Characterisation and structural studies of a superoxide dismutase and OmpA-like proteins from Borrelia burgdorferi sensu lato

Lyme borreliosis is the most common tick-borne, human infection across the Northern
hemisphere. The agent responsible, Borrelia burgdorferi sensu lato (s.l.) covers a family of
Spirochaetes with unique characteristics which are shared by both Gram-negative and Grampositive
bacteria. The outer membrane (OM) is rich in lipoproteins but contains a relatively
low density of integral membrane proteins (OMPs), of these OMPs very few have been
identified and even fewer are well characterised. The OmpA-like transmembrane domain
defined by the Pfam family PF01389 is a 8-stranded membrane spanning β-barrel and is well
conserved among Gram-negative bacteria but to date remains unknown in Spirochaetes.
Building from previous computational work which had sought to identify possible OMPs from
B. burgdorferi s.l. four OmpA-like proteins, BAPKO_0422 (Borrelia afzelii), BB_0562,
BB_0406 (B. burgdorferi) and BG0408 (Borrelia garinii) have been identified and structurally
characterised. The four proteins are encoded by chromosomal genes and highly conserved
between Borrelia species and may be of diagnostic or therapeutic value. Structural
characterisation by both circular dichroism and small angle X-ray scattering suggests these four
proteins adopt a compact globular structure rich in β-strand (~40%) with Ab initio molecular
envelopes resembling a cylindrical peanut shape with dimensions of ~25x45 Å consistent with
an 8-stranded β-barrel. The present work demonstrates that BAPKO_0422 can bind human
factor H (hfH) and some evidence for a further interaction between the BAPKO_0422 protein
and heparin. The interaction with hfH may contribute to the spirochaete’s immune evasion
mechanisms by the inhibition of the complement response.

The zoonotic life-cycle of Borrelia and challenges by the host’s immune system causes an ever
changing environment which often leads to fluctuations of O2 exposure. Although B.
burgdorferi s.l. have a distinct lack of metabolic systems including peroxidases and catalase
enzymes the Spirochaetes genome does encode a single superoxide dismutase gene (sodA -
bb_0157). Previously assigned as a Fe-SOD there has been some debate whether this protein
requires iron or manganese as a co-factor. The present work demonstrates that the B.
burgdorferi enzyme SodA requires manganese for activity and does not display cambialistic
behaviour. Structural and proteomic characterisation suggests the B. burgdorferi SodA enzyme
shares significant sequence similarity to a superoxide dismutase from Thermus thermophilus.