Structure of P46, an immunodominant surface protein from Mycoplasma hyopneumoniae: interaction with a monoclonal antibody

Mycoplasma hyopneumoniae is a prokaryotic pathogen that colonizes the respiratory ciliated epithelial cells in swine. Infected animals suffer respiratory lesions, causing major economic losses in the porcine industry. Characterization of the immunodominant membrane-associated proteins from M. hyopneumoniae may be instrumental in the development of new therapeutic approaches. Here, the crystal structure of P46, one of the main surface-antigen proteins, from M. hyopneumoniae is presented and shows N- and C-terminal α/β domains connected by a hinge. The structures solved in this work include a ligand-free open form of P46 (3.1 Å resolution) and two ligand-bound structures of P46 with maltose (2.5 Å resolution) and xylose (3.5 Å resolution) in open and closed conformations, respectively. The ligand-binding site is buried in the cleft between the domains at the hinge region. The two domains of P46 can rotate with respect to each other, giving open or closed alternative conformations. In agreement with this structural information, sequence analyses show similarities to substrate-binding members of the ABC transporter superfamily, with P46 facing the extracellular side as a functional subunit. In the structure with xylose, P46 was also bound to a high-affinity (K_d = 29 nM) Fab fragment from a monoclonal antibody, allowing the characterization of a structural epitope in P46 that exclusively involves residues from the C-terminal domain. The Fab structure in the complex with P46 shows only small conformational rearrangements in the six complementarity-determining regions (CDRs) with respect to the unbound Fab (the structure of which is also determined in this work at 1.95 Å resolution). The structural information that is now available should contribute to a better understanding of sugar nutrient intake by M. hyopneumoniae. This information will also allow the design of protocols and strategies for the generation of new vaccines against this important swine pathogen.