Crystal Structure of Borrelia turicatae protein, BTA121, a differentially regulated \xa0gene in the tick-mammalian transmission cycle of relapsing fever spirochetes

Zhipu Luo,Oluwatosin K Shittu,Alan J Kelleher,Rabih Darwiche,Elissa M Hudspeth,Oluwatoyin A Asojo,Job E Lopez,Aparna Krishnavajhala,Roger Schneiter

doi:10.1038/s41598-017-14959-9

Abstract

Tick-borne relapsing fever (RF) borreliosis is a neglected disease that is often misdiagnosed. RF species circulating in the United States include Borrelia turicatae, which is transmitted by argasid ticks. Environmental adaptation by RF Borrelia is poorly understood, however our previous studies indicated differential regulation of B. turicatae genes localized on the 150 kb linear megaplasmid during the tick-mammalian transmission cycle, including bta121. This gene is up-regulated by B. turicatae in the tick versus the mammal, and the encoded protein (BTA121) is predicted to be surface localized. The structure of BTA121 was solved by single-wavelength anomalous dispersion (SAD) using selenomethionine-derivative protein. The topology of BTA121 is unique with four helical domains organized into two helical bundles. Due to the sequence similarity of several genes on the megaplasmid, BTA121 can serve as a model for their tertiary structures. BTA121 has large interconnected tunnels and cavities that can accommodate ligands, notably long parallel helices, which have a large hydrophobic central pocket. Preliminary in-vitro studies suggest that BTA121 binds lipids, notably palmitate with a similar order of binding affinity as tablysin-15, a known palmitate-binding protein. The reported data will guide mechanistic studies to determine the role of BTA121 in the tick-mammalian transmission cycle of B. turicatae.

Highlights

Relapsing fever (RF) spirochetes cause recurrent febrile episodes, uncontrollable chills, nausea, vomiting, miscarriage, and potential death if untreated[1,2]
relapsing fever (RF) spirochetes are transmitted to the vertebrate host within seconds of tick bite, indicating that spirochetes colonizing the salivary glands are essential for establishing infection[24]
While BTA121 does not share appreciable sequence similarity to any known protein domains or motifs, its N-terminal region is enriched in proline, glycine, and glutamic acid and contains a potential procyclic acidic repetitive protein (PARP) domain that was previously reported as having roles in Trypanosoma brucei survival and successful replication in its insect vector, the tsetse fly[26]

Summary

Introduction

Relapsing fever (RF) spirochetes cause recurrent febrile episodes, uncontrollable chills, nausea, vomiting, miscarriage, and potential death if untreated[1,2]. Evaluation of primary amino acid sequences indicated that homologues were absent outside the Borrelia genus, most of these proteins are classified as domains of unknown function (DUFs). This limitation has significantly hindered progress in the understanding of the molecular mechanisms of RF spirochete pathogenesis. Efforts to clarify the function of BTA121 include determining if it shares any structural similarity with proteins of known function as these may offer insights towards possible functions Towards these ends we expressed, purified, and solved the crystal structure of BTA121 and present here the results of these studies

Methods

Results

Discussion

Conclusion