The membrane protein Rv0899 (OmpATb) from Mycobacterium tuberculosis, has been proposed to act as an outer membrane porin and to contribute to the bacterium's adaptation to the acidic environment of the phagosome during infection. The gene is restricted to pathogenic mycobacteria and, thus, is an attractive candidate for the development of anti-tuberculosis chemotherapy. The 326-residue protein contains three domains: an N-terminal domain (residues 1-72) that includes a sequence of 20 hydrophobic amino acids required for membrane translocation, a central B domain (residues 73-200) with homology to the conserved putative lipid-binding BON (bacterial OsmY and nodulation) superfamily, and a C domain (residues 201-326) with homology to the OmpA-C-like superfamily of periplasmic peptidoglycan-binding sequences, found in several types of bacterial membrane proteins, including in the C-terminus of the Escherichia coli outer membrane protein OmpA. We have characterized the structure and dynamics of the B and C domains and have determined the three-dimensional structure of the B domain. Rv0899 does not form a transmembrane beta-barrel. Residues 73-326 form a mixed alpha/beta-globular structure, encompassing two independently folded modules corresponding to the B and C domains connected by a flexible linker. The B domain folds with three parallel/antiparallel alpha-helices packed against six parallel/antiparallel beta-strands that form a flat beta-sheet. The core is hydrophobic, while the exterior is polar and predominantly acidic. The structure of a BON homology domain is revealed here for the first time. In light of this unexpected structure, it is hard to reconcile an outer membrane porin activity with the central domain of the protein. The structure of the B domain and the overall architecture of the protein suggest alternative modes of membrane association.