Immunotopological Analysis of the Major Surface Protein (Msp).

, one of several recognized periodontal pathogens, is a model organism for studying physiology and host-microbe interactions. Its major surface protein Msp (or MOSP) comprises an oligomeric outer membrane-associated complex that binds fibronectin, has cytotoxic pore-forming activity, and disrupts several intracellular responses. There are two hypotheses regarding native Msp structure and membrane topology. One hypothesis predicts that the entire Msp protein forms a β-barrel structure similar to that of well-studied outer membrane porins of Gram-negative bacteria. The second hypothesis predicts a bipartite Msp with distinct and separate periplasmic N-terminal and porin-like β-barrel C-terminal domains. The bipartite model, based on bioinformatic analysis of the orthologous Tpr proteins, is supported largely by studies of recombinant TprC and Msp polypeptides. The present study reports immunological studies in both and backgrounds to identify a prominent Msp surface epitope (residues 229 to 251 in ATCC 35405) in a domain that differs between strains with otherwise highly conserved Msps. These results were then used to evaluate a series of structural models of representative Msps. The data presented here are consistent with a model of Msp as a large-diameter β-barrel porin. This work adds to the knowledge regarding the diverse Msp-like proteins in oral treponemes and may contribute to an understanding of the evolutionary and potential functional relationships between Msps of oral and the orthologous group of Tpr proteins of is among a small subset of the oral microbiota contributing to severe periodontal disease. Due to its relative genetic tractability, is a model organism for studying physiology and host-microbe interactions. Msp is a highly expressed outer membrane-associated oligomeric protein that binds fibronectin, has cytotoxic pore-forming activity, and disrupts intracellular regulatory pathways. It shares homology with the orthologous group of Tpr proteins, one of which is implicated in antigenic variation. The outer membrane topologies of both Msp and the Tpr family proteins are unresolved, with conflicting reports on protein domain localization and function. In this study, we combined empirical immunological data derived both from diverse strains and from recombinant Msp expression in with predictive structural modeling of Msp membrane topology, to move toward resolution of this important issue in biology.