Borreliose-Gesellschaft e.V.


P13, an atypical porin of Borrelia burgdorferi

Iván Bárcena Uribarri
Rudolf Virchow Center for Experimental Biomedicine, Würzburg

Borrelia species possess a small genome. Many of their genes are related with their virulence and the adaptation to the different hosts. The absence of genes in Borrelia involved in the biosynthesis of amino acids, fatty acids or nucleotide is very remarkable. This metabolic deficiency makes Borrelia species dependent on substances produced by the host.

The first step in nutrient uptake is accomplished by porins. Bacterial porins are water-filled channels that facilitate the transport of essential molecules through the outer membrane. Four porins have been described in Borrelia up to this point. P66, P13 and Oms28 have been found in Borrelia burgdorferi, while Oms38 was discovered in relapsing fever spirochetes.

P13 is a small protein with an α-helical secondary structure while most porins posses a predominant β-sheet structure. Furthermore, its small molecular size is not enough to form a channel by itself and a protein complex is required. A C-terminal peptide which function remains unknown is cut in the periplasmic space and it is highly active in lipid membranes. As well, P13 has up to eight paralogues in the same cell, indicating the importance of this protein for Borrelia. All of these facts make of P13 a very extraordinary protein which does not fit into the general porin model.

P13 is an outer membrane protein with immunogenic potential present in LD and RF species and it does not have any other known bacterial homologue, even in the close related Treponema. This fact makes of P13 a perfect candidate to develop more accurate diagnosis tests and more effective therapeutic treatments. For such purpose, recombinant P13 was produced in two organisms. First, in the well known Escherichia coli were two different constructs were designed to establish the role of the periplasmic cleaved C-terminus. Second in tobacco plants, where a virus based vector was delivered by Agrobacterium tumefaciens into the plant cell nucleus. The vector replicates inside the plant cells spreading the infection to adjacent cells producing at the same time the recombinant protein. This second expression method should enable the production of large amounts of the recombinant protein reducing time, costs and the infection risk.

In order to gain some insight into the structure of this intriguing protein its oligomeric association was studied by Blue Native PAGE and second dimension SDS-Page. The P13 complex showed a high molecular weight and a possible association with some lipoprotein that could be involved in the biological function of the complex.

To sum up, P13 is a very unique and probably irreplaceable protein of Borrelia. Its structure could change the general model of porins described for Gram-negative bacteria, and its recombinant production could allow developing new diagnosis/treatment approaches as well as some crystallization experiments to determine its quaternary structure.