Binding of basement-membrane laminin by Escherichia coli

An invasive Escherichia coli (EIEC) isolate was found to bind basement-membrane laminin in a saturable and time-dependent manner. Excess of unlabelled laminin inhibited the binding of the radioactively labelled protein. Non-invasive E. coli K-12 exhibited only low-level laminin binding but introduction of the virulence-associated plasmid from the EIEC isolate led to high-level binding. Expression of a receptor for laminin on the bacteria was therefore associated with the presence of the virulence plasmid. Scatchard plot analysis indicated approximately 1000 receptors per bacterial cell, and a Kd of high-affinity binding of 0.5 pM. A laminin-binding protein which correlated with the presence of the plasmid was isolated and characterized. Its sequence of the eight amino-terminal amino acids was identical to that of the LamB protein of E. coli, although the molecular mass of the two in sodium dodecyl sulphate/polyacrylamide gel (SDS-PAGE) appeared to be slightly different. Both proteins reacted in immunoblot assays with polyclonal antisera raised against either protein, and both proteins bound laminin. Southern-blot hybridization analysis established that both the EIEC strain and the K-12 strains with or without the virulence plasmid contained one lamB gene only, and no laminin-binding protein appeared when the virulence plasmid was introduced into bacteria deleted for the lamB gene. On the basis of these results we suggest that native LamB protein of E. coli or a modified variant of it serves as a major receptor for laminin binding and is present at an increased level in invasive E. coli containing the virulence plasmid.