| Summary: | The Porphyromonas gingivalis type IX secretion system (T9SS) is a recently discovered protein secretion system that is widely distributed in Bacillus cereus. The T9SS is structurally complex and powerful. More than 20 T9SS components have been verified, and more than 30 virulence factors can be secreted by Porphyromonas gingivalis alone, which contributes significant to the pathogenicity of Porphyromonas gingivalis. T9SS is a large protein complex spanning the inner cell membrane, periplasm, and outer cell membrane. Through the structural and functional connections among its components, it forms a sophisticated functional complex that includes power provision, energy transduction, inner and outer membrane translocation, outer membrane modification, and regulatory systems to recognize, translocate, shear, and modify cargo proteins and translocate bacterial intracellular cargo proteins to the cell surface. In recent years, with advancements in X-ray diffraction and in situ cryoelectron microscopy, the exploration of T9SS has evolved from the functional study of single components to the in situ structural study of multiprotein complexes. Still, the structural resolution of the protein still has shortcomings such as low resolution and an inability to capture dynamic functional structures. Future research directions should focus more on exploring how T9SS interacts and functions with cargo proteins. In this paper, we review the research progress on Porphyromonas gingivalis T9SS on X-ray diffraction and cryoelectron microscopy structure resolution in order to gain a deeper understanding of the transport mechanism of T9SS.
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