Interaction of Plastocyanin with Photosystem I: A Chemical Cross-Linking Study of the Polypeptide That Binds Plastocyanin

Plastocyanin has been covalently cross-linked to photosystem I (PSI) by using a water-soluble cross-linker, N-ethyl-3-[3-(dimethylamino)propyl]carbodiimide. The cross-linking reaction is light stimulated and results in the disappearance of a single 19-kDa subunit of PSI with the formation of a new protein-staining component of 31 kDa. The new product at 31 kDa reacts with both plastocyanin and 19-kDa subunit antibodies. Carboxyl group modified plastocyanin does not form a cross-linked product with PSI, implying that the negatively charged surface-exposed groups on plastocyanin are necessary to stabilize binding. These results demonstrate a specific interaction of plastocyanin with PSI and further implicate a specific protein to which plastocyanin binds to facilitate electron transfer to the P700 reaction center.