The pilus-retraction protein the biological assembly 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 Angstrom outer diameter, 15-35 Angstrom 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 Angstrom) and diffract to a minimum Bragg spacing of 2.8 Angstrom when PilT is co-crystallized with adenine nucleotides. Initial phases to 3.5 Angstrom 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 Angstrom 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.