Membrane-directed Expression of BBA57 and Other Virulence Targets from Borrelia burgdorferi Reveals Structural Evidence of an Outer Membrane Oligomer in the Lyme Disease Pathogen

Borrelia burgdorferi (Bb), the causative agent of Lyme disease, is a unique pathogen, with a complex genome and unique immune evasion tactics. It lacks genes encoding proteins involved in nutrient synthesis and typical metabolic pathways, and therefore relies on the host for nutrients. The Bb genome encodes both an unusually high number of predicted outer surface lipoproteins of unknown function but with multiple complex roles in pathogenesis, and an unusually low number of predicted outer membrane proteins, given the necessity of bringing in the required nutrients for pathogen survival. Cellular processing of bacterial membrane proteins is complex, and structures of proteins from Bb have all been solved without the N-terminal signal sequence that directs the protein to proper folding and placement in the membrane. This dissertation presents the first membrane-directed expression in E. coli of several Bb proteins involved in the pathogenesis of Lyme disease. For the first time, I present evidence that the predicted lipoprotein, BBA57, forms a large alpha-helical homo-multimeric complex in the OM, is soluble in several detergents, and purifiable. The purified BBA57 complex forms homogeneous, 10 nm-diameter particles, visible by negative stain electron microscopy. Two-dimensional class averages from negative stain images reveal the first low-resolution particle views, comprised of a ring of subunits with a plug on top, possibly forming a porin or channel. These results provide the first evidence to support our theories that some of the predicted lipoproteins in Bb form integral-complexes in the outer membrane, and require proper membrane integration to form functional proteins.