Fig. 1

One-step secretion systems. Proteins (dark blue) are translocated directly from the cytosol to the extracellular space in an unfolded state, bypassing the inner and outer membranes (IM and OM, respectively). A C-terminal (T1SS, a) or an N-terminal (T3SS, b) secretion tag is required for translocation and remains attached to the cargo upon exiting the secretion apparatus. 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