Structure of the Pseudomonas aeruginosa Type IVa Pilus Secretin at 7.4 Å.

Type IVa pili (T4aP) function as bacterial virulence factors. T4aP pass through the outer membranes of Gram-negative bacteria via homo-oligomeric secretins. We present a 7.4 Å cryoelectron microscopy structure of the Pseudomonas aeruginosa PilQ secretin. Peripheral and internal features show that the secretin is composed of 14 subunits with C7 symmetry. The channel is a ribbed cylinder with central peripheral spokes and a central gate closed on the periplasmic side. The structure suggests that during pilus extrusion, the central gate is displaced to the interior walls and that no additional conformational changes are required, as the internal diameter can accommodate the pilus. The N1 domain was resolved, while the N0 and the N-terminal β-domains proposed to bind peptidoglycan were absent in class average images and the final 3D map, indicating a high flexibility. These data provide the highest-resolution structure to date of a T4aP secretin.