The type IVa pilus machinery is pre-installed during cell division
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ABSTRACTType IV pili (T4aP) are ubiquitous microbial appendages used for adherence, twitching motility, DNA uptake, and electron transfer. Many of these functions depend on dynamic assembly and disassembly of the pilus by a megadalton-sized, cell envelope-spanning protein complex located at the poles of rod-shaped bacteria. How the T4aP assembly complex becomes integrated into the cell envelope in the absence of dedicated peptidoglycan (PG) hydrolases is unknown. After ruling out potential involvement of housekeeping PG hydrolases in installation of the T4aP machinery inP. aeruginosa, we discovered that key components of inner (PilMNOP) and outer (PilQ) membrane subcomplexes are recruited to future sites of cell division. Mid-cell recruitment of a fluorescently tagged alignment subcomplex component, mCherry-PilO, depended on PilQ secretin monomers – specifically, their N-terminal PG-binding AMIN domains. PilP, which connects PilO to PilQ, was required for recruitment, while PilM, which is structurally similar to divisome component FtsA, was not. Recruitment preceded secretin oligomerization in the outer membrane, as loss of the PilQ pilotin, PilF, had no effect on localization. These results were confirmed in cells chemically blocked for cell division prior to outer membrane invagination. The hub protein FimV and a component of the Polar Organelle Coordinator complex – PocA – were independently required for mid-cell recruitment of PilO and PilQ. Together, these data reveal an integrated, energy-efficient strategy for the targeting and pre-installation – rather than retrofit – of the T4aP system into nascent poles, without the need for dedicated PG-remodelling enzymes.
