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.