Type IV pili (T4P) mediate surface motility, host interactions, and DNA uptake through cycles of extension and retraction. While the primary retraction ATPase PilT has been extensively characterized, its homolog PilU remains less well understood despite being demonstrated as a PilT-dependent retraction ATPase. Here, we determined six PilU structures by cryo-electron microscopy and x-ray crystallography. The structures reveal a homohexameric assembly stabilized by interactions between the C-terminal catalytic domain of one subunit and the N-terminal PAS-like domain of a neighboring subunit. PilU adopts multiple conformational states, exhibiting different combinations of open and closed interfaces even in the absence of nucleotide. Comparison with PilT highlights structural features that likely underlie PilU's weak ATPase activity and its dependence on PilT for function. Together, these findings provide a structural framework for understanding PilU's role within the T4P retraction machinery.
Guo et al. (Tue,) studied this question.