
The type IV pili (T4P) are long thin filaments present on the surfaces of many bacteria. Unlike other pilus types, which function exclusively in adhesion, the T4P are multifunctional. Their functions include twitching motility, microcolony formation, phage uptake and secretion. At least some of these functions require a dynamic pilus assembly that rapidly polymerizes and depolymerizes the major pilin protein. All T4P pili share a conserved multisubunit machinery that spans the cell envelope. The pilus filament is built from the inner membrane, with each pilin subunit addition powered by an assembly ATPase on the cytoplasmic side of this membrane. The filament grows through the periplasm and across the outer membrane through a secretin channel and requires as many as 40 additional proteins to assemble. Most T4P possess a second “retraction ATPase” that facilitates pilus depolymerization by poorly understood mechanism. The T4P assembly systems for the enteric pathogens Vibrio cholerae and enterotoxigenic E. coli (ETEC) are the simplest of all pilus systems and lack a retraction ATPase. Until recently these T4P were thought to be non-retractile. We have shown that the V. cholerae and ETEC T4P are in fact retractile and that retraction is necessary for all of their functions, which include bacterial aggregation, phage uptake and secretion. We have uncovered a mechanism to explain pilus retraction in these primitive pilus systems, which has implications for understanding pilus dynamics in the more complex systems as well as the closely related bacterial Type II secretion system.

The type IV pili (T4P) are long thin filaments present on the surfaces of many bacteria. Unlike other pilus types, which function exclusively in adhesion, the T4P are multifunctional. Their functions include twitching motility, microcolony formation, phage uptake and secretion. At least some of these functions require a dynamic pilus assembly that rapidly polymerizes and depolymerizes the major pilin protein. All T4P pili share a conserved multisubunit machinery that spans the cell envelope. The pilus filament is built from the inner membrane, with each pilin subunit addition powered by an assembly ATPase on the cytoplasmic side of this membrane. The filament grows through the periplasm and across the outer membrane through a secretin channel and requires as many as 40 additional proteins to assemble. Most T4P possess a second “retraction ATPase” that facilitates pilus depolymerization by poorly understood mechanism. The T4P assembly systems for the enteric pathogens Vibrio cholerae and enterotoxigenic E. coli (ETEC) are the simplest of all pilus systems and lack a retraction ATPase. Until recently these T4P were thought to be non-retractile. We have shown that the V. cholerae and ETEC T4P are in fact retractile and that retraction is necessary for all of their functions, which include bacterial aggregation, phage uptake and secretion. We have uncovered a mechanism to explain pilus retraction in these primitive pilus systems, which has implications for understanding pilus dynamics in the more complex systems as well as the closely related bacterial Type II secretion system.