RTX Adhesins are Key Bacterial Surface Megaproteins in the Formation of Biofilms

Gram-negative bacteria produce repeats-in-toxin adhesion proteins (RTX adhesins) to facilitate microbial adhesion. These large, multidomain proteins share a common architecture comprised of four regions. First to emerge from the bacterium, C terminal end leading, is the RTX export sequence that directs the protein through the type 1 secretion system (T1SS). This is followed by the ligand-binding region responsible for host adhesion and cohesion, which contains diverse ligand-binding domains. These serve a zip code function to direct bacteria to a particular environmental niche. Thereafter is a large extension region consisting of tens to hundreds of tandem bacterial immunoglobulin-like (BIg) domains, whose function is to extend the reach of the ligand-binding domains away from the bacterial surface. Lastly, there is a conserved N terminal cell-membrane-anchor region that retains the adhesin within the secretion system. This is also a site of in situ proteolysis, when nutrients are scarce, that enables the bacterium to leave the biofilm. In this review, the four regions of RTX adhesins are presented in the order in which they emerge from the cell during synthesis and retention.