Molecular mechanism of protein translocation in Escherichia coli

大肠杆菌中的蛋白质转运机制由SecA、易位ATPase和由SecY、SecE和SecG组成的膜包埋复合体组成。分泌蛋白通过由膜包埋复合体形成的亲水隧道，然后通过涉及SecD和SecF的机制释放到周质中。分泌蛋白在氨基末端含有一个信号序列。信号序列由带正电的氨基末端区域和疏水核心区组成。我们从分子水平上分析了蛋白质在大肠杆菌中的转运机制，得到了以下结果。结论1.在酸性磷脂存在的情况下，SECA与信号肽的疏水核心区相互作用，并能介导信号序列中不含带正电荷氨基酸残基的分泌蛋白的移位。在分泌蛋白proOmpA的成熟区，由4-5个氨基酸残基组成的短疏水片段是跨膜转运速率的决定因素。复制的短疏水片段可以作为停止转移序列。SecA的氨基末端区域和Asp-133分别在与SecG的相互作用和与ATP的相互作用中起重要作用。在动物细胞中，信号序列的疏水核心对于跨内质网膜的转运也很重要。

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.