Electron microscope image of distal end portion of flagellar filament.
The cap is a five fold pentamer of HAP2 protein with a shape of 5-legs table and bind at distal end of flagellar filament.
Tha cap complex firmly bind to the distal spiral staircase structure of the flagellum and yet always keep one hollow open to allow a flagellin molecule. protein assists the polymerization of flagellin to grow the flagellar filament. When a flagellin subunit is boune into position, the cap move its legs as if it were walk up a spiral staircase. It thus rotate the top plate to prepare the next site for binding flagellin.
Two-state protofilament model.
A protofilament can take two different repeat distances of flagellin, 52.7Å and 51.9Å. Combinations of two protofilament states (blue protofilament is 52.7Å period and red is 51.9Å period) give various herical structure.
