Chaddock A M, Mant A, Karnauchov I, Brink S, Herrmann R G, Klösgen R B, Robinson C
Department of Biological Sciences, University of Warwick, Coventry, UK.
EMBO J. 1995 Jun 15;14(12):2715-22. doi: 10.1002/j.1460-2075.1995.tb07272.x.
The delta pH-driven and Sec-related thylakoidal protein translocases recognise distinct types of thylakoid transfer signal, yet all transfer signals resemble bacterial signal peptides in structural terms. Comparison of known transfer signals reveals a single concrete difference: signals for the delta pH-dependent system contain a common twin-arginine motif immediately before the hydrophobic region. We show that this motif is critical for the delta pH-driven translocation process; substitution of the arg-arg by gln-gln or even arg-lys totally blocks translocation across the thylakoid membrane, and replacement by lys-arg reduces the rate of translocation by > 100-fold. The targeting information in this type of signal thus differs fundamentally from that of bacterial signal peptides, where the required positive charge can be supplied by any basic amino acid. Insertion of a twin-arg motif into a Sec-dependent substrate does not alter the pathway followed but reduces translocation efficiency, suggesting that the motif may also repel the Sec-type system. Other information must help to specify the choice of translocation mechanism, but this information is unlikely to reside in the hydrophobic region because substitution by a hydrophobic section from an integral membrane protein does not affect the translocation pathway.
由ΔpH驱动且与Sec相关的类囊体蛋白转运酶识别不同类型的类囊体转运信号，但所有转运信号在结构上都类似于细菌信号肽。对已知转运信号的比较揭示了一个具体差异：依赖ΔpH的系统的信号在疏水区域之前紧邻一个常见的双精氨酸基序。我们表明该基序对于由ΔpH驱动的转运过程至关重要；将精氨酸-精氨酸替换为谷氨酰胺-谷氨酰胺甚至精氨酸-赖氨酸会完全阻断跨类囊体膜的转运，而替换为赖氨酸-精氨酸会使转运速率降低100倍以上。因此，这种类型信号中的靶向信息与细菌信号肽的靶向信息根本不同，在细菌信号肽中所需的正电荷可由任何碱性氨基酸提供。将双精氨酸基序插入依赖Sec的底物中不会改变所遵循的途径，但会降低转运效率，这表明该基序也可能排斥Sec型系统。其他信息必定有助于确定转运机制的选择，但该信息不太可能存在于疏水区域，因为用来自整合膜蛋白的疏水片段进行替换不会影响转运途径。
