Division of Molecular Microbiology, College of Life Sciences, University of Dundee, Dundee DD1 5EH, UK.
Mol Microbiol. 2012 Jun;84(6):1108-23. doi: 10.1111/j.1365-2958.2012.08080.x. Epub 2012 May 17.
The twin arginine transport (Tat) system transports folded proteins across the prokaryotic cytoplasmic membrane and the plant thylakoid membrane. In Escherichia coli three membrane proteins, TatA, TatB and TatC, are essential components of the machinery. TatA from Providencia stuartii is homologous to E. coli TatA but is synthesized as an inactive pre-protein with an N-terminal extension of eight amino acids. Removal of this extension by the rhomboid protease AarA is required to activate P. stuartii TatA. Here we show that P. stuartii TatA can functionally substitute for E. coli TatA provided that the E. coli homologue of AarA, GlpG, is present. The oligomerization state of the P. stuartii TatA pro-protein was compared with that of the proteolytically activated protein and with E. coli TatA. The pro-protein still formed small homo-oligomers but cannot form large TatBC-dependent assemblies. In the absence of TatB, E. coli TatA or the processed form of P. stuartii TatA form a complex with TatC. However, this complex is not observed with the pro-form of P. stuartii TatA. Taken together our results suggest that the P. stuartii TatA pro-protein is inactive because it is unable to interact with TatC and cannot form the large TatA complexes required for transport.
双精氨酸转运（Tat）系统将折叠蛋白穿过原核细胞质膜和植物类囊体膜进行转运。在大肠杆菌中，三种膜蛋白 TatA、TatB 和 TatC 是该机制的必需组成部分。来自普罗维登斯菌的 TatA 与大肠杆菌 TatA 同源，但作为一种无活性的前体蛋白合成，其 N 端延伸有 8 个氨基酸。菱形蛋白酶 AarA 去除该延伸部分是激活 P. stuartii TatA 所必需的。在这里，我们表明，只要存在大肠杆菌同源物 GlpG（即 GlpG），P. stuartii TatA 就可以替代大肠杆菌 TatA 发挥功能。我们比较了 P. stuartii TatA 前蛋白的寡聚状态与蛋白水解激活的蛋白和大肠杆菌 TatA 的寡聚状态。前蛋白仍形成小同型寡聚体，但不能形成依赖 TatBC 的大型组装体。在没有 TatB 的情况下，大肠杆菌 TatA 或加工后的 P. stuartii TatA 与 TatC 形成复合物。然而，在没有 TatA 前体的情况下，不会观察到这种复合物。总之，我们的结果表明，P. stuartii TatA 前体蛋白是无活性的，因为它无法与 TatC 相互作用，并且不能形成运输所需的大型 TatA 复合物。
