Complement Regulator-Acquiring Surface Protein

Introduction

Lyme Disease is caused by the spirochete Borrelia burgdorferi, and is transferred into vertebrate hosts by zoonotic vectors such as Ixodes ticks ^[1]. Lyme disease can result in multisystemic disorders, including cardiovascular and neurological problems. There are thousands of cases of Lyme disease reported each year, making it a prevalent disease in North America and Eurasia ^[2]. In order for B. burgdorferi to survive in its host, it evades the host's immune system through the use of complement regulator-acquiring surface proteins. One such protein responsible for a successful initial infection is Borrelia burgdorferi complement regulator-acquiring surface protein 1, or BbCRASP-1 ^[1]. Because BbCRASP-1 binds host complement regulators to the spirochete's outer surface, B. burgdorferi remains undetected within the host ^[1]. BbCRASP-1 specifically binds to complement Factor H (FH) and Factor H-like proteins (FHL-1), which are responsible for the host's immune response and detection of pathogens ^[3]. Recently, it was found that BbCRASP-1 binds to several other proteins in the extra cellular matrix of a human cell, making it highly flexible and adaptive.

• BbCRASP-2 interacts preferentially with factor H^[4].
• BbCRASP-3 and BbCRASP-4 belong to different strains of Borrelia burgdorferi^[5].

Structure

In nature, BbCRASP-1 exists as a homodimer bound to the spirochete's surface ^[2]. BbCRASP-1 needs to be dimerized in order to bind FH/FHL-1 proteins. If its dimeric state is threatened, BbCRASP-1 would not be able to function.

Importance of the C-terminus

The C-terminus of BbCRASP-1 is a region crucial for its stability as a dimer. Previously,sequences of high conservation in the C-terminal regions of the protein’s monomers, , were of interest as a potential binding site ^[2]. Prior studies showed that deletion of these sites caused a complete inability of BbCRASP-1 to bind FH and FHL-1 regulators ^[3]. The role of the C-terminus was determined by mutating in this region of the dimer to aspartate ^[2]. Both the C-terminally truncated and mutated BbCRASP-1 proteins lost their ability to dimerize, inhibiting them from binding to their host’s regulatory factors. It was then concluded that the C-terminus is a structurally sensitive region rather than a direct binding site ^[2]. The C-terminus one monomer lies against the of the other and holds the in place ^[2].

Other Potential Binding Sites

Additional studies were done to investigate the FH and FHL-1 binding sites on the protein. As with previous research, areas of high conservation were investigated due to their relationship with upholding the structure and function of the protein ^[6]. Highly conserved areas of amino acid sequences among Borrelia CRASP-1 proteins encoded by the same gene were examined. The greatest homologous region was clustered on the of the protein. A binding site in this region would be probable because this area allows more contact with the regulator proteins and would aid in further evasion of the immune system by shielding the binding domain from potential detection ^[6].

Function

BbCRASP-1 can be found on the outer layer of the Lyme disease spirochete and is essential for the infiltration of the spirochete into the host ^[1]. BbCRASP-1 provides resistance for the spirochete against the host's complementary immune system as well as spreading of the spirochete within the host.

Host Immune Response Evasion

BbCRASP-1 has an affinity for Factor H (a regulatory protein secreted by the complement immune system) and Factor H-like proteins. Therefore, Factor H binds to BbCRASP-1, which is bound to the outer surface of the spirochete. Because there are multiple BbCRASP-1 proteins on the spirochete, the spirochete is coated with Factor H and effectively able to infiltrate the host and go undetected in the host's plasma^[1].

Relation to the Extra Cellular Matrix

Recently it was found that BbCRASP-1 not only binds to FH and FHL-1 proteins, but it also binds to several other human ligands such as BMP-2 and Extra cellular matrix ligands Collagen I, Collagen III, Collagen IV, fibronectin, laminin, and plasminogen ^[7]. As a result of this new finding, BbCRASP-1 is said to not only advocate the bypassing of the complementary immune system, but facilitate the dissemination of B. burgdorferi within the host's tissues ^[8]. Binding to the ECM of a host is common strategy used by pathogens to acquire contact within the host ^[9][10].

Discussion

In order for B. burgdorferi to successfully colonize its new and hostile environment, it depends on a complement of proteins to fend off the host’s immune system. BbCRASP-1 is extremely important as it serves as a “frontier” protein, helping to bring upon successful initial infection to which depends the entire course of the pathogen’s life cycle and existence within the host. As such it remains a protein of high interest to medical researchers who can use this valuable information to stem the tide of the colonization process before it develops into a full case of Lyme disease. BbCRASP-1’s high affinity for FH and FHL-1 complement factors and other ligands such as BMP-2, Collagen I, Collagen III, Collagen IV, fibronectin, laminin, and plasminogen make it a highly flexible and adaptive protein well suited to aiding the pathogen in colonization.

Future Studies

Further work needs to be done to determine the complement regulator protein and human ligand binding sites on BbCRASP-1. Knowing this information would aid in combating Lyme disease because it would give researchers a definite target for inhibitory drugs. However, with what is known about the protein, drugs that interfere with the C-terminus region of the dimer would also aid in mediating the effects of the disease because once the dimeric state of the protein is disrupted, it cannot function. These methods should also be applied to other CRASPs so FH/FHL-1 binding would be suppressed and the host's immune system can mount an effective defense against the invading spirochete.