Exploring the roles of FK506-binding proteins in chloroplast function and plant development

The FK506-binding proteins (FKBPs) are a class of peptidyl-prolyl cis/trans isomerase (PPIase) enzymes found in all organisms, comprising large protein families that are localised throughout the cell. FKBPs possess a well conserved FK506-binding domain that forms the active site for PPIase activity, although many isoforms are PPIasedeficient. The FKBP families of higher plants include over twenty members with established roles as molecular chaperones in hormone trafficking and stress response. The chloroplast thylakoid contains half of the plant FKBP population, which have been tentatively linked to regulating photosynthetic membrane function. The aim of this study was to characterise the FKBP families occurring in wheat and other cereals, with particular focus on elucidating their functions in the thylakoid. Identification of the FKBP multigenes in the rice genome uncovered the largest FKBP family so far reported. Bioinformatics characterisation of their proteins revealed unique isoforms in rice and showed that gene duplications have driven expansion of the plant FKBP families. Ten isoforms predicted in the thylakoid of rice, wheat and other cereals were highly conserved with their counterparts in other plants, suggesting specialised and conserved roles for this subfamily. Of these, FKBP13, FKBP16-1 and FKBP16-3 were isolated from the wheat genome and found to be expressed in leaf tissue. Transcription regulation analyses identified a novel promoter element that linked FKBP16-1 with chloroplast biogenesis, while FKBP16-3 expression indicated a role in carbohydrate synthesis. In vitro PPIase assays demonstrated an absence of enzyme activity in both FKBP16-1 and FKBP16-3, suggesting alternative roles in the thylakoid for these isoforms, while FKBP13 was shown to be the first active PPIase reported in a cereal chloroplast. FKBP13, FKBP16-1 and FKBP16-3 were found to interact with specific chloroplast proteins in yeast two-hybrid analysis, indicating roles as specialised chaperones in the thylakoid that may serve to regulate photosynthetic acclimation processes. Mechanisms linking these FKBPs with photosynthetic state transitions and cyclic electron flow around photosystem one are described here, and a hypothesis linking FKBP function to protein phosphorylation is suggested.