The hook is the universal joint, and is also largely made of a single protein. Diameter is about 25 nm in E. coli . The basal bodies have rings for each layer of the cell wall, and they must act as both bushings and stators. About 1000 protons are required for one complete turn of the motor. E. coli has about 100-300 pili per cell.
Targets are often sugars of polysaccharides or glycoproteins.
Assembly occurs from base of structure, the core diameters of pili are too small to allow subunits to pass through. SEX PILI play a role in conjugation. Agrobacterium tumefaciens requires a pilus to deliver DNA to plant cells. Coupling of transcription and translation used in some regulation processes. About 25 nm in diameter, roughly heart-shaped.