Molecular, antigenic, and functional analyses of Omp2b porin size variantsof Brucella spp.

Omp2a and Omp2b are highly homologous porins present in the outer membrane of the bacteria from the genus Brucella, a facultative intracellular pathog en. The genes coding for these proteins are closely linked in the Brucella genome and oriented in opposite directions. In this work, we present the cl oning, purification, and characterization of four Omp2b size variants found in various Brucella species, and we compare their antigenic and functional properties to the Omp2a and Omp2b porins of Brucella melitensis reference strain 16M. The variation of the Omp2a and Omp2b porin sequences among the various strains of the genus Brucella seems to result mostly from multiple gene conversions between the two highly homologous genes. As shown in this study, this phenomenon has led to the creation of natural Omp2a and Omp2b c himeric proteins in Omp2b porin size variants. The comparison by liposome s welling assay of the porins sugar permeability suggested a possible functio nal differences between Omp2a and Omp2b, with Omp2a showing a more efficien t pore in sugar diffusion. The sequence variability in the Omp2b size varia nts was located in the predicted external loops of the porin. Several epito pes recognized by anti-Omp2b monoclonal antibodies were mapped by compariso n of the Omp2b size variants antigenicity, and two of them were located in the most exposed surface loops. However, since variations are mostly driven by simple exchanges of conserved motifs between the two genes (except for an Omp2b version from an atypical strain of Brucella suis biovar 3), the po rin variability does not result in major antigenic variability of the Bruce lla surface that could help the bacteria during the reinfection of a host. Porin variation in Brucella seems to result mainly in porin conductivity mo difications.