Koussevitzky S, Ne'eman E, Sommer A, Steffens J C, Harel E
Department of Plant Sciences, the Hebrew University, Jerusalem 91904, Israel.
J Biol Chem. 1998 Oct 16;273(42):27064-9. doi: 10.1074/jbc.273.42.27064.
Polyphenol oxidases (PPOs) are nuclear-encoded chloroplast proteins that are targeted to the thylakoid lumen by a bipartite presequence. The N-terminal part of this sequence is removed by a stromal processing peptidase (SPP), and the resulting intermediate is translocated across the thylakoid and processed to the mature protein. A 4800-fold-purified SPP processed a PPO precursor (pPPO) at a site identical to that occurring in organelle. The in vitro product of SPP action on pPPO was further processed and translocated by thylakoids. This SPP processed other precursors but was inactive toward those of light-harvesting chlorophyll binding proteins. The enzyme appeared to be a metalloendopeptidase, like previously reported SPPs. However, it differed in substrate specificity, apparent size, and, most significantly, cleavage site of pPPO. Whereas the processing sites of lumen proteins determined so far were relatively distant from the hydrophobic core of the thylakoid targeting domain, pPPO was cleaved immediately before this domain. Cleavage removed the twin arginine motif characteristic of thylakoid targeting domains of lumen proteins, which are translocated by the DeltapH-dependent pathway. The possible significance of these observations to PPO translocation mechanism is discussed. It is suggested that several SPPs may exist in chloroplasts with preferences for different subsets of precursors.
多酚氧化酶（PPOs）是由细胞核编码的叶绿体蛋白，通过一个双元前导序列靶向运输到类囊体腔。该序列的N端部分被一种基质加工肽酶（SPP）切除，产生的中间产物穿过类囊体进行加工成为成熟蛋白。一种经过4800倍纯化的SPP在与细胞器中相同的位点加工PPO前体（pPPO）。SPP作用于pPPO的体外产物进一步被类囊体加工并转运。这种SPP能加工其他前体，但对捕光叶绿素结合蛋白的前体无活性。该酶似乎是一种金属内肽酶，与先前报道的SPP类似。然而，它在底物特异性、表观大小以及最显著的是pPPO的切割位点方面有所不同。到目前为止确定的腔蛋白加工位点相对远离类囊体靶向结构域的疏水核心，而pPPO在该结构域之前紧邻处被切割。切割去除了腔蛋白类囊体靶向结构域特有的双精氨酸基序，这些腔蛋白是通过依赖ΔpH的途径转运的。讨论了这些观察结果对PPO转运机制的可能意义。有人提出叶绿体中可能存在几种SPP，它们对不同的前体亚群有偏好。
