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Function(s) and Biological Relevance

Bap1 (Biofilm-Associated protein 1) is an extracellular matrix protein from the bacterium Vibrio cholerae that aids in biofilm architecture and adhesion affinity to living surfaces. Bap1 is also responsible for the biofilm resistance in aqueous environments, acidic conditions, antibiotics and immune systems making the V. cholerae biofilm very versatile and difficult to eradicate. Vibrio cholerae biofilms are made up of Vibrio polysaccharides, nucleic acids and matrix proteins RBmA, RbmC, and Bap1. V. cholerae biofilms are linked to increased transmission, virulence and resistance to various environments, which resulted in pandemic cholera. The biofilm resistance and adhesion affinity promote survival in a multitude of environments including mammalian stomachs. The increased resistance and adhesion affinity increase the virulence of the bacterium resulting in a greater probability of contraction, infection and disease.

Broader Implications

It is important for medical professionals and scientists alike to understand how Vibrio cholerae interact with the host they are infecting. “Cholera is a disease spread by drinking water or eating food contaminated with cholera bacteria.” ^[1] The main implication cholera causes in humans is dehydration from loose stools. Pushing fluids to maintain hydration will lower your risk of death below 1%. The main implication of treating cholera is breaking down the V. cholerae biofilms which are resistant to antibiotics, stomach acid and antibiotics. Understanding how to break down this resistant biofilm could help many 3rd world countries and travelers avoid illness.

Structural highlights and structure-function relationships

Bap1 from V. cholerae is composed of a beta-prism that attaches to blade six of the eight-bladed beta-propeller. The beta-prism is an accessory domain to the eight-bladded beta-propeller domain, these two domains make up the tertiary structure of the protein.

Bap1 is an insoluble protein at full length and requires a deletion of a hydrophobic 57-amino acid chain, which forms the beta-prism domain within blade-six of the eight-bladed beta-propeller.

is located in the beta-prism of Bap1 between residues 414-473. Bap1 eight-bladed beta-propeller is insoluble until the beta-prism is attached in the loop of blade six of the beta-propeller. Once the insertion and beta-prism are bound to the beta-propeller Bap1 improves in soluble expression.

The ligands that make up Bap1 are FLC, GOL, NA and CA. Citrate (FLC) binds carbohydrates and promotes structural qualities. Glycerol (GOL) promotes adhesion. Sodium (NA) has one molecule per blade on the beta-propeller. Finally, calcium (CA) provides structure for the eight-bladed crystal structure.

Bap1 is formed by a two-domain structure, which is made up of an eight-bladed beta-propeller with a beta-prism domain within the sixth blade by a flexible linker.

The eight-bladed beta-propeller has a four-stranded antiparallel beta-sheet making up each blade.

The beta-prism, which is the smaller domain, is an insertion domain within blade six via a flexible linker. The beta-prism is responsible for binding citrates to promote a crystal structure and also bind carbohydrates in the same binding sites.

Blade six and blade eight don’t appear to be ion binding sites, however all sites appear to bind calcium ions. During purification of the protein, all cations were replaced by sodium ions.

Bap1 active site appears to be outside of the central cavity of the eight-bladed beta-propeller. There seems to be no catalytic triad associated within Bap1.

The citrate molecule in Bap1 binds in the carbohydrate-binding site in the beta-prism domains. Citrate is required to produce optimal crystal form due to the extensive crystal contacts in the area.

The beta-propeller utilizes lectins, called PropLecs, which are found in the beta-propeller folds that contain carbohydrate-binding sites. Asp348 forms essential contacts with bound carbohydrates in the beta-prism lectin domain on blade-six of the eight-bladed beta-propeller.

Energy Transformation

Bap1 does not transform energy as its main purpose is binding and promote structural integrity of the Vibrio cholerae biofilm.

References

1. ↑ . PMID:216315890657

3. “Travelers' Health.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 3 Oct. 2019, wwwnc.cdc.gov/travel/diseases/cholera.

4. Kaus, Katherine, et al. The 1.9 Å Crystal Structure of the Extracellular Matrix Protein Bap1 from Vibrio Cholerae Provides Insights into Bacterial Biofilm Adhesion. The American Society for Biochemistry and Molecular Biology, 2019.