PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Functional characterization of the evolutionarily divergent fern plastocyanin. José Navarro^*,1, Christian Lowe², Reinout Amons³, Takamitsu Kohzuma⁴, Gerard Canters², Miguel de la Rosa¹, Marcellus Ubbink², Manuel Hervás¹, ¹ Instituto de Bioquímica Vegetal y Fotosíntesis, Sevilla, Sevilla, Spain² Leiden Institute of Chemistry, Leiden, The Netherlands³ Department of Molecular Cell Biology, Leiden, The Netherlands⁴ Department of Chemistry, Mito, Japan

ABSTRACT- Plastocyanin is a soluble copper protein that transfers electrons from cytochrome b₆f to photosystem I, two protein complexes localized in the thylakoid membranes in chloroplasts. The surface electrostatic potential distribution of plastocyanin plays a key role in complex formation with the membrane-bound partners. It is practically identical for plastocyanins from plants and green algae, but is quite different for plastocyanin from ferns. Here we report on a laser flash kinetic analysis of photosystem I reduction by plastocyanin from various eukaryotic and prokaryotic organisms. The reaction of fern plastocyanin with fern photosystem I follows a two-step kinetic model, consisting of complex formation and electron transfer, whereas other plant systems exhibit a mechanism that requires an additional intra-complex rearrangement step. The fern plastocyanin interacts inefficiently with spinach photosystem I, showing no detectable complex formation. This can be explained by assuming that the unusual surface charge distribution of fern plastocyanin impairs the interaction. Fern photosystem I behaves in a similar way as spinach photosystem I in reaction with other plastocyanins. The reactivity of fern plastocyanin towards several soluble c-type cytochromes, including cytochrome f, has been analyzed by flavin-photosensitized laser flash photolysis, demonstrating that the specific surface motifs for the interaction with cytochrome f are conserved in fern plastocyanin.

KEY WORDS: fern, plastocyanin, cytochrome, photosystem I