Controlled Proteolysis of an Essential Virulence Determinant Dictates Infectivity of Lyme Disease Pathogens

The Borrelia burgdorferi BB0323 protein undergoes a complex yet poorly defined proteolytic maturation event that generates N-terminal and C-terminal proteins with essential functions in cell growth and infection. Here, we report that a borrelial protease, B. burgdorferi high temperature requirement A protease (BbHtrA), cleaves BB0323 between asparagine (N) and leucine (L) at positions 236 and 237, while the replacement of these residues with alanine in the mutant protein prevents its cleavage, despite preserving its normal secondary structure. The N-terminal BB0323 protein binds BbHtrA, but its cleavage site mutant displays deficiency in such interaction. An isogenic borrelial mutant with NL-to-AA substitution in BB0323 (referred to as Bb^bb0323NL) maintains normal growth yet is impaired for infection of mice or transmission from infected ticks. Notably, the BB0323 protein is still processed in Bb^bb0323NL, albeit with lower levels of mature N-terminal BB0323 protein and multiple aberrantly processed polypeptides, which could result from nonspecific cleavages at other asparagine and leucine residues in the protein. The lack of infectivity of Bb^bb0323NL is likely due to the impaired abundance or stoichiometry of a protein complex involving BB0238, another spirochete protein. Together, these studies highlight that a precise proteolytic event and a particular protein-protein interaction, involving multiple borrelial virulence determinants, are mutually inclusive and interconnected, playing essential roles in the infectivity of Lyme disease pathogens.