Wild J, Altman E, Yura T, Gross C A
Department of Bacteriology, University of Wisconsin-Madison 53706.
Genes Dev. 1992 Jul;6(7):1165-72. doi: 10.1101/gad.6.7.1165.
In Escherichia coli secreted proteins must be maintained in an export-competent state before translocation across the cytoplasmic membrane. This function is carried out by a group of proteins called chaperones. SecB is the major chaperone that interacts with precursor proteins before their secretion. We report results indicating that the DnaK and DnaJ heat shock proteins are also involved in the export of several proteins, most likely by acting as their chaperones. Translocation of alkaline phosphatase, a SecB-independent protein, was inhibited in dnaK- and dnaJ- mutant strains, suggesting that export of this protein probably involves DnaK and DnaJ. In addition, DnaK and DnaJ play a critical role in strains lacking SecB. They are required both for viability and for the residual processing of the SecB-dependent proteins LamB and maltose-binding protein (MBP) seen in secB null strains. Furthermore, overproduction of DnaK and DnaJ permits strains lacking SecB to grow in rich medium and accelerates the processing of LamB and MBP. These results suggest that under conditions where SecB becomes limiting, DnaK and DnaJ probably substitute for SecB and facilitate protein export. This provides the cell with a mechanism to overcome a temporary imbalance in the secretion process caused by an abrupt expansion in the pool of precursor proteins.
在大肠杆菌中，分泌蛋白在穿过细胞质膜转运之前必须保持可输出的状态。这一功能由一组称为伴侣蛋白的蛋白质来执行。SecB是主要的伴侣蛋白，在分泌前与前体蛋白相互作用。我们报告的结果表明，DnaK和DnaJ热休克蛋白也参与了几种蛋白质的输出，很可能是作为它们的伴侣蛋白发挥作用。碱性磷酸酶是一种不依赖SecB的蛋白，其转运在dnaK和dnaJ突变菌株中受到抑制，这表明该蛋白的输出可能涉及DnaK和DnaJ。此外，DnaK和DnaJ在缺乏SecB的菌株中起关键作用。它们对于secB缺失菌株的存活以及SecB依赖蛋白LamB和麦芽糖结合蛋白（MBP）的残余加工都是必需的。此外，DnaK和DnaJ的过量表达使缺乏SecB的菌株能够在丰富培养基中生长，并加速LamB和MBP的加工。这些结果表明，在SecB变得有限的条件下，DnaK和DnaJ可能替代SecB并促进蛋白质输出。这为细胞提供了一种机制，以克服由前体蛋白池突然扩大导致的分泌过程中的暂时失衡。
