Investigations into plasmid and chromosomally encoded outer membrane proteins of Borrelia

Lyme disease is the most common vector-borne infection across temperate zones of the Northern Hemisphere and is caused by the spirochete, Borrelia burgdorferi sensu lato (s.l). The outer membrane (OM) of Borrelia possesses many unique features, including an abundance of lipoproteins and few integral β-barrel membrane proteins. The predicted small β-barrel OMPs from Borrelia burgdorferi s.s; BB_0027, BB_0405, BB_0406 and BB_0562, are all predicted to form an 8-stranded transmembrane β-barrel. Three truncations were made to each OMP to increase the chance of crystal formation and these, along with potentially soluble targets, were cloned and recombinantly expressed in E. coli. Following purification, a BB_0406 truncation was soluble in DDM and showed promise for crystal formation. Factor-H binding was investigated using a far-Western blot. No binding was detected, in agreement with the most recent literature.

The Borrelia genome consists of a linear chromosome and numerous linear and circular plasmids. While the chromosome of Borrelia has been subject to various searches for β-barrel OM proteins, the plasmids have so far remained unexplored. This research utilised a computational-framework approach to identify potential OM β-barrel proteins in the plasmid proteome of B. burgdorferi s.l. This approach identified two plasmid-encoded proteins: the annotated porin OMS28, and a previously uncharacterized protein BBJ25. Orthologs of BBJ25 are detected in two other major spirochaete families, Brachyspira and Treponema. Within the Borrelia species complex, BBJ25 is found within a predicted 7-gene operon at either the 3rd or 7th position, resulting in two allelic variants. Phylogenetic analysis of the allelic variants is inconsistent with vertical descent alongside the chromosome suggesting that horizontal transfer and/or recombination has occurred after the divergence of Relapsing Fever/Lyme Disease-type Borrelia. While the precise role of BBJ25 remains unknown, the identification of a novel conserved outer membrane protein is of interest as a diagnostic or therapeutic target.