Mechanism of the action of active oxygen species and proteases in the degradation of photosystem II complexes

当叶绿体受到强光照射时，光系统II(PS II)反应中心结合的D1蛋白被破坏和降解。在蛋白质降解过程中，PS II受体侧产生的活性氧分子和PS II供体侧产生的内源性阳离子自由基参与了蛋白质的降解。研究还表明，PS II(OEC亚基)的外源蛋白在D1蛋白的降解过程中起着S的调节作用。在本研究中，我们分析了氧自由基和阳离子自由基对D1蛋白降解的作用。我们还研究了OEC亚基的二级结构。主要研究结果如下：(1)光损伤的D1蛋白在PSⅡ中容易与周围的多肽发生交联，我们发现这种交联产物不是末端产物，而是被一种基质蛋白酶(S)所消化。该酶(S)对十二烷基硫酸钠具有抗性。发现基质中有一个15 kDa的因子具有该酶活性。(2)在交联物中，D_1蛋白与CP43之间的交联物的产生受OEC33的调控。这些结果表明OEC亚基起着类似分子伴侣的作用。(3)利用傅里叶变换红外光谱分析了OEC24和OEC18的二级结构。结果表明，这些蛋白质具有大量的β-Sheet结构，对热处理较为稳定。

When chloroplasts are illuminated with strong light, the reaction-center binding D1 protein of photosystem II (PS II) Is damaged and degraded. In the protein degradation, active oxygen molecules produced at the acceptor-side of PS II and the endogenous cationic radicals generated at the donor-side of PS II are involved. It was also shown that the extrinsic proteins of PS II (OEC subunits) play s regulatory role in the degradation of the D1 protein. In the present study, we analyzed the action of the oxygen- and cationic radicals on the degradation of the D1 protein. We also studied the secondary structure of the OEC subunits. The results are summarized below.(1) Photodamaged D1 protein easily cross-links with the surrounding polypeptides in PS II. We found that the cross-linked products are not the dead-end products, but are digested by a stromal protease(s). The protease(s) is SDS-resistant. A 15-kDa factor in the stroma was found to have the protease activity.(2) Among the cross-linked products, generation of those between the D1 protein and CP43 were shown to be regulated by the OEC33. These results suggest that the OEC subunit plays a molecular chaperone-like role.(3) The secondary structures of OEC24 and OEC18 were analyzed by Fourier transform infrared spectroscopy. The results obtained show that these proteins have a large amount of β-sheet structure and they are relatively stable to heat treatment.