The Type 1 secretion pathway — The hemolysin system and beyond

Type 1 secretion systems (T1SS) are wide-spread among Gram-negative bacteria. An important example is the secretion of the hemolytic toxin HlyA from uropathogenic strains. Secretion is achieved in a single step directly from the cytosol to the extracellular space. The translocation machinery is composed of three indispensable membrane proteins, two in the inner membrane, and the third in the outer membrane. The inner membrane proteins belong to the ABC transporter and membrane fusion protein families (MFPs), respectively, while the outer membrane component is a porin-like protein. Assembly of the three proteins is triggered by accumulation of the transport substrate (HlyA) in the cytoplasm, to form a continuous channel from the inner membrane, bridging the periplasm and finally to the exterior. Interestingly, the majority of substrates of T1SS contain all the information necessary for targeting the polypeptide to the translocation channel — a specific sequence at the extreme C-terminus. Here, we summarize our current knowledge of regulation, channel assembly, translocation of substrates, and in the case of the HlyA toxin, its interaction with host membranes. We try to provide a complete picture of structure function of the components of the translocation channel and their interaction with the substrate. Although we will place the emphasis on the paradigm of Type 1 secretion systems, the hemolysin A secretion machinery from E. coli, we also cover as completely as possible current knowledge of other examples of these fascinating translocation systems. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.