Wang Hongyun, Na Bing, Yang Hsiuchin, Tai Phang C
Department of Biology, Georgia State University, Atlanta, GA 30303, USA.
J Bacteriol. 2008 Feb;190(4):1413-8. doi: 10.1128/JB.01633-07. Epub 2007 Dec 7.
SecA is an essential component in the Sec-dependent protein translocation pathway and, together with ATP, provides the driving force for the transport of secretory proteins across the cytoplasmic membrane of Escherichia coli. Previous studies established that SecA undergoes monomer-dimer equilibrium in solution. However, the oligomeric state of functional SecA during the protein translocation process is controversial. In this study, we provide additional evidence that SecA functions as a dimer in the membrane by (i) demonstration of the capability of the presumably monomeric SecA derivative to be cross-linked as dimers in vitro and in vivo, (ii) complementation of the growth of a secA(Ts) mutant with another nonfunctional SecA or (iii) in vivo complementation and in vitro function of a genetically tandem SecA dimer that does not dissociate into monomers, and (iv) formation of similar ring-like structures by the tandem SecA dimer and SecA in the presence of lipid bilayers. We conclude that SecA functions as a dimer in the membrane and dissociation into monomers is not necessary during protein translocation.
SecA是依赖Sec的蛋白质转运途径中的一个必需成分，它与ATP一起为分泌蛋白穿过大肠杆菌细胞质膜的运输提供驱动力。先前的研究表明，SecA在溶液中存在单体-二聚体平衡。然而，在蛋白质转运过程中功能性SecA的寡聚状态存在争议。在本研究中，我们提供了额外的证据，证明SecA在膜中作为二聚体发挥作用，具体如下：（i）证明推测的单体SecA衍生物在体外和体内能够交联形成二聚体；（ii）用另一种无功能的SecA互补secA（Ts）突变体的生长；（iii）不分解成单体的基因串联SecA二聚体的体内互补和体外功能；以及（iv）在脂质双层存在的情况下，串联SecA二聚体和SecA形成类似的环状结构。我们得出结论，SecA在膜中作为二聚体发挥作用，在蛋白质转运过程中分解成单体并非必要。
