The pilus-retraction protein PilT: ultrastructure of the biological assembly.

PilT is a biological motor required for the retraction of bacterial type IV pili. Nesseria gonorrhoeae PilT has been purified and its ultrastructure has been examined by freeze-etch electron microscopy, revealing a 115 A outer diameter, 15-35 A inner diameter ring. Aquifex aeolicus PilT crystals were obtained in a primitive hexagonal space group (unit-cell parameters a = b = 107.3, c = 68.5 A) and diffract to a minimum Bragg spacing of 2.8 A when PilT is co-crystallized with adenine nucleotides. Initial phases to 3.5 A resolution have been determined by multiwavelength anomalous dispersion and density modification. Resulting electron-density maps show a hexameric A. aeolicus PilT ring 105 A wide by 55 A high, with an inner cavity that varies in shape and width from 20 to 40 A over the height of the complex. Both PilT ultrastructures are very similar to type II and type IV secretion ATPases in overall shape, size and assembly.