Type IV pilli: paradoxes in form and function

Type IV pili are filaments on the surfaces of many Gram-negative bacteria that mediate an extraordinary array of functions, including adhesion, motility, microcolony formation and secretion of proteases and colonization factors. Their prominent display on the surfaces of many bacterial pathogens, their vital role in virulence, and their ability to elicit an immune response make Type IV pilus structures particularly relevant for study as targets for component vaccines and therapies. Structural studies of the pili and components of the pilus assembly apparatus have proven extremely challenging, but new approaches and methods have produced important breakthroughs that are advancing our understanding of pilus functions and their complex assembly mechanism. These structures provide insights into the biology of Type IV pili as well as that of the related bacterial secretion and archaeal flagellar systems. This review will summarize the most recent structural advances on Type IV pili and their assembly components and highlight their significance.