Twitching motility is understood to arise from pilus extension, tethering of the pilus tip, and subsequent pilus retraction that pulls the cell body towards the point of attachment. We also know that there are two or more ATPases involved; PilB, the pilin polymerase (think ‘B’ for build) and PilT, the pilin depolymerase (think ‘T’ for tear down). Some bacteria, including P. aeruginosa, have a third ATPase, PilU, that appears to affect pilus retraction but it’s specific function is not clear (think ‘U’ for unknown function).
We have studied the localization of the ATPases in P. aeruginosa by fusing them to fluorescent reporters. This image is a screenshot from a movie of twitching cells, and shows that a YFP-fusion to PilU localizes to the leading pole during motility, and may therefore be involved in identifying the active pole. Similar studies of PilB and PilT showed that they localize to both poles.