Type two secretion systems secretins are necessary for exopolymeric slime secretion in cyanobacteria and myxobacteria

While protein translocation in Gram-negative bacteria is well understood, our knowledge about the translocation of other high-molecular-weight substances is limited. Nozzle-like structures that secrete exopolymeric substances during gliding motility have previously been observed in the outer membranes of cyanobacteria and myxobacteria. Here, we show that these nozzles are composed of the secretins PilQ/GspD, the outer membrane component of the type II and III secretion systems, the type IV pilus apparatus, and filamentous phage extrusion machinery. Our results show for the first time that secretins may be used for secretion of non-proteinaceous polymers in some bacteria, considerably expanding the repertoire of substrates of these multifunctional outer membrane gates. Moreover, we show that gspD is an essential gene in Myxococcus xanthus, which, when depleted, renders this bacterium defective in slime secretion and gliding motility. SignificanceMany bacteria exhibit gliding motility, movement across surfaces. This motility has been correlated with the deposit of slime trails in their wake. To date, the mechanism of slime secretion has not been understood, and no cell envelope-structures have been identified that are involved in slime secretion during gliding motility. Here, we show that cyanobacteria and myxobacteria use the secretins PilQ/GspD, the outer membrane channels of the T2SS, for slime secretion, which demonstrates a novel cargo transport capacity of these multifunctional outer membrane gates.