Structure: Bacterial adhesins are long filamentous proteins that attach host bacteria to surfaces. Attachment is an early event in formation of biofilms. Guo et al., 2017 have recently pieced together the first complete structure of an RTX adhesin. This giant 1.5 MDa protein is a single polypeptide chain that folds into ~130 domains with different functions in different regions along the chain (Fig. 1). At the C terminus is the Type I secretion signal that directs the export of the adhesin. Next, is the ligand-binding region, which in this example has domains to bind ice, terminal sugars, and a specific peptide sequence. The majority of the adhesin domains are in the extender (or stalk) region that puts distance between the ligand-binding domains and the N-terminal membrane-anchoring region, which is a novel structure responsible for retention of the adhesin in the outer membrane of its bacterial host. Bioinformatic analyses of other RTX adhesins has shown that they adopt similar structures, but with different binding domains and different lengths.
Fig. 1 Domain structure of the ice-binding RTX adhesin from a marine bacterium
Function: This particular adhesin comes from a marine bacterium (Marinomonas primoryensis) isolated from a salt water lake in Antarctica that is covered in ice for most of the year. We reasoned that this aerobic bacterium has developed an ice-binding domain at the end of the adhesin to locate itself just under the ice where photosynthetic microorganisms like algae and diatoms get the most sunlight (Fig. 2 left panel). But what are the functions of the other ligand-binding domains? It turns out that they bind to the surface of an Antarctic diatom, Chaetoceros neogracile – one of the diatoms we received from our colleague, Dr. Eon-Seon Jin in S. Korea. The motile M. primoryensis home in on the diatoms and collectively swim them to the ice where they attach the cell mass via the ice-binding domain on the adhesin to form a mixed microorganism biofilm (Fig. 2 right panel). This is a neat example of symbiosis. The non-motile diatoms are brought by the bacteria to the best place for photosynthesis. The bacteria benefit by receiving oxygen and other metabolic waste products from the diatoms.
Fig. 2 Marinomonas primoryensis binding to ice and diatoms to form a mixed species biofilm under marine ice in Antarctica
Applications: We are being guided by this model system to deduce how other RTX adhesins bind their host bacteria to different surfaces. Some of these bacteria are human and animal pathogens. By learning how to block these interactions we might be able to better control bacterial infections and biofilm formation.
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