Identification of fibronectin-binding proteins in Mycoplasma gallisepticum strain R

We have determined that virulent Mycoplasma gallisepticum strain R-low is capable of binding the extracellullar matrix protein fibronectin. Fibronectin was found to be present in M. gallisepticum R-low protein extracts by Western blotting and peptide sequencing. Mycoplasma gallisepticum R-high, the attenuated, high-passage derivative of R-low, is deficient in this ability. MGA_1199, the M. gallisepticum homologue of the cytadherence-associated protein P65 from Mycoplasma pneumoniae, and MGA_0928, the M. gallisepticum homologue of the M. pneumoniae cytoskeletal protein HMW3, were identified as fibronectin-binding proteins. Peptides from the regions of MGA_1199 and MGA_0928 exhibiting the highest degree of homology with known fibronectin-binding proteins were shown to bind the gelatin/heparin-binding domain of fibronectin. MGA_1199 and MGA_0928 were shown to be absent and aberrant, respectively, in R-high, explaining its lack of fibronectin-binding capability. Consistent with its M. pneumoniae counterpart, MGA_1199 (renamed PlpA) was demonstrated to be surface exposed, despite a lack of classical membrane-spanning domains. Due to its demonstrated topology and the strength of interaction between its binding peptide and fibronectin, we propose that PlpA functions as a fibronectin-binding protein in vivo and may possess atypical transmembrane domains.