Hatzixanthis Kostas, Palmer Tracy, Sargent Frank
Centre for Metalloprotein Spectroscopy and Biology, School of Biological Sciences, University of East Anglia, Norwich, NR4 7TJ, UK.
Mol Microbiol. 2003 Sep;49(5):1377-90. doi: 10.1046/j.1365-2958.2003.03642.x.
A group of bacterial exported proteins are synthesized with N-terminal signal peptides containing a SRRxFLK 'twin-arginine' amino acid motif. Proteins bearing twin-arginine signal peptides are targeted post-translationally to the twin-arginine translocation (Tat) system which transports folded substrates across the inner membrane. In Escherichia coli, most integral inner membrane proteins are assembled by a co-translational process directed by SRP/FtsY, the SecYEG translocase, and YidC. In this work we define a novel class of integral membrane proteins assembled by a Tat-dependent mechanism. We show that at least five E. coli Tat substrate proteins contain hydrophobic C-terminal transmembrane helices (or 'C-tails'). Fusions between the identified transmembrane C-tails and the exclusively Tat-dependent reporter proteins TorA and SufI render the resultant chimeras membrane-bound. Export-linked signal peptide processing and membrane integration of the chimeras is shown to be both Tat-dependent and YidC-independent. It is proposed that the mechanism of membrane integration of proteins by the Tat system is fundamentally distinct from that employed for other bacterial inner membrane proteins.
一组细菌输出蛋白在合成时带有含SRRxFLK“双精氨酸”氨基酸基序的N端信号肽。带有双精氨酸信号肽的蛋白在翻译后被靶向双精氨酸转运（Tat）系统，该系统将折叠好的底物转运穿过内膜。在大肠杆菌中，大多数整合内膜蛋白是通过由信号识别颗粒/ FtsY、SecYEG转运体和YidC指导的共翻译过程组装而成。在这项工作中，我们定义了一类通过Tat依赖性机制组装的新型整合膜蛋白。我们表明，至少五种大肠杆菌Tat底物蛋白含有疏水的C端跨膜螺旋（或“C尾”）。已鉴定的跨膜C尾与仅依赖Tat的报告蛋白TorA和SufI之间的融合使所得嵌合体与膜结合。嵌合体的与输出相关的信号肽加工和膜整合显示既依赖Tat又不依赖YidC。有人提出，Tat系统介导的蛋白膜整合机制与其他细菌内膜蛋白所采用的机制根本不同。
