Caf1 of Yersinia pestis Forms Complex Highly Stable Protein Polymers and Hydrogel Scaffolds

Abstract

Caf1 is a polymeric protein from the plague bacterium Y. pestis which is secreted via the chaperone-usher pathway and protects the pathogen from macrophages of the host's immune system by forming a protective layer around the cell. The 15.5kDa monomer has beta structure and resembles the extracellular matrix protein fibronectin. The polymer is expressed recombinantly in Escherichia coli, secreted by the bacteria forming a flocculent layer above the cell pellet that can be harvested and the polymer extracted in large quantities. An NHS-PEG crosslinker was used to form a stable hydrogel which has potential for development in 3D-tissue culture and regenerative medicine applications due to its low cost, high stability and biodegradable nature. We have selectively reversed the natural non-stick behaviour of the WT polymer by introducing an integrin binding sequence, RGDS, into loop5 and can promote fibroblast adhesion and growth. Here we describe the high stability of the WT protein which confirms its potential as a medical polymer. The polymer is stable on SDS-PAGE gels and this analysis revealed it's resistance to a range of proteases. Circular dichroism spectroscopy and differential scanning calorimetry revealed it is extremely thermostable from pH2.0 to 11.0 with maximum stability at pH6 of >90°C. It is also stable at high ionic strength and in a range of detergents. Several cell binding motifs have been introduced into the polymer; bone morphogenic protein and collagen motifs (for promoting bone cell adhesion) and laminin motifs (for keratinocyte applications such as wound healing). Degradation sites such as matrix metalloproteinase and thrombin cleavage motifs can be located in regions which promote efficient breakdown of the hydrogel. Co-polymers have been produced by expressing selectively designed monomers off separate plasmids, giving almost limitless possibilities for hydrogel design.