Khatib Karim, Belin Dominique
Department of Pathology, University of Geneva, Switzerland.
Genetics. 2002 Nov;162(3):1031-43. doi: 10.1093/genetics/162.3.1031.
The murine plasminogen activator inhibitor 2 (PAI2) signal sequence inefficiently promotes the export of E. coli alkaline phosphatase (AP). High-level expression of PAI2::AP chimeric proteins from the arabinose P(BAD) promoter is toxic and confers an Ara(S) phenotype. Most Ara(R) suppressors map to secA, as determined by sequencing 21 independent alleles. Mutations occur throughout the gene, including both nucleotide binding domains (NBDI and NBDII) and the putative signal sequence binding domain (SSBD). Using malE and phoA signal sequence mutants, we showed that the vast majority of these secA suppressors exhibit weak Sec phenotypes. Eight of these secA mutations were further characterized in detail. Phenotypically, these eight suppressors can be divided into three groups, each localized to one domain of SecA. Most mutations allow near-normal levels of wild-type preprotein export, but they enhance the secretion defect conferred by signal sequence mutations. Interestingly, one group exerts a selective effect on the export of PAI2::AP when compared to that of AP. In conclusion, this novel class of secA mutations, selected as suppressors of a toxic signal sequence, differs from the classical secA (prlD) mutations, selected as suppressors of defective signal sequences, although both types of mutations affect signal sequence recognition.
小鼠纤溶酶原激活物抑制剂2（PAI2）信号序列促进大肠杆菌碱性磷酸酶（AP）输出的效率较低。从阿拉伯糖P（BAD）启动子高水平表达PAI2::AP嵌合蛋白具有毒性，并赋予Ara（S）表型。通过对21个独立等位基因进行测序确定，大多数Ara（R）抑制子定位于secA。突变发生在整个基因中，包括两个核苷酸结合结构域（NBDI和NBDII）以及假定的信号序列结合结构域（SSBD）。使用malE和phoA信号序列突变体，我们表明这些secA抑制子中的绝大多数表现出较弱的Sec表型。对其中8个secA突变进行了进一步详细表征。从表型上看，这8个抑制子可分为三组，每组定位于SecA的一个结构域。大多数突变允许野生型前体蛋白输出达到接近正常水平，但它们增强了信号序列突变所导致的分泌缺陷。有趣的是，与AP相比，其中一组对PAI2::AP的输出具有选择性作用。总之，这类作为毒性信号序列抑制子而被选择的新型secA突变，不同于作为缺陷信号序列抑制子而被选择的经典secA（prlD）突变，尽管这两种类型的突变都会影响信号序列识别。
