The prodomain of the B ordetella two‐partner secretion pathway protein FhaB remains intracellular yet affects the conformation of the mature C ‐terminal domain

Summary Two‐partner secretion ( TPS ) systems use β‐barrel proteins of the Omp 85‐ TpsB superfamily to transport large exoproteins across the outer membranes of Gram‐negative bacteria. The Bordetella FHA / FhaC proteins are prototypical of TPS systems in which the exoprotein contains a large C ‐terminal prodomain that is removed during translocation. Although it is known that the FhaB prodomain is required for FHA function in vivo , its role in FHA maturation has remained mysterious. We show here that the FhaB prodomain is required for the extracellularly located mature C ‐terminal domain ( MCD ) of FHA to achieve its proper conformation. We show that the C ‐terminus of the prodomain is retained intracellularly and that sequences within the N ‐terminus of the prodomain are required for this intracellular localization. We also identify sequences at the C ‐terminus of the MCD that are required for release of mature FHA from the cell surface. Our data support a model in which the intracellularly located prodomain affects the final conformation of the extracellularly located MCD . We hypothesize that maturation triggers cleavage and degradation of the prodomain.