Jung
authors.forename: Werner
authors.initials: W
authors.lastname: Kühlbrandt
mesh_terms: D017920:Antiporters / Q000737:chemistry*; D001692:Biological Transport; D002331:Carnitine / Q000737:chemistry / Q000378:metabolism*; D002845:Chromatography; D002850:Chromatography, Gel; D003432:Cross-Linking Reagents / Q000494:pharmacology; D003460:Crystallization; D003902:Detergents / Q000494:pharmacology; D019281:Dimerization; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000737:chemistry*; D005583:Fourier Analysis; D007091:Image Processing, Computer-Assisted; D008051:Lipid Bilayers / Q000737:chemistry; D008854:Microscopy, Electron; D011487:Protein Conformation; D020836:Protein Structure, Quaternary
chemical_list: D017920:Antiporters; C469350:CaiT protein, E coli; D003432:Cross-Linking Reagents; D003902:Detergents; D029968:Escherichia coli Proteins; D008051:Lipid Bilayers; D002331:Carnitine