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Fig. 2 | Microbial Cell Factories

Fig. 2

From: Developing Gram-negative bacteria for the secretion of heterologous proteins

Fig. 2

Two-step secretion systems. a Proteins (dark blue) are exported across the inner membrane (IM) via either Sec or Tat before passively diffusing into the extracellular space. b An example of transport via a fusion partner. Export pathway specificity is unknown for many fusion partners, but YebF (purple box) is secreted only when it is exported through Sec. It is believed to translocate the outer membrane (OM) via a porin (orange). c Proteins are exported through either Sec or Tat before entering the pseudopilus apparatus (pink) that transports cargo across the OM. d The translocation domain-passenger domain fusion is exported through the Sec pathway. The translocation domain (yellow) inserts in the outer membrane and the passenger domain (green) is secreted through the pore. An autocleavage event releases the passenger domain in the class of T5SS discussed here. e Proteins fused to the curli subunit (teal) are exported through Sec and are thought to traverse the outer membrane via an entropy gradient in a chaperonin-like structure (magenta). In the absence of the protein that anchors curli subunits to the outer membrane, fibers spontaneously polymerize and aggregate into networks in the extracellular space. A list of engineering features for each secretion system is listed below the diagrams, and those highlighted in green are considered advantages of each system. “Substrate range” and the level of characterization refer specifically to heterologous protein secretion, and “complexity” describes the secretion machinery

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