Haase J, Lanka E
Max-Planck-Institut für Molekulare Genetik, Berlin, Germany.
J Bacteriol. 1997 Sep;179(18):5728-35. doi: 10.1128/jb.179.18.5728-5735.1997.
TraF, an essential component of the conjugative transfer apparatus of the broad-host-range plasmid RP4 (IncP), which is located at the periplasmic side of the cytoplasmic membrane, encodes a specific protease. The traF gene products of IncP and Ti plasmids show extensive similarities to prokaryotic and eukaryotic signal peptidases. Mutational analysis of RP4 TraF revealed that the mechanism of the proteolytic cleavage reaction resembles that of signal and LexA-like peptidases. Among the RP4 transfer functions, the product of the Tra2 gene, trbC, was identified as a target for the TraF protease activity. TrbC is homologous to VirB2 of Ti plasmids and thought to encode the RP4 prepilin. The maturation of TrbC involves three processing reactions: (i) the removal of the N-terminal signal peptide by Escherichia coli signal peptidase I (Lep), (ii) a proteolytic cleavage at the C terminus by an as yet unidentified host cell enzyme, and (iii) C-terminal processing by TraF. The third reaction of the maturation process is critical for conjugative transfer, pilus synthesis, and the propagation of the donor-specific bacteriophage PRD1. Thus, cleavage of TrbC by TraF appears to be one of the initial steps in a cascade of processes involved in export of the RP4 pilus subunit and pilus assembly mediated by the RP4 mating pair formation function.
TraF是广宿主范围质粒RP4（IncP）接合转移装置的一个必需组分，位于细胞质膜的周质侧，编码一种特异性蛋白酶。IncP和Ti质粒的traF基因产物与原核和真核信号肽酶有广泛的相似性。对RP4 TraF的突变分析表明，蛋白水解切割反应的机制类似于信号肽酶和LexA样肽酶的机制。在RP4的转移功能中，Tra2基因的产物trbC被确定为TraF蛋白酶活性的作用靶点。TrbC与Ti质粒的VirB2同源，被认为编码RP4前菌毛蛋白。TrbC的成熟涉及三个加工反应：（i）由大肠杆菌信号肽酶I（Lep）去除N端信号肽；（ii）由一种尚未鉴定的宿主细胞酶在C端进行蛋白水解切割；（iii）由TraF进行C端加工。成熟过程的第三个反应对于接合转移、菌毛合成以及供体特异性噬菌体PRD1的繁殖至关重要。因此，TraF对TrbC的切割似乎是由RP4交配型对形成功能介导的RP4菌毛亚基输出和菌毛组装所涉及的一系列过程中的初始步骤之一。
