The Yersinia pestis Ail protein mediates binding and Yop delivery to host cells required for plague virulence.

Although adhesion to host cells is a critical step in the delivery of cytotoxic Yop proteins by Yersinia pestis, the mechanism has not been defined. To identify adhesins critical for Yop delivery, we initiated two transposon mutagenesis screens using the mariner transposon. To avoid redundant cell binding activities, we initiated the screen with a strain deleted for two known adhesins, pH 6 antigen and the autotransporter, YapC, as well as the Caf1 capsule, which is known to obscure some adhesins. The mutants that emerged contained insertions within the ail (attachment and invasion locus) gene of Y. pestis. A reconstructed mutant with a single deletion in the ail locus (y1324) was severely defective for delivery of Yops to HEp-2 human epithelial cells and significantly defective for delivery of Yops to THP-1 human monocytes. Specifically, the Yop delivery defect was apparent when cell rounding and translocation of an ELK-tagged YopE derivative into host cells were monitored. Although the ail mutant showed only a modest decrease in cell binding capacity in vitro, the KIM5 Deltaail mutant exhibited a >3,000-fold-increased 50% lethal dose in mice. Mice infected with the Deltaail mutant also had 1,000-fold fewer bacteria in their spleens, livers, and lungs 3 days after infection than did those infected with the parental strain, KIM5. Thus, the Ail protein is critical for both Y. pestis type III secretion in vitro and infection in mice.