Secretins of type-two secretion systems are necessary for exopolymeric slime secretion in cyanobacteria and myxobacteria

Cyanobacteria and myxobacteria display gliding motility associated with the secretion of an exopolymeric slime through nozzle-like structures. Here, we use biochemical and structural assays to show that these nozzles are composed of secretins of the PilQ/GspD family, which are known to form outer membrane gates in type-two secretion systems (T2SSs) and other bacterial protein secretion systems. We show that gspD is an essential gene in Myxococcus xanthus, and its downregulation by conditional knockdown renders this bacterium defective in both slime secretion and gliding motility. In cyanobacteria, available data suggest that the exopolymeric slime is a polysaccharide, although the precise nature of the slime in myxobacteria remains unclear. Our results, therefore, indicate that secretins may be required for the secretion of non-proteinaceous polymers in certain bacteria.