Type IV pili (T4P) are extraordinarily widespread surface appendages made by many species of extant archaea and bacteria as well as by the last universal common ancestor. T4Ps are assembled by a complex, multi-component machine that consumes energy from ATP hydrolysis. Many human pathogens require T4P for virulence. Volunteer studies have proven that T4P of enteropathogenic Escherichia coli (EPEC) and Vibrio cholerae, which make type IVb pili (T4bP) that diverged from other T4P early in evolutionary history, are required for illness in people. Despite remarkable progress, the molecular details of T4P assembly remain obscure. The overall structure of the T4P machinery is remarkably conserved, but closer investigation reveals fundamental differences among systems. Which findings from convenient model systems can be extrapolated to pathogens remains unclear. For example, we recently solved the structure of the BfpD extension ATPase from EPEC to 3.0 Å resolution using cryo-EM. In contrast to the two-fold symmetry and complex enzyme kinetics of the distantly-related extension ATPases from thermophiles, the BfpD ATPase has six-fold symmetry and simple kinetics, consistent with a concerted mechanism of catalysis rather than the symmetric rotary mechanism proposed for its counterparts. In the current brief, circumscribed multi-investigator proposal, we aim to determine to what extent the structure and mechanism of catalysis exhibited by BfpD is representative of other potentially lethal human pathogens. To do so, we have purified the extension ATPases from four other pathogens. Since initial (A0) submission of this revised (A1) proposal, we collected cryo-EM images of the PilB extension ATPase from Pseudomonas aeruginosa. The 2D classes from these images demonstrate unambiguous two-fold symmetry like the structures obtained from thermophiles with which it is closely related. Furthermore, the structure of one such enzyme, which was derived from X-ray crystallography, fits well into our 3D PilB model, currently at 3.4 Å resolution. In the first aim of the current proposal, we will use cryo-EM to determine the structure of the TcpT ATPase from Vibrio cholerae, which is more closely related to EPEC BfpD than to those from thermophiles or P. aeruginosa. This structure will reveal whether it has two-fold, six-fold, or perhaps even three-fold symmetry. In Aim 2 we will perform careful enzyme kinetic studies to determine whether each of the four additional extension ATPases that we purified exhibits classic Michaelis-Menten monophasic kinetics like BfpD or complex multiphasic kinetics like the extension ATPase described for a thermophilic bacterium. Differences among diverse T4P systems indicate that efforts to develop new approaches to prevent and treat lethal bacterial pathogens will benefit from a better understanding of which essential features of T4P are specific to a subset of the organisms and which are universal. As the extension ATPase is a potential t...