Sec-dependent preprotein translocation in bacteria
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© Springer-Verlag 1996
MINI-REVIEW
Tanneke den Blaauwen · Arnold J. M. Driessen
Sec-dependent preprotein translocation in bacteria
Received: 4 August 1995 / Accepted: 9 October 1995
Abstract Translocation of precursor proteins across the cytoplasmic membrane in bacteria is mediated by a multisubunit protein complex termed translocase, which consists of the integral membrane heterotrimer SecYEG and the peripheral homodimeric ATPase SecA. Preproteins are bound by the cytosolic molecular chaperone SecB and targeted in a complex with SecA to the translocation site at the cytoplasmic membrane. This interaction with SecYEG allows the SecA/preprotein complex to insert into the membrane by binding of ATP to the high affinity nucleotide binding site of SecA. At that stage, presumably recognition and proofreading of the signal sequence occurs. Hydrolysis of ATP causes the release of the preprotein in the translocation channel and drives the withdrawal of SecA from the membrane-integrated state. Hydrolysis of ATP at the low-affinity nucleotide binding site of SecA converts the protein into a compact conformational state and releases it from the membrane. In the absence of the proton motive force, SecA is able to complete the translocation stepwise by multiple nucleotide modulated cycles. Key words Escherichia coli · ATPase · Energetics · Membrane protein · Protein folding · Sec Proteins · Transport
Proteins designed for transport across the cytoplasmic membrane of Escherichia coli and other bacteria are synthesized with a cleavable signal peptide at the amino terminus. This signal sequence is recognized by the translocation machinery and acts as the targeting signal that guides preproteins to the translocase at the cytoplasmic
T. den Blaauwen · A. J. M. Driessen (Y) Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands Tel. +31-50-632164; Fax +31-50-632154 e-mail: [email protected]
membrane. The translocase is a multi-subunit protein complex with the integral membrane subunits SecY, SecE, and SecG, and with SecA as the dissociable peripheral subunit. These subunits have been found in all prokaryotes studied thus far and also in eukaryotic organelles evolved from bacteria, such as the thylakoidal membrane of the plant chloroplast (Yuan et al. 1994). Preproteins can be bound as nascent chains at the ribosome by the cytosolic chaperone SecB. SecB promotes the interaction of the preprotein with SecA to form a ternary complex. SecA is an ATPase that exists both in a cytosolic and membranebound form. At the membrane surface, SecA interacts with negatively charged phospholipids and with the integral subunits SecY, SecE, and SecG of the translocase. The translocation of a preprotein across the cytoplasmic membrane is an energy-requiring event driven by the hydrolysis of ATP and the proton motive force (∆p). Translocation is initiated by the ATP-dependent co-insertion of SecA and the preprotein in the translocase. Int
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