Formation of the β-sheet at the extracellular opening of the intimin β-barrel domain is necessary for stability and efficient passenger secretion

Attaching and effacing pathogens, such as enteropathogenic and enterohaemorrhagic E. coli , rely on an adhesin, intimin, for attachment to enterocytes and downstream effects leading to actin pedestal formation. Intimin belongs to the inverse autotransporter family (type 5e secretion systems), which secrete the extracellular adhesive domain or passenger via a hairpin intermediate. While many of the next steps in secretion are now understood, how the hairpin initially forms is not known. We sought to investigate this by making point mutations at several positions in the β-barrel domain of intimin, as this domain forms at least part of the secretion pore. We made the mutations in a wild-type background and in a stalled intermediate caught in the hairpin conformation, which allowed us to uncouple passenger secretion from hairpin formation. Surprisingly, most of the point mutations did not have an appreciable effect on hairpin formation or passenger secretion, and larger changes such as replacing the entire linker region with a flexible glycine-serine stretch showed only a modest reduction in passenger secretion. By contrast, mutations affecting a small β-sheet at the extracellular face of the intimin β-barrel between two extracellular loops and the C-terminus of the linker had a more pronounced effect on secretion, and abolishing this β-sheet prevented hairpin formation and led to complete loss of the wild-type protein. Our results show that the intimin β-barrel is remarkably tolerant to changes and that the β-sheet on the extracellular side of this domain plays a central role in passenger secretion and protein stability.