STRUCTURE-FUNCTION ANALYSIS OF A FAMILY OF BORRELIAL C1r INHIBITORS FROM LYME DISEASE AND RELAPSING FEVER SPIROCHETES
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Date
2023-06-01
Access
2025-07-01
Authors
Booth, Charles Edward
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Publisher
East Carolina University
Abstract
The complement system is an ancient part of the innate immune system consisting of fluid-phase proteins that contribute to inflammation, homeostasis, and pathogen clearance. A group of vector-borne spirochetal pathogens are known as the Borrelia spp., are vector-borne bacterial pathogens transmitted the hard- or soft-bodied ticks. The most prominently studied spirochete pathogen in the U.S. is Borreliella burgdorferi, the causative agent of Lyme disease. Lyme disease is a debilitating sickness with an estimated 476,000 cases per year. Within the U.S., other borrelial pathogens are known to cause the diseases relapsing fever and Borrelia miyamotoi disease with the etiological agents being B. hermsii and B. turicatae, and B. miyamotoi respectively.
The Lyme spirochete B. burgdorferi was previously found to express the lipoprotein BBK32 in the mammalian host. BBK32 was first discovered to interact with host fibronectin for adhesion and extravasation from the vasculature system. Later BBK32 was
found to bind and inhibit the protease, C1r, of the classical pathway of complement for immune evasion purposes. Thus, BBK32 was indicated to be a multifunctional bacterial protein by interacting with multiple host proteins.
The etiologic agents of both relapsing fever and B. miyamotoi disease were discovered to encode BBK32 orthologs denoted as the fibronectin binding proteins (Fbps). These orthologs are broken into three separate families termed FbpA, FbpB, and FbpC. Previous work indicated that FbpC from B. hermsii and B. recurrentis interacts with fibronectin and complement regulators, but neither FbpA nor FbpB fibronectin interactions have been characterized. More importantly, no work has investigated the C1r inhibitory potential for either Fbp protein. Herein we characterize the fibronectin interactions for FbpA and FbpB of B. miyamotoi and implement a structure-function approach to determine if the Fbps are capable of interacting with C1r.
Utilizing biochemical and biophysical assays we determined that the Fbps demonstrate a differentially bind fibronectin, and C1r active states. Moreover, structural data of the complement inhibitory domains for BBK32 and the Fbps indicate that there are structural differences within the key C1r interacting domains. Molecular dynamic simulations of the complement inhibitory domains of BBK32 and its orthologs indicate that these proteins showcase differences in protein dynamics that may contribute to their C1r inhibition. Overall, our data supports that this family of borrelial C1r inhibitors exhibit similar, but non-overlapping functions. This work directly increases our understanding or host-pathogen interactions and immune evasion along with guiding the synthesis of specific small molecule inhibitors for complement-mediated pathologies.