Structural Analysis of the Outer Membrane Lipoprotein BBA14 (OrfD) and the Corresponding Paralogous Gene Family 143 (PFam143) from Borrelia burgdorferi

Lyme disease is caused by the spirochete Borrelia burgdorferi, which can be transmitted to a mammalian host when infected Ixodes ticks feed. B. burgdorferi has many unique characteristics, such as the presence of at least 130 different lipoproteins, which is considerably more than any other known bacterium. Moreover, the B. burgdorferi genome is relatively small (1.5 Mbp) but at the same time it is quite complicated because it comprises a chromosome and 21 linear and circular plasmids. B. burgdorferi is also rich in paralogous proteins; in total, there are approximately 150 paralogous gene families. Equally important is the fact that there is still no vaccine against the Lyme disease. To better understand the role of lipoproteins in this unique bacterium, we solved the crystal structure of the outer membrane lipoprotein BBA14, which is coded on the relatively stable linear plasmid 54 (lp54). BBA14 does not share sequence identity with any other known proteins, and it is one of the ten members of the paralogous gene family 143 (PFam143). PFam143 members are known as orfD proteins from a genetic locus, designated 2.9. The obtained crystal structure revealed similarity to the antitoxin from the epsilon/zeta toxin-antitoxin system. The results of this study help to characterize BBA14 and to clarify the role of PFam143 in the lifecycle of B. burgdorferi.