title: Membrane topology of Borrelia burgdorferi and Treponema pallidum lipoproteins.
abstract: A critical issue regarding the molecular architectures of Treponema pallidum and Borrelia burgdorferi, the agents of venereal syphilis and Lyme disease, respectively, concerns the membrane topologies of their major lipoprotein immunogens. A related question is whether these lipid-modified membrane proteins form intramembranous particles during freeze fracture electron microscopy. To address these issues, native borrelial and treponemal lipoproteins were reconstituted into liposomes of diverse composition. The importance of the covalently associated lipids for membrane association of lipoproteins was revealed by the observation that nonlipidated recombinant forms of both B. burgdorferi OspA and the T. pallidum 47-kDa immunogen (Tpp47) showed very weak or no binding to model bilayer vesicles. In contrast to control liposomes reconstituted with bacteriorhodopsin or bovine rhodopsin, two well-characterized transmembrane proteins, none of the lipoprotein-liposomes contained particles when examined by freeze fracture electron microscopy. To extend these findings to prokaryotic lipoproteins with relatively amphiphilic polypeptides, similar experiments were conducted with a recombinant nonlipidated form of Escherichia coli TraT, a lipoprotein which has putative transmembrane domains. The nonlipidated TraT oligomers bound vesicles derived from E. coli lipids but, surprisingly, did not form particles in the freeze-fractured liposomes. These findings support (i) a proposed topology of spirochetal lipoproteins in which the polypeptide is extrinsic to the membrane surface and (ii) the contention that particles visualized in freeze-fractured spirochetal membranes represent poorly characterized transmembrane proteins.
authors.affiliation: Department of Microbiology, University of Texas, Southwestern Medical Center at Dallas 75235, USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Jones
authors.forename: K W
authors.initials: KW
authors.lastname: Bourell
authors.forename: M V
authors.initials: MV
authors.lastname: Norgard
authors.forename: J D
authors.initials: JD
authors.lastname: Radolf
mesh_terms: D000954:Antigens, Surface / Q000648:ultrastructure; D001425:Bacterial Outer Membrane Proteins / Q000648:ultrastructure; D001428:Bacterial Vaccines; D001436:Bacteriorhodopsins / Q000648:ultrastructure; D001483:Base Sequence; D015748:Borrelia burgdorferi Group / Q000648:ultrastructure*; D002462:Cell Membrane / Q000648:ultrastructure*; D005614:Freeze Fracturing; D008051:Lipid Bilayers; D008074:Lipoproteins / Q000648:ultrastructure*; D008081:Liposomes; D008969:Molecular Sequence Data; D015345:Oligonucleotide Probes / Q000737:chemistry; D011994:Recombinant Proteins; D014210:Treponema pallidum / Q000648:ultrastructure*
chemical_list: D000954:Antigens, Surface; D001425:Bacterial Outer Membrane Proteins; D001428:Bacterial Vaccines; D008051:Lipid Bilayers; D008074:Lipoproteins; D008081:Liposomes; D015345:Oligonucleotide Probes; C072045:OspA protein; D011994:Recombinant Proteins; D001436:Bacteriorhodopsins