Twitching motility

Mechanisms

Pilus structure

The type IV pilus complex consists of both the pilus itself and the machinery required for its construction and motor activity. The pilus filament is largely composed of the PilA protein, with more uncommon minor pilins at the tip. These are thought to play a role in initiation of pilus construction. Under normal conditions, the pilin subunits are arranged as a helix with five subunits in each turn, but pili under tension are able to stretch and rearrange their subunits into a second configuration with around 1+2⁄3 subunits per turn.

Three subcomplexes form the apparatus responsible for assembling and retracting the type IV pili. The core of this machinery is the motor subcomplex, consisting of the PilC protein and the cytosolic ATPases PilB and PilT. These ATPases drive pilus extension or retraction respectively, depending on which of the two is currently bound to the pilus complex. Surrounding the motor complex is the alignment subcomplex, formed from the PilM, PilN, PilO and PilP proteins. These proteins form a bridge between the inner and outer membranes and create a link between the inner membrane motor subcomplex and the outer membrane secretion subcomplex. This consists of a pore formed from the PilQ protein, through which the assembled pilus can exit the cell.

Regulation

Regulatory proteins associated with the twitching motility system have strong sequence and structural similarity to those that regulate bacterial chemotaxis using flagellae. In P. aeruginosa for example, a total of four homologous chemosensory pathways are present, three regulating swimming motility and one regulating twitching motility. These chemotactic systems allow cells to regulate twitching so as to move towards chemoattractants such as phospholipids and fatty acids. In contrast to the run-and-tumble model of chemotaxis associated with flagellated cells however, movement towards chemoattractants in twitching cells appears to be mediated via regulation of the timing of directional reversals.

Motility patterns

Twitching motility is capable of driving the movement of individual cells. The pattern of motility that results is highly dependent upon cell shape and the distribution of pili over the cell surface. In N. gonorrhoeae for example, the roughly spherical cell shape and uniform distribution of pili results in cells adopting a 2D random walk over the surface they are attached to. In contrast, species such as P. aeruginosa and M. xanthus exist as elongated rods with pili localised at their poles, and show much greater directional persistence during crawling due to the resulting bias in force generation direction. P. aeruginosa and M. xanthus are also able to reverse direction during crawling by switching the pole of pilus localization. Type IV pili also mediate a form of walking motility in P. aeruginosa, where pili are used to pull the cell rod into a vertical orientation and move it at much higher speeds than during horizontal crawling motility.

The existence of many pili pulling simultaneously on the cell body results in a balance of forces determining the movement of the cell body. This is known as the tug-of-war model of twitching motility. Sudden changes in the balance of forces caused by detachment or release of individual pili results in a fast jerk (or 'slingshot') that combines fast rotational and lateral movements, in contrast to the slower lateral movements seen during the longer periods between slingshots.