Structure of Clostridium difficile PilJ Exhibits Unprecedented Divergence from Known Type IV Pilins

Background: Type IV pili are non-covalently assembled appendages, characterized now in both Gram-negative and Gram-positive bacteria. Results:Clostridium difficile produces Type IV pili containing PilJ, a pilin with a novel dual-pilin fold. Conclusion: Models suggest that the C-terminal pilin domain is exposed in pili, providing a unique interaction surface. Significance: The novel fold of PilJ suggests a new mode for Type IV pilus function. Type IV pili are produced by many pathogenic Gram-negative bacteria and are important for processes as diverse as twitching motility, cellular adhesion, and colonization. Recently, there has been an increased appreciation of the ability of Gram-positive species, including Clostridium difficile, to produce Type IV pili. Here we report the first three-dimensional structure of a Gram-positive Type IV pilin, PilJ, demonstrate its incorporation into Type IV pili, and offer insights into how the Type IV pili of C. difficile may assemble and function. PilJ has several unique structural features, including a dual-pilin fold and the incorporation of a structural zinc ion. We show that PilJ is incorporated into Type IV pili in C. difficile and present a model in which the incorporation of PilJ into pili exposes the C-terminal domain of PilJ to create a novel interaction surface.