Pogliano J A, Beckwith J
Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02115.
EMBO J. 1994 Feb 1;13(3):554-61. doi: 10.1002/j.1460-2075.1994.tb06293.x.
We show here that the rate of protein translocation in the bacterium Escherichia coli depends on the levels of the SecD and SecF proteins in the cell. Overexpression of SecD and SecF stimulates translocation in wild type cells and improves export of proteins with mutant signal sequences. Depletion of SecD and SecF from the cell greatly reduces but does not abolish protein translocation. A secDF::kan null mutant deleted for the genes encoding both proteins is cold-sensitive for growth and protein export, has a severe export defect at 37 degrees C and is barely viable. The phenotypes of a secD null mutant and a secF null mutant are identical to the secDF::kan double null mutant. These results partially resolve the conflict between genetic studies and results from in vitro translocation systems which do not require SecD and SecF for activity, affirm the importance of these proteins to the export process, and suggest that SecD and SecF function together to stimulate protein export in a role fundamentally different from other Sec proteins. Our results provide additional support for the notion that an early step in protein export is cold-sensitive.
我们在此表明，细菌大肠杆菌中的蛋白质转运速率取决于细胞中SecD和SecF蛋白的水平。SecD和SecF的过表达刺激野生型细胞中的转运，并改善具有突变信号序列的蛋白质的输出。从细胞中去除SecD和SecF会大大降低但不会消除蛋白质转运。缺失编码这两种蛋白质的基因的secDF::kan无效突变体对生长和蛋白质输出是冷敏感的，在37摄氏度时有严重的输出缺陷，并且几乎无法存活。secD无效突变体和secF无效突变体的表型与secDF::kan双无效突变体相同。这些结果部分解决了遗传研究与体外转运系统结果之间的冲突，体外转运系统的活性不需要SecD和SecF，证实了这些蛋白质对输出过程的重要性，并表明SecD和SecF共同发挥作用以刺激蛋白质输出，其作用与其他Sec蛋白根本不同。我们的结果为蛋白质输出的早期步骤对寒冷敏感这一观点提供了额外的支持。
