progress has been made in our understanding of how proteins are translocated across the bacterial cytoplasmic membrane. Most proteins that reside in the periplasmic space or outer membrane of Escherichia coli are synthesized as precursors with a hydrophobic amino-terminal extension, the signal sequence. These preproteins are exported across the inner membrane via the general secretion pathway, which involves proteins encoded by the secretion (sec) genes (for reviews see Refs 1-3). The six known Sec proteins, SecA, SecB, SecD, SecE, SecF and SecY, catalyse each of the essential steps in the protein translocation process. Secretory proteins are recognized in the cytosol, maintained in a translocationcompetent state, targeted to the membrane surface, translocated across the membrane and finally released at the periplasmic face of the inner membrane. Other proteins are required for preprotein processing and covalent modifications. Proteins that are exported across the outer membrane and released into the medium use a mechanism involving specialized components and/or ancillary proteins. Functional studies have shed new light on the energetics and catalysis of protein export. The roles of the Sec proteins Bacterial protein export is mediated by soluble chaperone proteins and a multimeric membrane protein complex, termed the translocase 2,4. The SecB protein is the major chaperone for protein export and prevents preprotein aggregation and misfolding 5. SecB is required for only a subset of preproteins, which it directs to the SecA protein bound at the membrane surface 6. SecA is the peripheral domain of the translocase, and performs a central role in protein translocation 7. SecA, an ATPase, drives the ATP-dependent translocation of preproteins across the membrane 8. Interactions with preproteins, acidic phospholipids and the SecY/E protein complex activates SecA for ATP hydrolysis 8,9. The SecY/E protein complex constitutes the membrane-inte
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