Plugging interactions of HAP2 pentamer into the distal end of flagellar filament revealed by electron microscopy

Bacterial flagellum has a cap structure tightly attached to its distal end. The cap is an oligomeric assembly of HAP2 protein (also called FliD) and plays an essential role in the filament growth in vivo by preventing flagellin monomers from leaking out without polymerization. Electron micrographs of the HAP2 complex formed in solution showed exclusively a pentagonal shape, called "star-cap", which was thought to be the end-on view of the cap. The molecular mass roughly corresponded to a dodecamer of HAP2, and therefore a double-layered star-cap was modeled to be the cap. Here, we have observed the side view of the complex in electron micrographs. The images clearly show a rectangular shape, about 80 A wide and 180 A long, with a bipolar feature in its long axis, indicating that the complex is a bipolar pair of pentamers. A thin plate feature is identified at each end of the particle, which looks exactly like the one observed as the structure of the native filament cap. Together with the structure of the filament previously analyzed by electron cryomicroscopy, the results suggest that the cap is a pentamer with its thin plate exposed to the solvent and the other half plugged into the hole at the distal end of the filament, which is almost twice wider than its central channel. This also allows us to model the axial domain arrangement of flagellin subunit in the filament.