The Dynamic Structures of the Type IV Pilus

The fundamental type IVa pilus (T4aP)‐like architecture includes a retractable pilus fiber, a motor, an alignment subcomplex, and—in Gram‐negative bacteria—an outer membrane secretin pore (Fig. 1). The pilus is an extracellular polymer of pilins. Pilins subunits are stored in the inner membrane and the motor powers their polymerization (extension) and depolymerization (retraction) at the pilus base. The alignment subcomplex connects the secretin with the motor and controls pilus dynamics. Finally, the secretin pore allows the pilus to extend through the outer membrane. Since publication of previous T4aP reviews (1‐4), discoveries made using cryo‐electron microscopy (cryo‐EM), cryo‐electron tomography (cryo‐ET), X‐ray crystallography, and nuclear magnetic resonance (NMR) have dramatically reshaped our understanding of T4P‐like systems. Here we put these discoveries in context with the structure and function of the T4aP, using the Pseudomonas aeruginosa T4aP system nomenclature.