Protein secretion in the absence of ATP: The autotransporter, two-partner secretion and chaperone/usher pathways of Gram-negative bacteria

Bacteria secrete a wide variety of proteins, many of which play important roles in virulence. In Gram-negative bacteria, these proteins must cross the cytoplasmic or inner membrane, periplasm, and outer membrane to reach the cell surface. Gram-negative bacteria have evolved multiple pathways to allow protein secretion across their complex envelope. ATP is not available in the periplasm and many of these secretion pathways encode components that harness energy available at the inner membrane to drive secretion across the outer membrane. In contrast, the autotransporter, two-partner secretion and chaperone/usher pathways are comparatively simple systems that allow secretion across the outer membrane without the need for input of energy from the inner membrane. This review will present overviews of these 'self-sufficient' pathways, focusing on recent advances and secretion mechanisms. Similarities among the pathways and with other protein translocation mechanisms will be highlighted.