Structure and Function of the Type IX Secretion System for Protein Secretion and Cellular Motility

The type IX secretion system (T9SS) is a Bacteroidota -specific multiprotein machine that supports a wide range of biological processes, from nutrient acquisition and surface modification to host interaction and gliding motility. T9SS effectors represent a structurally diverse repertoire of enzymes, adhesins, and surface proteins that all possess a C-terminal domain that addresses them to their final destination. Recent structural and mechanistic information has revealed the modular organization of the T9SS and molecular details governing effector selection, transport, processing, and sorting. In motile Bacteroidota , the T9SS has been co-opted, evolved, and specialized for gliding motility. In this review, we summarize current knowledge on T9SS architecture and function, describe the embedded gliding machinery, and highlight conceptual advances and open questions regarding the mechanisms, dynamics, and ecological implications of this unique system.