Characterization of surface layer proteins in Lactobacillus crispatus isolate ZJ001

Lactobacillus crispatus (L. crispatus) ZJ001 is highly adhesive to epithelial cells and expresses S-layer proteins. In this study, S-layer genes were sequenced and expressed in E. coli to characterize the function of S-layer proteins with this particular strain. L. crispatus ZJ001 harbored two Slayer genes slpA and slpB, and only slpA gene was expressed in the bacterium, as revealed by RT-PCR and immunoassays. The mature SlpA showed 47% amino acid sequence identity to SlpB. The SlpA and SlpB of L. crispatus ZJ001 were highly homologous at the C-terminal region to other Lactobacillus S-layer proteins, but were substantially variable at N-terminal and middle regions. Electron microscopic analysis indicated that His-slpA expressed in E. coli was able to form a sheet-like structure similar to the natural S-layer, but His-slpB formed as disclike structures. In the cell binding experiments, HeLa cells were able to bind to both recombinant His-slpA and HisslpB proteins to the extent similar to the natural S-layer. The cell binding domains remain mostly in the N-terminal regions in SlpA and SlpB, as shown by high binding of truncated peptides SlpA2-228 and SlpB2-249. Our results indicated that SlpA was active and high binding to HeLa cells, and that the slpA gene could be targeted to display foreign proteins on the bacterial surface of ZJ001 as a potential mucosal vaccine vector.