Immunotopological analysis of the Treponema denticola major surface protein (Msp).

Treponema denticola , one of several recognized periodontal pathogens, is a model organism for studying Treponema 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 ß-barrel structure similar to well-studied outer membrane porins of Gram-negative bacteria. The second predicts a bipartite Msp with distinct and separate periplasmic N-terminal and porin-like ß-barrel C-terminal domains. The bipartite model, based on bioinformatic analysis of the orthologous T. pallidum Tpr proteins, is supported largely by studies of recombinant TprC and Msp polypeptides. The present study reports on immunological studies in both T. denticola and E. coli backgrounds to identify a prominent Msp surface epitope (residues 229-251 in ATCC 35405) in a domain that differs between strains with otherwise highly conserved Msp's. These results were then used to evaluate a series of in silico structural models of representative T. denticola Msp's. The data presented here are consistent with a model of Msp as a large-diameter ß-barrel porin. This work adds to knowledge of the diverse Msp-like proteins in oral treponemes and may contribute to understanding the evolutionary and potential functional relationships between Msp's of oral Treponema and the orthologous group of Tpr proteins of T. pallidum. IMPORTANCE Treponema denticola is among a small subset of the oral microbiota contributing to severe periodontal disease. Due to its relative genetic tractability, T. denticola is a model organism for studying Treponema physiology and host-microbe interactions. T. denticola 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 T. pallidum Tpr proteins, one of which is implicated in T. pallidum in vivo 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 T. denticola strains and from recombinant Msp expression in E. coli with in silico predictive structure modeling of T. denticola Msp membrane topology to move toward resolution of this important issue in Treponema biology.