Further characterization of the chloroplast grana margins: the non-detergent preparation of granal Photosystem I cannot reduce ferredoxin in the absence of NADP+ reduction

The chloroplast grana margins of spinach thylakoids were isolated by sonication and aqueous-two-phase partitioning and their electron transport properties examined. Photosystem II and I electron transport activities were measured and compared to the appressed and non-appressed grana core and stroma lamellae, respectively, as well as to whole thylakoids. The results show that the PS II complexes in the grana margins are of the PS II subtype with respect to antenna size, but are QB reducing with respect to the acceptor side properties, while the PS I centers in the grana margins have slightly larger antennae as compared to the PS I centers in the stroma lamellae and are more like the PS I centers located in the grana domain. The ability to reduce ferredoxin and NADP+ was also tested and it was found that the grana margin membrane fraction was unable to reduce ferredoxin, even in the presence of added artificial electron donors. The stroma lamellae and whole thylakoid fractions both reduced ferredoxin at high rates. However, the grana margins could catalyze the reduction of NADP+ when supplied with the necessary components (ferredoxin, ferredoxin:NADP+ oxidoreductase, and an electron source). The results suggest that the PS I populations located in the margins of the grana domain are functionally different from the PS I centers located in the stroma lamellae. The data support a model whereby the PS I centers in the grana are primarily involved in non-cyclic electron transport, while the PS I centers located in the stroma lamellae are capable of participating in both cyclic and non-cyclic electron transport.