Specificity and role of the Borrelia burgdorferi CtpA protease in outer membrane protein processing

To further characterize the function of the Borrelia burgdorferi C-terminal protease CtpA, we used site-directed mutagenesis to alter the putative CtpA cleavage site of one of its known substrates, the outer membrane (OM) porin P13. These mutations resulted in only partial blockage of P13 processing. Ectopic expression of a C-terminally truncated P13 in B. burgdorferi indicated that the C-terminal peptide functions as a safeguard against misfolding or mislocalization prior to its proteolytic removal by CtpA. In a parallel study of Borrelia burgdorferi lipoprotein sorting mechanisms, we observed a lower-molecular-weight variant of surface lipoprotein OspC that was particularly prominent with OspC mutants that mislocalized to the periplasm or contained C-terminal epitope tags. Further investigation revealed that the variant resulted from C-terminal proteolysis by CtpA. Together, these findings indicate that CtpA rather promiscuously targets polypeptides that lack structurally constrained C termini, as proteolysis appears to occur independently of a specific peptide recognition sequence. Low-level processing of surface lipoproteins such as OspC suggests the presence of a CtpA-dependent quality control mechanism that may sense proper translocation of integral outer membrane proteins and surface lipoproteins by detecting the release of C-terminal peptides.