Congo red binding by Porphyromonas gingivalis is mediated by a 66 kDa outer-membrane protein

Congo red was bound from solution by strains of Porphyromonas gingivalis including W50, HG189, HG184, NCTC 11834, Bg 381, WPH35, the slower brown pigmenting colonial variant W50/BR1, and the avirulent mutant W50/BE1, and by Porphyromonas endodontalis HG370 and Porphyromonas asaccharolytica B537. SDS-PAGE of whole cells of all species examined displayed a 66 kDa Congo-red-binding component which was also detected in the outer membranes of P. gingivalis W50 grown in the chemostat under both haemin limitation and haemin excess, and which corresponded to a Coomassie-blue-stained band of the same mobility. Pretreatment of haemin-excess batch-grown cells of P. gingivalis W50 with polymyxin B, which binds to lipid A, did not inhibit binding, whilst binding was enhanced in the presence of 2 M ammonium sulphate, suggesting the involvement of non-specific hydrophobic interactions. Binding was also reduced by pretreatment with trypsin and papain, and by 8-anilino-1-naphthalenesulphonic acid, which binds to hydrophobic amino acids. The 66 kDa binding component was sensitive to proteinase K digestion, and loss of Congo red staining of this band correlated with the quantitative reduction in Congo red binding by whole cells. These data, and our previous work, show that Congo red and iron protoporphyrin IX (haemin) are bound to different outer-membrane components, and that Congo red binding may be of little value as a marker to detect virulent strains of P. gingivalis or those expressing haemin-binding proteins.