Takahashi S, Seki Y, Uchida A, Nakayama K, Satoh H
Department of Biomolecular Science, Toho University, Funabashi, Chiba, Japan.
Plant Biol (Stuttg). 2015 May;17(3):632-8. doi: 10.1111/plb.12274. Epub 2015 Jan 8.
Non-photosynthetic and hydrophilic chlorophyll (Chl) proteins, called water-soluble Chl-binding proteins (WSCPs), are distributed in various species of Chenopodiaceae, Amaranthaceae, Polygonaceae and Brassicaceae. Based on their photoconvertibility, WSCPs are categorised into two classes: Class I (photoconvertible) and Class II (non-photoconvertible). Chenopodium album WSCP (CaWSCP; Class I) is able to convert the chlorin skeleton of Chl a into a bacteriochlorin-like skeleton under light in the presence of molecular oxygen. Potassium iodide (KI) is a strong inhibitor of the photoconversion. Because KI attacks tyrosine residues in proteins, tyrosine residues in CaWSCP are considered to be important amino acid residues for the photoconversion. Recently, we identified the gene encoding CaWSCP and found that the mature region of CaWSCP contained four tyrosine residues: Tyr13, Tyr14, Tyr87 and Tyr134. To gain insight into the effect of the tyrosine residues on the photoconversion, we constructed 15 mutant proteins (Y13A, Y14A, Y87A, Y134A, Y13-14A, Y13-87A, Y13-134A, Y14-87A, Y14-134A, Y87-134A, Y13-14-87A, Y13-14-134A, Y13-87-134A, Y14-87-134A and Y13-14-87-134A) using site-directed mutagenesis. Amazingly, all the mutant proteins retained not only chlorophyll-binding activity, but also photoconvertibility. Furthermore, we found that KI strongly inhibited the photoconversion of Y13-14-87-134A. These findings indicated that the four tyrosine residues are not essential for the photoconversion.
非光合亲水性叶绿素（Chl）蛋白，即水溶性Chl结合蛋白（WSCPs），分布于藜科、苋科、蓼科和十字花科的多种植物中。根据其光转换特性，WSCPs可分为两类：I类（可光转换）和II类（不可光转换）。藜（Chenopodium album）WSCP（CaWSCP；I类）在分子氧存在下，光照时能够将Chl a的二氢卟吩骨架转化为类细菌叶绿素骨架。碘化钾（KI）是这种光转换的强抑制剂。由于KI会攻击蛋白质中的酪氨酸残基，因此CaWSCP中的酪氨酸残基被认为是光转换的重要氨基酸残基。最近，我们鉴定了编码CaWSCP的基因，发现CaWSCP的成熟区域含有四个酪氨酸残基：Tyr13、Tyr14、Tyr87和Tyr134。为了深入了解酪氨酸残基对光转换的影响，我们利用定点诱变构建了15种突变蛋白（Y13A、Y14A、Y87A、Y134A、Y13 - 14A、Y13 - 87A、Y13 - 134A、Y14 - 87A、Y14 - 134A、Y87 - 134A、Y13 - 14 - 87A、Y13 - 14 - 134A、Y13 - 87 - 134A、Y14 - 87 - 134A和Y13 - 14 - 87 - 134A）。令人惊讶的是，所有突变蛋白不仅保留了叶绿素结合活性，还保留了光转换能力。此外，我们发现KI强烈抑制Y13 - 14 - 87 - 134A的光转换。这些发现表明，这四个酪氨酸残基对光转换并非必不可少。
