In the photosynthetic light reactions of plants and cyanobacteria, plastocyanin plays a crucial role as an electron carrier and shuttle protein between two membrane protein complexes: cytochrome
b6f and photosystem I. Using transferred cross-saturation method, we demonstrated that the hydrophobic patch residues of plastocyanin are in close proximity to PSI and cytochrome
b6f, whereas the acidic patch residues of plastocyanin do not form stable salt bridges with either PSI or cyt
b6f, in the electron transfer complexes. The transient characteristics of the interactions on the acidic patch facilitate the rapid association and dissociation of plastocyanin.
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