Calcium stabilizes strong protein fold

Bacterial surface proteins called adhesins help pathogenic bacteria stick to their targets. So-called B domains in some of these adhesins are by far the most mechanically stable proteins known to date. For example, unfolding the B1 domain in a staphylococcal adhesin known as SdrG requires more than 2 nN of force, more than double the value of the next-strongest-known protein. Lukas F. Milles, Hermann E. Gaub, and coworkers at Ludwig Maximilian University Munich now report that three calcium ions are crucial for SdrG’s B1 domain’s mechanical stability (Nat. Commun. 2018, DOI: 10.1038/s41467-018-07145-6). Removing the calcium ions with the chelating agent ethylenediaminetetraacetic acid reduces the strength of the protein fold to 600 pN. Replacing the calcium ions restores the strength. The researchers tuned the force response by mutating amino acids around the calcium-binding sites. One of the binding sites is more important than the others for stabilizing the domain. The researchers propose using B domains to design new biomaterials. “B domains are small and can easily be produced in large quantities,” Milles says. “One could use these folds to assemble a smart protein hydrogel, a stimuli-responsive material that would turn extremely stiff and rigid when calcium is present but loose and flexible when it is chelated.”