Identification and Structural Characterisation of Novel Outer Membrane Proteins in the genus Borrelia

Although considered Gram-negative the outer membrane (OM) of Borrelia is unique, consisting of a variety of glycolipids, a significant number of surface exposed lipoproteins and a smaller number of integral membrane-spanning β-barrels. Many of these proteins are known to act as adhesins and invasins binding to regulators of the host immune response and extracellular matrix proteins. Many Gram-negative bacteria have the evolutionary conserved OmpA-like transmembrane domain consisting of an eight-stranded membrane spanning β- barrel. The prototypical example is E. coli OmpA; a multifunctional protein involved in a wide variety of physiological and pathological functions. To date the OmpA-like transmembrane domain has not been identified in the Spirochaete phylum. An approach based on hidden Markov models and topology and fold prediction identified a group of homologous genes in the genus Borrelia (BAPKO_0026, BAPKO_0422, BAPKO_0423 and BAPKO_0571), which are predicted to form an eight stranded OM-spanning β-barrel structure with similar structure to E. coli OmpA/W. One of these orthologues in B. afzelii (BAPKO_0422) has been expressed, purified and characterised in vitro; circular dichroism studies show a large percentage of β-strand. A low resolution molecular envelope generated from small-angle X-ray scattering data is consistent with an eight-stranded β-barrel.