Lima Amparo, Lima Santiago, Wong Joshua H, Phillips Robert S, Buchanan Bob B, Luan Sheng
Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720, USA.
Proc Natl Acad Sci U S A. 2006 Aug 15;103(33):12631-6. doi: 10.1073/pnas.0605452103. Epub 2006 Aug 7.
Photosystem II (PSII) catalyzes the first of two photosynthetic reactions that convert sunlight into chemical energy. Native PSII is a supercomplex consisting of core and light-harvesting chlorophyll proteins. Although the structure of PSII has been resolved by x-ray crystallography, the mechanism underlying its assembly is poorly understood. Here, we report that an immunophilin of the chloroplast thylakoid lumen is required for accumulation of the PSII supercomplex in Arabidopsis thaliana. The immunophilin, FKBP20-2, belongs to the FK-506 binding protein (FKBP) subfamily that functions as peptidyl-prolyl isomerases (PPIases) in protein folding. FKBP20-2 has a unique pair of cysteines at the C terminus and was found to be reduced by thioredoxin (Trx) (itself reduced by NADPH by means of NADP-Trx reductase). The FKBP20-2 protein, which contains only two of the five amino acids required for catalysis, showed a low level of PPIase activity that was unaffected on reduction by Trx. Genetic disruption of the FKBP20-2 gene resulted in reduced plant growth, consistent with the observed lower rate of PSII activity determined by fluorescence (using leaves) and oxygen evolution (using isolated chloroplasts). Analysis of isolated thylakoid membranes with blue native gels and immunoblots showed that accumulation of the PSII supercomplex was compromised in mutant plants, whereas the levels of monomer and dimer building blocks were elevated compared with WT. The results provide evidence that FKBP20-2 participates specifically in the accumulation of the PSII supercomplex in the chloroplast thylakoid lumen by means of a mechanism that has yet to be determined.
光系统II(PSII)催化将阳光转化为化学能的两个光合作用反应中的第一步。天然PSII是一种由核心叶绿素蛋白和捕光叶绿素蛋白组成的超复合体。尽管PSII的结构已通过X射线晶体学解析,但对其组装的潜在机制了解甚少。在这里,我们报道拟南芥叶绿体类囊体腔中的一种亲环蛋白是PSII超复合体积累所必需的。该亲环蛋白FKBP20 - 2属于FK - 506结合蛋白(FKBP)亚家族,在蛋白质折叠中作为肽基脯氨酰异构酶(PPIase)发挥作用。FKBP20 - 2在C末端有一对独特的半胱氨酸,并且被发现可被硫氧还蛋白(Trx)还原(硫氧还蛋白本身通过NADP - Trx还原酶被NADPH还原)。FKBP20 - 2蛋白仅包含催化所需的五个氨基酸中的两个,显示出低水平的PPIase活性,该活性不受Trx还原的影响。FKBP20 - 2基因的遗传破坏导致植物生长减缓,这与通过荧光(使用叶片)和放氧(使用分离的叶绿体)测定的PSII活性较低的观察结果一致。用蓝色天然凝胶和免疫印迹分析分离的类囊体膜表明,突变植物中PSII超复合体的积累受到损害,而与野生型相比,单体和二聚体构建模块的水平升高。结果提供了证据,表明FKBP20 - 2通过一种尚未确定的机制特异性地参与叶绿体类囊体腔中PSII超复合体的积累。