Structural and biomolecular analyses of Borrelia burgdorferi BmpD reveal a substrate-binding protein of an ABC-type nucleoside transporter family

Borrelia burgdorferi sensu lato (sl), the causative agent of the tick-borne Lyme borreliosis (LB), has a limited metabolic capacity and needs to acquire nutrients, such as amino acids, fatty acids, and nucleic acids, from the host environment. Using X-ray crystallography, liquid chromatography-mass spectrometry, microscale thermophoresis, and cellular localization studies, we show that Basic membrane protein D (BmpD) is a periplasmic substrate-binding protein of an ABC transporter system binding to purine nucleosides. Nucleosides are essential for bacterial survival in the host organism and these studies suggest a key role for BmpD in the purine salvage pathway of B. burgdorferi sl. As B. burgdorferi sl lacks the enzymes required for de novo purine synthesis, BmpD may play a vital role in ensuring access to the purines needed for sustaining an infection in the host. Further, we show that although human LB patients develop anti-BmpD antibodies, immunization of mice with BmpD does not confer protection against B. burgdorferi sl infection.