SECRETION OF IPA PROTEINS BY SHIGELLA-FLEXNERI - INDUCER MOLECULES AND KINETICS OF ACTIVATION

The type III Mxi-Spa secretion machinery of Shigella flexneri is respo nsible for secretion of Ipa proteins, which are involved in the entry of bacteria into epithelial cells. Ipa proteins accumulate within bact eria growing in laboratory media, and their secretion is activated upo n contact of bacteria with eukaryotic cells. In this study, we have id entified a group of chemical compounds, including Congo red, Evans blu e, and direct orange, which are able to induce secretion of Ipa protei ns by bacteria suspended in phosphate-buffered saline. Parameters of k inetics of activation of Ipa secretion by Congo red were determined by measuring by enzyme-linked immunosorbent assay the amount of IpaC sec reted and by investigating the increase in susceptibility of Ipa prote ins to proteinase K degradation. Ipa secretion occurred at 37 degrees C, was obtained with 5 to 10 mu M Congo red, and was complete within 3 0 min, In addition, activation of Ipa secretion by Congo red was obser ved with bacteria harvested throughout the exponential phase of growth but not with bacteria in the stationary phase. The interactions of Co ngo red and Congo red-related compounds with the Mxi-Spa secretion app aratus might be specific hydrophobic interactions similar to those inv olved in binding of Congo red to amyloid proteins.