The type IV pilus is a long and sturdy grappling hook that bacteria use to attach to a surface and then pull themselves closer to the surface. They are important for virulence in many pathogens, including those that cause cholera, gonorrhoea, food-borne diseases, and multi-drug resistant hospital acquired infections. The molecular mechanism of the motors involved in throwing out and pulling in these grappling hooks was deduced by the Howell Lab, in conjunction with the Burrows lab at McMaster University and published recently in Nature Communications.
The structure of the motor ATPase PilB was determined in the presence of non-saturating amounts of either an ATP analogue or ADP, representing “before” and “after” states of hydrolysis. This allowed for the deduction of the movements made during the hydrolysis cycle. Each time a pair of ATP molecules is hydrolyzed, the pore in the centre of the ring rotates in a clockwise direction while scooping upwards. Based on previous findings, the rotation and scooping motions of PilB would rotate and scoop inner membrane protein, PilC, that would in turn push successive pilin subunits out of the inner membrane leading to pilin polymerization. Rapid pilin polymerization into the pilus can be thought of as throwing out the grappling hook.
This ATPase mechanism was extended to the previously solved structure of a second ATPase, PilT, which is responsible for the opposite function: pulling-in the grappling hook by disassembling the pilus into pilin subunits. The pore inside PilT would rotate in a counterclockwise direction and pull at each step – the opposite of PilB. Thus PilT could act as an anti-PilB, pulling pilins out of the pilus to facilitate rapid depolymerisation, i.e. pulling the grappling hook in.
Both PilB and PilT are essential for pilus function, so the information can be used to design ways to inhibit their activity and render the bacteria less pathogenic to their hosts. The molecular mechanisms are also pertinent to homologous virulence factors, including the type II secretion system that is thought to operate using the extension of the pilus as a piston to push out toxins. Due to the relatedness of the type IV pilus and the type II secretion system, inhibitors could have the potential to block the function of both systems simultaneously.