Analysis of the assembly apparatus of photosystem I reaction center complex

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Chloroplast-encoded Ycf4 is a thylakoidal protein and is essential for the assembly of photosystem I (PSI) reaction center complex. This protein is a part of a large complex that is an assembly apparatus of PSI complex. In this project, the Ycf4-complex was highly purified by two steps affinity chromatography from the thylakoid extracts of the chloroplast transformant of Chlamydomonas reinhardtii in which Ycf4 fused with TAP-tag is expressed. The resultant Ycf4-complex was found to contain Cop2, the retinal binding protein, and MASAP (mitochondrial ATP synthase associated protein) as well as PsaF, one of the PSI subunits and Ycf4. It was found from separate biochemical experiments that all Cop2 present in the thylakoids is co-purified with Ycf4, indicating that this retinal binding protein is an authentic component of the Ycf4-complex. The functional role of Cop2 is not yet known. Absorption and fluorescence emission spectra revealed that the purified Ycf4-complex contains chlorophyll and carotenoid. These pigments may be supplied to an assembly intermediate of PSI complex.