Type II Secretory Pathway for Surface Secretion of DraD Invasin from the Uropathogenic Escherichia coli Dr + Strain

The virulence of the uropathogenicEscherichia coli Dr+ IH11128 strain is associated with the presence of Dr fimbrial structures and a DraD invasin which can act as a fimbrial capping domain at the bacterial cell surface. However, a recent study suggests that the DraD protein is surface exposed in two forms: fimbria associated and fimbria nonassociated (prone to interaction with the N-terminal extension of the DraE protein located on the fimbrial tip). The actual mechanism of DraD surface secretion is presently unknown. We identified a previously unrecognized type II secretory pathway (secreton) in the uropathogenicE. coli Dr+ strain which is well conserved among gram-negative bacteria and used mainly for secretion of virulence determinants. An active secreton is composed of 12 to 15 different proteins, among which GspD functions as an outer-membrane channel to permit extrusion of proteins in a folded state. Therefore, we inactivated the pathway by inserting the group II intron into agspD gene of the type II secretion machinery by site-specific recombination. DraD secretion by theE. coli Dr+ andgspD mutant strains was determined by immunofluorescence microscopy (with antibodies raised against DraD) and an assay of cell binding between bacteria and HeLa cells. The specificity of DraD-mediated bacterial binding for the integrin receptor was confirmed by examination of the adhesion of DraD-coated beads to HeLa cells in the presence and absence of α5 β1 monoclonal antibodies. The investigations that we performed showed that type II secretion inE. coli Dr+ strains leads to DraD translocation at the bacterial cell surfaces.