G12SecL02Tpc3

From Proteopedia

Introduction

Borrelia burgdorferi is a gram-negative spirochete that is the causative agent of Lyme disease. It’s transmitted to humans through hard-bodied ticks of the genus Ixodes. In order for this microbe to survive and proliferate in its various environments, it possesses many different lipid anchored outer surface proteins, including OspB.

In conjunction with OspA, OspB allows B. burgdorferi to adhere to the midgut of the tick host. Once inside the human host, the epitopes on OspB are recognized by the immune system. Certain complement independent antibodies, H6831 and CB2, are able to lyse the microbes without help from WBCs. Recognition of the OspB epitope by either antibody is dependent on the lys-253 side chain; mutations that lead to any other amino acid in that position reduces binding affinity dramatically. Studies have shown that binding to the epitope induces conformational changes in the liporotein that may lead to the lysing of the cell.

Structure

Crystallization and X-Ray diffraction were methods used to determine the structure of OspB, although some sections of the protein were not able to be determined. During crystallization it was found that the protein dimerizes with perfect symmetry. OspB contains 12 anti-parallel strands and one α- helix. The β-strands 1-4 form a sheet near the N terminus, and strands 5-12 form two additional sheets that form a barrel-like domain with the C terminal α-helix. The epitope is comprised of three surface exposed loops near the C terminus that will eventually come in contact with the Fabs. (Becker 2005). The Lys-253 residue is critical for the binding of the Fabs to the OspB epitope. Lys-253 is located on loop 2 and is exposed to solvent when the protein is uncomplexed. When OspB is complexed however, Lys253 has extensive interaction with the Fab. Lys253 adopts a Trans configuration and is wedged between Tyrosine and Tryptophan (aromatic residues of the Fab). If the Lys residue is replaced with any other residue at the 253 position, the binding affinity is decreased substantially. (Becker 2005).

The 1st b-sheet (strands 1-4) transforms when complexed. It is likely that this structure play a role in anchoring OspB to the lipid membrane. It is most likely proteolytically or auto proteolytically removed.

There is a set of three residues (thr-166, arg-162,glu-184) located near the N-terminus that resembles a catalytic triad of serine proteases. It is possible that this triad is activated in some conformational change and this could be where the autoproteolysis takes place.

Examining the global dynamics of the antigen + Fab by NMR methods have shown that the effects of binding can be propagated to regions of the antigen that are distant to the epitope, which would be critical to the auto proteolytic removal of strands 1-4.