Calcium-mediated Protein Folding and Stabilization of Salmonella Biofilm-associated Protein A

Biofilm-associated proteins (BAPs) are important for early biofilm formation (adhesion) by bacteria and are also found in mature biofilms. BapA from Salmonella is a similar to 386-kDa surface protein, comprising 27 tandem repeats predicted to be bacterial lg-like (Big) domains. Such tandem repeats are conserved for BAPs across different bacterial species, but the function of these domains is not completely understood. In this work, we report the first study of the mechanical stability of the BapA protein. Using magnetic tweezers, we show that the folding of BapA Blg domains requires calcium binding and the folded domains have differential mechanical stabilities. Importantly, we identify that >100 nM concentration of calcium is needed for folding of the Blg domains, and the stability of the folded Blg domains is regulated by calcium over a wide concentration range from sub-micromolar (mu M) to millimolar (mM). Only at mM calcium concentrations, as found in the extracellular environment, do the Blg domains have the saturated mechanical stability. BapA has been suggested to be involved in Salmonella invasion, and it is likely a crucial mechanical component of biofilms. Therefore, our results provide new insights into the potential roles of BapA as a structural maintenance component of Salmonella biofilm and also Salmonella invasion. (C) 2018 Elsevier Ltd. All rights reserved.