Pugsley A P, Francetic O, Possot O M, Sauvonnet N, Hardie K R
Unité de Génétique Moléculaire, CNRS URA 1149, Institut Pasteur, Paris, France.
Gene. 1997 Jun 11;192(1):13-9. doi: 10.1016/s0378-1119(96)00803-7.
The main terminal branch (MTB) of the general secretory pathway is used by a wide variety of Gram- bacteria to transport exoproteins from the periplasm to the outside milieu. Recent work has led to the identification of the function of two of its 14 (or more) components: an enzyme with type-IV prepilin peptidase activity and a chaperone-like protein required for the insertion of another of the MTB components into the outer membrane. Despite these important discoveries, little tangible progress has been made towards identifying MTB components that determine secretion specificity (presumably by binding to cognate exoproteins) or which form the putative channel through which exoproteins are transported across the outer membrane. However, the idea that the single integral outer membrane component of the MTB could line the wall of this channel, and the intriguing possibility that other components of the MTB form a rudimentary type-IV pilus-like structure that might span the periplasm both deserve more careful examination. Although Escherichia coli K-12 does not normally secrete exoproteins, its chromosome contains an apparently complete set of genes coding for MTB components. At least two of these genes code for functional proteins, but the operon in which twelve of the genes are located does not appear to be expressed. We are currently searching for conditions which allow these genes to be expressed with the eventual aim of identifying the protein(s) that E. coli K-12 can secrete.
多种革兰氏阴性菌利用一般分泌途径的主要终末分支（MTB）将外蛋白从周质转运到细胞外环境。最近的研究已鉴定出其14个（或更多）组分中两个组分的功能：一种具有IV型前菌毛肽酶活性的酶和一种伴侣样蛋白，后者是将MTB的另一个组分插入外膜所必需的。尽管有这些重要发现，但在鉴定决定分泌特异性（可能通过与同源外蛋白结合）的MTB组分或形成外蛋白穿过外膜所经假定通道的组分方面，几乎没有取得切实进展。然而，MTB的单一整合外膜组分可能构成该通道壁的观点，以及MTB的其他组分形成可能跨越周质的基本IV型菌毛样结构的有趣可能性，都值得更仔细的研究。虽然大肠杆菌K-12通常不分泌外蛋白，但其染色体含有一套显然完整的编码MTB组分的基因。其中至少两个基因编码功能蛋白，但十二个基因所在的操纵子似乎不表达。我们目前正在寻找能使这些基因表达的条件，最终目的是鉴定大肠杆菌K-12能够分泌的蛋白。
