A unique borrelial protein facilitates microbial immune evasion.

the pathogen of Lyme disease, encodes many conserved proteins of unknown structure or function, including ones that serve essential roles in microbial infectivity. One such protein is BB0238, which folds into a two-domain protein, as we have determined by X-ray crystallography and AlphaFold analysis. The N-terminal domain begins with a helix-turn-helix motif (HTH), previously referred to as a tetratricopeptide repeat (TPR) motif, known to mediate protein-protein interactions. The fold of the C-terminal domain has been seen in proteins with a range of unrelated activities and thus does not infer function. In addition to its previously known binding partner BB0323, another essential borrelial virulence determinant, we show that BB0238 also binds a second protein, BB0108, a borrelial ortholog of the chaperone protein SurA and the peptidyl-prolyl / isomerase protein PrsA. An enzymatic assay confirmed the catalytic activity. We also determined the crystal structure of the catalytic domain of BB0108, which revealed the parvulin-type organization of the key catalytic residues. We show that BB0238 influences the proteolytic processing of BB0323, although the TPR/HTH motif is not involved in the process. Instead, we show that the motif stabilizes BB0238 in the host environment and facilitates tick-to-mouse pathogen transmission by aiding spirochete evasion of early host cellular immunity. Taken together, these studies highlight the biological significance of BB0238 and its interactions with multiple proteins essential for microbial infection. IMPORTANCE Lyme disease is a major tick-borne infection caused by a bacterial pathogen called , which is transmitted by ticks and affects hundreds of thousands of people every year. These bacterial pathogens are distinct from other genera of microbes because of their distinct features and ability to transmit a multi-system infection to a range of vertebrates, including humans. Progress in understanding the infection biology of Lyme disease, and thus advancements towards its prevention, are hindered by an incomplete understanding of the microbiology of , partly due to the occurrence of many unique borrelial proteins that are structurally unrelated to proteins of known functions yet are indispensable for pathogen survival. We herein report the use of diverse technologies to examine the structure and function of a unique protein, annotated as BB0238-an essential virulence determinant. We show that the protein is structurally organized into two distinct domains, is involved in multiplex protein-protein interactions, and facilitates tick-to-mouse pathogen transmission by aiding microbial evasion of early host cellular immunity. We believe that our findings will further enrich our understanding of the microbiology of potentially impacting the future development of novel prevention strategies against a widespread tick-transmitted infection.