| Type IV pili (T4P) are the major adhesins of Pseudomonas aeruginosa. The pilus is composed of single repeating subunits of pilin that are assembled into a filamentous polymer. The assembly of T4P is attributed to the function of PilB, an ATPase that provides the energy required for the biogenesis and extension of T4P appendages. PilT, also an ATPase, contributes the energy required to retract the T4P. Alternate cycles of pilus extension and retraction enable the cell to carry out a form of surface-associated locomotion called twitching motility. A third ATPase, PilU also participates in pilus retraction and may be involved in modulating the strength of T4P during twitching. Under irrigated conditions, T4P are important adhesins for the initial surface attachment required for the formation of multi-cellular biofilm communities, but the twitching function was shown to have no direct role in initiating biofilm formation. However, twitching motility appears to affect mature biofilm morphology, possibly due to the role of twitching motility in the latter dissemination stages. Fluorescent protein fusions to T4P ATPases showed that PilB and PilT are localized to both poles of the rod-shaped bacterium, while PilU localizes to the single flagellated end of the cell. The bipolar localization of ATPases suggests a possible mechanism for the rapid changes in the direction of twitching motility observed in P. aeruginosa. T4P ATPases belong to a superfamily of AAA+ proteins (A&barbelow;TPases a&barbelow;ssociated with cellular a&barbelow;ctivity) that contain conserved Walker A and B boxes required for ATP binding and hydrolysis, respectively, while the function of additional conserved Asp and His boxes are unclear. Twitching motility assay of site-directed AAA motif mutants and ATPases assays of purified proteins showed that while PilU is an ATPase in vitro, it does not require Walker B for twitching motility, suggesting that PilU does not play an energetic function in vivo. In the Asp and His boxes, only a subset of conserved residues are needed for twitching motility and ATPase activity. Based on our structural analysis, the Asp box likely plays a structural role, while the His box might sense ATP, thus contributing to the ATPase function of T4P AAA+ proteins. |
