Molecular mechanism of protein translocation in Escherichia coli

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The protein translocation machinery in Escherichia coli consists of SecA,translocation ATPase, and the membrane-embedded complex comprising SecY,SecE and SecG.Secretory proteins are passed through a hydrophilic tunnel formed by the membrane-embedded complex, and then released into the periplasm via a mechanism involving SecD and SecF.Secretory proteins contain a signal sequence at the amino-terminus. Signal sequences consist of positively charged amino-terminal region and a hydrophobic core region. We analyzed the mechanism of protein translocation in E.coli at the molecular level and obtained the following results.1. SecA interacts with the hydrophobic core region of the signal peptide in the presence of acidic phospholipids and can mediate the translocation of secretory proteins that do not contain positively charged amino acid residues in the signal sequence.2. Short hydrophobic segments consisting of 4-5 amino acid residues in the mature region of a secretory protein, proOmpA,are a determinant for the rate of translocation across the membrane. A duplicated short hydrophobic segment can act as a stop-transfer sequence.3. The amino-terminal region and Asp-133 of SecA are important for the interaction with SecG and ATP hydrolysis, respectively.4. The hydrophobic core of the signal sequence is also important for translocation across the endoplasmic reticulum membrane in animal cells.