Immunophilins: Residents of the thylakoid lumen.

摘要

Photosynthetic light reactions rely on the proper function of large protein complexes that reside in the thylakoid membrane. Although their composition, structure, and function are known, the repertoire of assembly and maintenance factors is still being determined. Here we show that two immunophilins, an FKBP-type and a cyclophilin-type play a crucial role in the assembly of photosystem II (PSII) and Cytochrome b6f (Cyt b6f).;FKBP20-2 belongs to the FK-506 binding protein subfamily and, functions as peptidyl-prolyl isomerase (PPIase). FKBP20-2 has a unique pair of cysteines at the C terminus and was found to be reduced by thioredoxin (Trx). 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 and oxygen evolution. 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.;CYP26-2 belongs to the cyclophilin-type subfamily of immunophilins and is located in the thylakoid lumen of the chloroplasts. Silencing CYP26-2 ( cyp26-2i) in Arabidopsis thaliana caused stunned growth and an intense accumulation of anthocyanins in the leaf veins. Chlorophyll fluorescence analysis revealed that the electron transport rates and the redox state of the plastoquinone pool were unaffected but the plants capacity to dissipate energy or non-photochemical energy was significantly lower in cyp26-2i plants. Analysis of isolated thylakoid membranes with immunoblots showed that accumulation of the Cyt b 6f polypeptide subunits: cytochrome f, cytochrome b6, Rieske and plastocyanin were all reduced in the absence of CYP26-2. Analysis of hemagglutinin tagged CYP26-2 plants corroborated the possible interaction between CYP26-2 and Cyt b6 f. The specific nature of these interactions have yet to be determined.