The Gram-negative Fibrobacteres-Chlorobi-Bacteroidetes superphylum has a protein export pathway called the type 9 secretion system (T9SS), which is considered as an important determinant of pathogenicity in serious periodontal disease. These bacteroidetes are famous for the quick and unique gliding motility where their cell surface adhesins move on helical tracks (Shrivastava, Roland, and Berg 2016). Cryo-electron microscopy revealed that SprA is the translocon of T9SS. Furthermore, SprA consists of a large transmembrane ß barrel of 36 strands. The SprA barrel has a lateral entry to the external membrane surface because the barrel pore on the extracellular end is closed.

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