Lee H C, Bernstein H D
Genetics and Biochemistry Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Building 10, Room 9D-20, Bethesda, MD 20892-1810, USA.
Proc Natl Acad Sci U S A. 2001 Mar 13;98(6):3471-6. doi: 10.1073/pnas.051484198. Epub 2001 Feb 27.
Previous studies have demonstrated that presecretory proteins such as maltose binding protein (MBP) and outer membrane protein A (OmpA) are targeted to the Escherichia coli inner membrane by the molecular chaperone SecB, but that integral membrane proteins are targeted by the signal recognition particle (SRP). In vitro studies have suggested that trigger factor binds to a sequence near the N terminus of the mature region of OmpA and shunts the protein into the SecB pathway by blocking an interaction between SRP and the signal peptide. By contrast, we have found that the targeting pathway of a protein under physiological conditions is dictated by the composition of its targeting signal. Replacement of the MBP or OmpA signal peptide with the first transmembrane segment of AcrB abolished the dependence on SecB for transport and rerouted both proteins into the SRP targeting pathway. More modest alterations of the MBP signal peptide that simply increase its hydrophobicity also promoted SRP binding. Furthermore, we obtained evidence that SRP has a low affinity for typical signal peptides in vivo. These results imply that different classes of E. coli proteins are targeted by distinct pathways because bacterial SRP binds to a more restricted range of targeting signals than its eukaryotic counterpart.
先前的研究表明，诸如麦芽糖结合蛋白（MBP）和外膜蛋白A（OmpA）等分泌前蛋白由分子伴侣SecB靶向至大肠杆菌内膜，但整合膜蛋白则由信号识别颗粒（SRP）靶向。体外研究表明，触发因子结合到OmpA成熟区域N端附近的一个序列上，并通过阻断SRP与信号肽之间的相互作用将该蛋白分流到SecB途径中。相比之下，我们发现蛋白质在生理条件下的靶向途径由其靶向信号的组成决定。用AcrB的第一个跨膜片段替换MBP或OmpA信号肽消除了转运对SecB的依赖性，并将这两种蛋白重新导向SRP靶向途径。仅增加其疏水性的MBP信号肽的适度改变也促进了SRP结合。此外，我们获得的证据表明，SRP在体内对典型信号肽的亲和力较低。这些结果意味着，不同类别的大肠杆菌蛋白通过不同的途径靶向，因为细菌SRP与其真核对应物相比，结合的靶向信号范围更窄。
