MEMBRANE TOPOLOGY OF BORRELIA-BURGDORFERI AND TREPONEMA-PALLIDUM LIPOPROTEINS

A critical issue regarding the molecular architectures of Treponema pa llidum and Borrelia burgdorferi, the agents of venereal syphilis and L yme disease, respectively, concerns the membrane topologies of their m ajor lipoprotein immunogens. A related question is whether these lipid -modified membrane proteins form intramembranous particles during free ze fracture electron microscopy. To address these issues, native borre lial and treponemal lipoproteins were reconstituted into liposomes of diverse composition. The importance of the covalently associated lipid s for membrane association of lipoproteins was revealed by the observa tion that nonlipidated recombinant forms of both B. burgdorferi OspA a nd the T. pallidum 47-kDa immunogen (Tpp47) showed very weak or no bin ding to model bilayer vesicles. In contrast to control liposomes recon stituted with bacteriorhodopsin or bovine rhodopsin, two well-characte rized transmembrane proteins, none of the lipoprotein-liposomes contai ned particles when examined by freeze fracture electron microscopy. To extend these findings to prokaryotic lipoproteins with relatively amp hiphilic polypeptides, similar experiments were conducted with a recom binant nonlipidated form of Escherichia coli TraT, a lipoprotein which has putative transmembrane domains. The nonlipidated TraT oligomers b ound vesicles derived from E. coli lipids but, surprisingly, did not f orm particles in the freeze-fractured liposomes. These findings suppor t (i) a proposed topology of spirochetal lipoproteins in which the pol ypeptide is extrinsic to the membrane surface and (ii) the contention that particles visualized in freeze-fractured spirochetal membranes re present poorly characterized transmembrane proteins.