Abstract
The type IX secretion system (T9SS) is a multiprotein machine distributed in Bacteroidota and responsible for the secretion of various proteins across the outer membrane. Secreted effectors can be either delivered into the medium or anchored to the cell surface. The T9SS is composed of a transenvelope complex consisting of proton-motive force-dependent motors connected to a membrane-associated ring and outer membrane translocons, and a cell-surface anchoring complex that processes effectors once translocated. The T9SS is involved in pathogenesis, metal acquisition, carbohydrate degradation, S-layer biogenesis and gliding motility. The broad spectrum of functions is linked to a highly versatile repertoire of effectors including metallophores, enzymes, toxins and adhesins, that all possess specific signatures to be recruited and transported by the apparatus. This review summarizes the current knowledge on T9SS substrate secretion signals, transport, processing and activities.