Bringing Structure to Biology
A powerful little motor
The image for March in our 2018 calendar shows a pathogenic bacterium, the structures that enable it to interact with the environment, and the powerful molecular motor that drives those structures.
Type IV Pili
All organisms, from humans to bacteria, need to interact with their environment. Many bacteria have long, rod-shaped organelles on their surface, called pili, which enable them to interact with the environment or other cells. Pili come in different types, and the pathogenic bacterium Pseudomonas aeruginosa possesses type IV pili (TFP). P. aeruginosa causes various diseases in plants and animals which are difficult to treat due to its natural resistance to antibiotics. TFP have a multitude of functions including locomotion, cell adhesion and DNA uptake. Their primary function in pathogenesis appears to be adhesion of the bacterium to the host cell by attaching to the host and pulling the bacterium towards the cell via retraction of the pilus in a powerful ‘twitching’ motion.
TFP are composed of a helical polymeric arrangement of a protein called pilin that forms thin rods of around 7 nm in diameter but which can reach up to 1 μm in length. Despite their appearance, these delicate fibres are surprisingly strong and can tolerate the significant forces involved in bacterial motility. For such large structures, they can be assembled and disassembled rapidly, at a rate of over 1000 pilin subunits per second. There are multiple motor proteins involved in the process of pilus formation and breakdown, however none of them can match the power of the PilT complex that drives the motion of the full pilus.
Hexameric PilT complex shown from the top (shown on the left), and side (shown on the right). Subunits are shown in blue except for one subunit. The N and C-terminal domains of this single PilT protein are displayed in magenta and purple, respectively.
The structure shown on the right hand side of the calendar image is that of the PilT motor protein (PDB entry 3jvv) which drives pilus motion in P. aeruginosa. This complex is the main driving force of the pilus pulling power, generating over 100 pN of force per ‘twitching’ motion, more than any other molecular motor. If scaled up, this would be the equivalent of a flea with the strength to lift a blue whale! The complex is formed from six subunits of pilT, which has ATPase enzyme activity and two distinct domains connected by a flexible linker. There are a pair of arginine residues, positioned at the interface of the two domains, which interact with the phosphates of the ATP molecule and drive conformational change in the motor as ATP is bound. Subsequent hydrolysis of the ATP then releases the phosphate group and the interaction with arginine is lost. The domains can return to the original conformation.
Interface of the PilT domains with arginine residues shown in orange sticks and ATP analog shown in green at the bottom on the image.
The paper describing this structure suggests a three-state, “Ready, Active, Release” mechanism to drive the PilT motor. The Ready state is the unliganded structure, with the domains in an open conformation. Binding of ATP initiates the Active state, pulling the N-terminal domain down towards the C-terminal domain, before hydrolysis of ATP returns the structure to a more open Release state. The Ready, Active and Release conformations are adjacent to each other in the hexameric complex, with the monomers opposite to these in the equivalent conformation. Therefore these pairs of subunits take turns to pull down the pilus, each removing a pilin in the process, while the remaining subunits prevent slippage of the pilus. This complex really shows that teamwork is the best way to get things done!
The calendar image shows an intaglio print of a P. aeruginosa bacterium, with its pili shown on the surface. It was created by Rachel Brierton from Impington Village College. Overlaid on the right is a PyMol image of the PilT motor protein complex (PDB entry 3jvv).