The PilB-PilZ-FimX regulatory complex of the Type IV pilus from Xanthomonas citri

Type IV pili (T4P) are thin and flexible filaments found on the surface of a wide range of Gram-negative bacteria that undergo cycles of extension and retraction and participate in a variety of important functions related to lifestyle, defense and pathogenesis. During pilus extensions, the PilB ATPase energizes the polymerization of pilin monomers from the inner membrane. In Xanthomonas citri, two cytosolic proteins, PilZ and the c-di-GMP receptor FimX, are involved in the regulation of T4P biogenesis through interactions with PilB. In vivo fluorescence microscopy studies show that PilB, PilZ and FimX all colocalize to the leading poles of X. citri cells during twitching motility and that this colocalization is dependent on the presence of all three proteins. We demonstrate that full-length PilB, PilZ and FimX can interact to form a stable complex as can PilB N-terminal, PilZ and FimX C-terminal fragments. We present the crystal structures of two binary complexes: i) that of the PilB N-terminal domain, encompassing sub-domains ND0 and ND1, bound to PilZ and ii) PilZ bound to the FimX EAL domain within a larger fragment containing both GGDEF and EAL domains. Evaluation of PilZ interactions with PilB and the FimX EAL domain in these and previously published structures, in conjunction with mutagenesis studies and functional assays, allow us to propose an internally consistent model for the PilB-PilZ-FimX complex and its interactions with the PilM-PilN complex in the context of the inner membrane platform of the X. citri Type IV pilus. Author summary Bacteria have to adapt their lifestyles to changing environments, at times deciding to hunker down and establish compact multicellular colonies with intricate 3-dimensional structures, called biofilms, that provide protection against hostile conditions; while at other times deciding to go out on their own to explore new habitats. Both of these lifestyles rely on Type IV pili, long extendable and retractable surface filaments that allow the bacteria to attach to and move across surfaces. Pilus extension and retraction are powered by two ATPases, PilB and PilT respectively, and there is significant interest in understanding the means by which the activities of these ATPases are controlled. In this study we explore this theme in the phytopathogen Xanthomonas citri that causes citrus canker disease. In X. citri, the ATPase responsible for pilus extension, PilB, can form a complex with two regulatory proteins, PilZ and the c-di-GMP receptor FimX. We show that the in vitro ATPase activity of the PilB-PilZ complex is enhanced by FimX and that these three proteins co-localize at the leading poles of X. citri cells moving across surfaces. The X-ray structures of PilZ in complex with the N-terminal domain of PilB and with the C-terminal domain of FimX allow us to propose a model for the three-way interaction between these proteins.