van der Wolk J P, Fekkes P, Boorsma A, Huie J L, Silhavy T J, Driessen A J
Department of Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands.
EMBO J. 1998 Jul 1;17(13):3631-9. doi: 10.1093/emboj/17.13.3631.
In Escherichia coli, precursor proteins are translocated across the cytoplasmic membrane by translocase. This multisubunit enzyme consists of a preprotein-binding and ATPase domain, SecA, and the SecYEG complex as the integral membrane domain. PrlA4 is a mutant of SecY that enables the translocation of preproteins with a defective, or missing, signal sequence. Inner membranes of the prlA4 strain efficiently translocate Delta8proOmpA, a proOmpA derivative with a non-functional signal sequence. Owing to the signal sequence mutation, Delta8proOmpA binds to the translocase with a lowered affinity and the recognition is not restored by the prlA4 SecY. At the ATP-dependent initiation of translocation, the binding affinity of SecA for SecYEG is lowered causing the premature loss of bound preproteins from the translocase. The prlA4 membranes, however, bind SecA with a much higher affinity than the wild-type, and during initiation, the SecA and preprotein remain bound at the translocation site allowing an improved efficiency of translocation. It is concluded that the prlA4 strain prevents the rejection of defective preproteins from the export pathway by stabilizing SecA at the SecYEG complex.
在大肠杆菌中，前体蛋白通过转位酶跨细胞质膜转运。这种多亚基酶由一个前蛋白结合和ATP酶结构域SecA以及作为整合膜结构域的SecYEG复合体组成。PrlA4是SecY的一种突变体，它能使信号序列有缺陷或缺失的前蛋白进行转运。prlA4菌株的内膜能有效地转运Delta8proOmpA，一种信号序列无功能的proOmpA衍生物。由于信号序列突变，Delta8proOmpA与转位酶的结合亲和力降低，且prlA4 SecY不能恢复这种识别。在依赖ATP的转运起始阶段，SecA与SecYEG的结合亲和力降低，导致转位酶上结合的前蛋白过早丢失。然而，prlA4膜与SecA的结合亲和力比野生型高得多，并且在起始阶段，SecA和前蛋白仍结合在转位位点，从而提高了转运效率。得出的结论是，prlA4菌株通过在SecYEG复合体处稳定SecA来防止有缺陷的前蛋白从输出途径中被排斥。
