Higgs Penelope I, Larsen Ray A, Postle Kathleen
School of Molecular Biosciences, Washington State University, Pullman, WA 99164-4234, USA.
Mol Microbiol. 2002 Apr;44(1):271-81. doi: 10.1046/j.1365-2958.2002.02880.x.
The TonB-dependent energy transduction system couples cytoplasmic membrane proton motive force to active transport of iron-siderophore complexes across the outer membrane in Gram-negative bacteria. In Escherichia coli, the primary players known in this process to date are: FepA, the TonB-gated transporter for the siderophore enterochelin; TonB, the energy-transducing protein; and two cytoplasmic membrane proteins with less defined roles, ExbB and ExbD. In this study, we report the per cell numbers of TonB, ExbB, ExbD and FepA for cells grown under iron-replete and iron-limited conditions. Under iron-replete conditions, TonB and FepA were present at 335 +/- 78 and 504 +/- 165 copies per cell respectively. ExbB and ExbD, despite being encoded from the same operon, were not equimolar, being present at 2463 +/- 522 and 741 +/- 105 copies respectively. The ratio of these proteins was calculated at one TonB:two ExbD:seven ExbB under all four growth conditions tested. In contrast, the TonB:FepA ratio varied with iron status and according to the method used for iron limitation. Differences in the method of iron limitation also resulted in significant differences in cell size, skewing the per cell copy numbers for all proteins.
在革兰氏阴性菌中，依赖TonB的能量转导系统将细胞质膜质子动力与铁-铁载体复合物跨外膜的主动运输偶联起来。在大肠杆菌中，迄今为止已知参与这一过程的主要蛋白有：FepA，铁载体肠螯合素的TonB门控转运蛋白；TonB，能量转导蛋白；以及两个作用不太明确的细胞质膜蛋白ExbB和ExbD。在本研究中，我们报告了在铁充足和铁限制条件下生长的细胞中TonB、ExbB、ExbD和FepA的细胞数量。在铁充足条件下，TonB和FepA分别以每个细胞335±78和504±165个拷贝的数量存在。ExbB和ExbD尽管由同一个操纵子编码，但并非等摩尔，分别以2463±522和741±105个拷贝的数量存在。在所测试的所有四种生长条件下，这些蛋白的比例为一个TonB:两个ExbD:七个ExbB。相比之下，TonB:FepA的比例随铁状态以及用于铁限制的方法而变化。铁限制方法的差异也导致细胞大小有显著差异，从而使所有蛋白的每个细胞拷贝数产生偏差。
