Tracking the function of the cytochrome c6-like protein in higher plants

The contention that plastocyanin is the only mobile electron donor to photosystem I in higher plants was recently shaken by the discovery of a cytochrome c6-like protein in Arabidopsis and other flowering plants. However, the genetic and biochemical data presented in support of the idea that the cytochrome c6 homologue can replace plastocyanin have now been challenged by two complementary studies. This re-opens the debate on the real function(s) of cytochrome c in the chloroplasts of higher plants. During the past few years, the genetic analysis of photosynthesis has identified novel photosynthetic polypeptides, as well as unexpected protein–function relationships [1] and [2]. In 2002, a major surprise was reported by two groups – the identification of a cytochrome c6 (cyt c6)-like protein in higher plants [3] and [4]. Before this, it was generally accepted that this protein had been lost during the evolution of angiosperms, and only algae and cyanobacteria were thought to use either plastocyanin or cyt c6 as electron donors to photosystem I (PSI) [5]. Using database analyses, Christopher Howe's group identified cyt c6-like sequences in the genomes and transcriptomes of several higher-plant species [4]. In parallel, Sheng Luan's group discovered the Arabidopsis cyt c6 homologue in a screen for proteins that interacted with chloroplast immunophilins [3]. From biochemical and genetic analyses, Sheng Luan and co-workers concluded that the cyt c6-like protein is targeted to the thylakoid lumen where it can replace plastocyanin in reducing PSI [3]. Two more-recent studies [6] and [7] now challenge the contention that the higher-plant cyt c6 homologue donates electrons to PSI and is capable of functionally replacing plastocyanin. In turn, the new analyses raise the question of what function(s) the cyt c6-like protein serves in higher plants.