The Type I Export Mechanism

Abstract

In gram-negative bacteria such as Escherichia coli, proteins destined for the cell surface or surrounding medium must cross the cytoplasmic (inner) and outer membranes and the periplasm between them. Several mechanisms employ periplasmic intermediates, e.g., for assembly of adhesion pili, or utilize a large number of proteins to span the envelope, e.g., in the assembly of flagella. The type I export mechanism contrasts with these as it does not generate periplasmic intermediates and employs a dedicated secretory apparatus of just three proteins. Although acylation is essential for toxin interaction with mammalian cell target membranes, it is not required for export across the prokaryotic envelope. The type I export machinery requires only three export components. These are all integral membrane proteins, a traffic ATPase, an accessory or ‘‘adaptor’’ protein (HlyD), and the outer membrane protein TolC. Cell membrane traffic ATPases provide energy from ATP hydrolysis for movement of various molecules, large polypeptides to small ions, across membranes. The hemolysin export ATPase HlyB has 707 residues and is assumed to function as a homodimer. Putatively, six transmembrane helices between amino acids 158 and 432 interact with the bacterial membrane, whereas the C-terminal ca. 200 residues form the ATPase domain located in the cytoplasm. Each TolC monomer contributes four antiparallel β-strands and four antiparallel α-helical strands to form the channel and tunnel domains, respectively. Whereas a β-barrel is a typical feature of outer membrane proteins, the TolC channel domain is different in that the three monomers form a single β-barrel.