The Peptidyl–Prolyl Isomerase and Chaperone Par27 of Bordetella pertussis as the Prototype for a New Group of Parvulins

Proteins that pass through the periplasm in an unfolded state are highly sensitive to proteolysis and aggregation and, therefore, often require protection by chaperone-like proteins. The periplasm of Gram-negative bacteria is well equipped with ATP-independent chaperones and folding catalysts, including peptidyl–prolyl isomerases (PPIases). The filamentous hemagglutinin of Bordetella pertussis, which is secreted by the two-partner secretion pathway, crosses the periplasm in an unfolded conformation. By affinity chromatography, we identified a new periplasmic PPIase of the parvulin family, Par27, which binds to an unfolded filamentous hemagglutinin fragment. Par27 differs from previously characterized bacterial and eukaryotic parvulins. Its central parvulin-like domain is flanked by atypical N- and C-terminal extensions that are found in a number of putative PPIases present mostly in β proteobacteria. Par27 displays both PPIase and chaperone activities in vitro. In vivo, Par27 might function as a general periplasmic chaperone in B. pertussis.