Components and dynamics of fiber formation define a ubiquitous biogenesis pathway for bacterial pili

Type IV pili (Tfp) are a unique class of multifunctional surface organelles in Gram-negative bacteria, which play important roles in prokaryotic cell biology. Although components of the Tfp biogenesis machinery have been char acterized, it is not clear how they function or interact. Using Neisseria g onorrhoeae as a model system, we report here that organelle biogenesis can be resolved into two discrete steps: fiber formation and translocation of t he fiber to the cell surface. This conclusion is based on the capturing of an intermediate state in which the organelle is retained within the cell ow ing to the simultaneous absence of the secretin family member and biogenesi s component PilQ and the twitching motility/pilus retraction protein PilT. This finding is the first demonstration of a specific translocation defect associated with loss of secretin function, and additionally confirms the ro le of PilT as a conditional antagonist of stable pilus fiber formation. The se findings have important implications for Tfp structure and function and are pertinent to other membrane translocation systems that utilize a highly related set of components.