Oertel Dan, Schmitz Sabrina, Freudl Roland
Institut für Bio- und Geowissenschaften 1, IBG1: Biotechnologie, Forschungszentrum Jülich GmbH, Jülich, Germany.
PLoS One. 2015 Apr 2;10(4):e0123413. doi: 10.1371/journal.pone.0123413. eCollection 2015.
The twin-arginine translocation (Tat) system transports folded proteins across the cytoplasmic membrane of bacteria and the thylakoid membrane of plant chloroplasts. Escherichia coli and other Gram-negative bacteria possess a TatABC-type Tat translocase in which each of the three inner membrane proteins TatA, TatB, and TatC performs a mechanistically distinct function. In contrast, low-GC Gram-positive bacteria, such as Bacillus subtilis, use a TatAC-type minimal Tat translocase in which the TatB function is carried out by a bifunctional TatA. In high-GC Gram-positive Actinobacteria, such as Mycobacterium tuberculosis and Corynebacterium glutamicum, tatA, tatB, and tatC genes can be identified, suggesting that these organisms, just like E. coli, might use TatABC-type Tat translocases as well. However, since contrary to this view a previous study has suggested that C. glutamicum might in fact use a TatAC translocase with TatB only playing a minor role, we reexamined the requirement of TatB for Tat-dependent protein translocation in this microorganism. Under aerobic conditions, the misassembly of the Rieske iron-sulfur protein QcrA was identified as a major reason for the severe growth defect of Tat-defective C. glutamicum mutant strains. Furthermore, our results clearly show that TatB, besides TatA and TatC, is strictly required for unimpaired aerobic growth. In addition, TatB was also found to be essential for the secretion of a heterologous Tat-dependent model protein into the C. glutamicum culture supernatant. Together with our finding that expression of the C. glutamicum TatB in an E. coli ΔtatB mutant strain resulted in the formation of an active Tat translocase, our results clearly indicate that a TatABC translocase is used as the physiologically relevant functional unit for Tat-dependent protein translocation in C. glutamicum and, most likely, also in other TatB-containing Actinobacteria.
双精氨酸转运（Tat）系统可将折叠好的蛋白质转运穿过细菌的细胞质膜以及植物叶绿体的类囊体膜。大肠杆菌和其他革兰氏阴性菌拥有一种TatABC型Tat转运酶，其中内膜蛋白TatA、TatB和TatC各自发挥着机制上不同的功能。相比之下，低GC含量的革兰氏阳性菌，如枯草芽孢杆菌，使用一种TatAC型最小Tat转运酶，其中TatB的功能由双功能的TatA执行。在高GC含量的革兰氏阳性放线菌中，如结核分枝杆菌和谷氨酸棒杆菌，可鉴定出tatA、tatB和tatC基因，这表明这些生物体可能像大肠杆菌一样，也使用TatABC型Tat转运酶。然而，与这种观点相反，之前的一项研究表明谷氨酸棒杆菌实际上可能使用TatAC转运酶，而TatB仅起次要作用，因此我们重新研究了谷氨酸棒杆菌中TatB对依赖Tat的蛋白质转运的需求。在有氧条件下， Rieske铁硫蛋白QcrA的错误组装被确定为Tat缺陷型谷氨酸棒杆菌突变株严重生长缺陷的主要原因。此外，我们的结果清楚地表明，除了TatA和TatC外，TatB对于有氧生长不受损害也是严格必需的。此外，还发现TatB对于将一种依赖Tat的异源模型蛋白分泌到谷氨酸棒杆菌培养上清液中也是必不可少的。连同我们发现谷氨酸棒杆菌TatB在大肠杆菌ΔtatB突变株中的表达导致形成活性Tat转运酶，我们的结果清楚地表明，TatABC转运酶是谷氨酸棒杆菌中依赖Tat的蛋白质转运的生理相关功能单位，并且很可能在其他含TatB的放线菌中也是如此。
