Proteus mirabilis mannose-resistant, Proteus-like fimbriae: MrpG is located at the fimbrial tip and is required for fimbrial assembly.
The mannose-resistant, Proteus-like (MR/P) fimbria, responsible for mannose-resistant hemagglutination, is a virulence factor for uropathogenic Proteus mirabilis. Based on known fimbrial gene organization, we postulated that MrpG, a putative minor subunit of the MR/P fimbria, functions as an adhesin responsible for hemagglutination, while MrpA serves as the major structural subunit for the filamentous structure. To test this hypothesis, an mrpG mutant was constructed by allelic-exchange mutagenesis and verified by PCR and Southern blotting. The mrpG mutant was found to be negative for hemagglutination, while wild-type strain H14320 and the complemented mutant were positive. Western blots with antiserum raised against an overexpressed MrpG'-His6 fusion protein showed that MrpG was present in the fimbrial preparations of both the wild-type strain and the complemented mutant but absent in that of the mrpG mutant. The mrpG mutant was significantly less virulent in a CBA mouse model of ascending urinary tract infection. Western blots with antiserum to whole MR/P fimbriae showed that MrpA protein was also missing from the fimbrial preparation of the mrpG mutant. Using immunogold electron microscopy, we found that the normal MR/P-fimbrial structure was absent in the mrpG mutant, suggesting that MrpG is essential for initiation of normal fimbrial formation. In the wild-type strain, MrpG protein was localized to the tips of the fimbriae or at the surface of the cell when antiserum raised against overexpressed MrpG was used. Given the tip localization, MrpG may be required for initiation of assembly of MR/P fimbriae but does not appear to be the fimbrial adhesin.