Newitt J A, Ulbrandt N D, Bernstein H D
Genetics and Biochemistry Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
J Bacteriol. 1999 Aug;181(15):4561-7. doi: 10.1128/JB.181.15.4561-4567.1999.
The signal recognition particle (SRP) targeting pathway is required for the efficient insertion of many polytopic inner membrane proteins (IMPs) into the Escherichia coli inner membrane, but in the absence of SRP protein export proceeds normally. To define the properties of IMPs that impose SRP dependence, we analyzed the targeting requirements of bitopic IMPs that are structurally intermediate between exported proteins and polytopic IMPs. We found that disruption of the SRP pathway inhibited the insertion of only a subset of bitopic IMPs. Studies on a model bitopic AcrB-alkaline phosphatase fusion protein (AcrB 265-AP) showed that the SRP requirement for efficient insertion correlated with the presence of a large periplasmic domain (P1). As previously reported, perturbation of the SRP pathway also affected the insertion of a polytopic AcrB-AP fusion. Even exhaustive SRP depletion, however, failed to block the insertion of any AcrB derivative by more than 50%. Taken together, these data suggest that many proteins that are normally targeted by SRP can utilize alternative targeting pathways and that the structure of both hydrophilic and membrane-spanning domains determines the degree to which the biogenesis of a protein is SRP dependent.
信号识别颗粒（SRP）靶向途径是许多多跨膜内膜蛋白（IMP）高效插入大肠杆菌内膜所必需的，但在没有SRP的情况下，蛋白质输出仍能正常进行。为了确定导致SRP依赖性的IMP特性，我们分析了双跨膜IMP的靶向需求，这些双跨膜IMP在结构上介于输出蛋白和多跨膜IMP之间。我们发现，SRP途径的破坏仅抑制了一部分双跨膜IMP的插入。对模型双跨膜AcrB-碱性磷酸酶融合蛋白（AcrB 265-AP）的研究表明，SRP对高效插入的需求与大的周质结构域（P1）的存在相关。如先前报道，SRP途径的扰动也影响多跨膜AcrB-AP融合蛋白的插入。然而，即使彻底耗尽SRP，也无法将任何AcrB衍生物的插入阻断超过50%。综上所述，这些数据表明，许多通常由SRP靶向的蛋白质可以利用替代靶向途径，并且亲水结构域和跨膜结构域的结构决定了蛋白质生物合成依赖SRP的程度。
