Structural Biochemistry/Type IV Pili
Type IV pili is a type of pili that uses twitching motility as a means of bacterial movement rather than the more common swimming motility. These pili are generally located at the poles of a bacterial cell. The usage of type IV pili as a means of transportation is predominantly seen in bacterial colonies in which twitching motility is present. Isolated cells that are not located to any inert surfaces have not been observed to use type IV pili. Movement reversals often times are related to alternating usage of type IV pili at opposite poles of the bacteria cell. This is better known as twitching motility.
Movement resulting from type IV pili is often observed to be sporadic and erratic rather than the smoother swimming motility. The principle force behind twitching motility results from pili retraction. Pili retraction on a cell are independent from each other. Movement via type IV pili is described as being similar to a “grappling hook” because type IV pili are too flexible to be used to push and propel a cell forward. Twitching motility is predominantly observed in a colonial setting. Colonies of cells use type IV pili and the proximity of neighboring cells as a means to move as a whole.
Type IV pili bind to inert surfaces through ambiguous and unclear adhesion at the tip of the structure. The adhesion resulting from the contact of type IV pili and a surface are not significant, weak, and only occur at the tips of the pili. On the other hand, type IV pili bind to mammalian cells and other cell types through specific receptors. These receptors bind with the tip of the pili. However, there are many structural variations of type IV pili, which results in different binding specificities. Essentially, binding varies based on the structural variation of the pili itself as well as the receptors of that which it is attempting to bind to. Pili retraction can be attributed to a protein that is specific to type IV pili known as PilT. It is believed that when pili retract, the pili are partially disassembled into smaller subunits. This action, the disassembly of the pili, is controlled by PilT. Likewise, PilT is responsible for pilin degredation. Type IV pili can bind to either surfaces or other cells. Specifically, type IV pili form a sort of “grappling hook” for the bacteria cell, in which the cell can pull itself around using the curved pili. As such, a proximity to other objects is an important aspect of using type IV pili. Studies of P. aeruginosa have indicated that the retraction of type IV pili seems to be partially reflexive. Specifically, exposure to an attachable surface has resulted in the retraction of the type IV pili, even if the pili was not directly attached to the cell.
Type IV pili are 5-7 nanometers in width and multiple micrometers in length. Type IV pili are predominantly composed of a small subunit known as pilin. Pilin is a protein that is 145-160 amino acids in length, depending on the species. The majority of the pilin is hydrophilic and experience structural variation. Structural variation of type IV pili is thought to occur as a result of changes in the environment, changes in the behavior and the needs of the host cell, changes in selection pressure from bacteriophages, and evolutionary drift. Through analysis of three-dimensional models of the crystal structure of different bacteria, pilin is shown to be extremely assymetrical. Pilin contain an alpha helical spine, which contain a sugar portion, an anti-parallel beta sheet, and a c-terminal beta sheet. This structure is stabilized through the existence of disulfide bridges and the attractive close-range forces of nearby residues. Another key characteristic of type IV pili is the fact that the pilin of one bacteria is often times interchangeable with the pilin of another bacteria. Though variation occurs in type IV pili, all type IV pili have extremely similar quarternary structures and fairly similar tertiary structures.
Mattick JS. Type IV pili and twitching motility. Annual Review of Microbiology. 2002;56:289-314. Epub 2002 Jan 30. PMID 12142488. Accessed 2012 Dec 6.