Isolation of wheat FK506-binding proteins (FKBPs): exploring roles for FKBPs in the wheat thylakoid

The FK506-binding proteins (FKBPs) belong to the peptidyl-prolyl cis/trans isomerase (PPIase) class of enzymes that catalyse the rotation of the proline peptide bond, while they also operate as molecular chaperones for specific partner proteins. The FKBPs are ubiquitous to all organisms studied and they vary in protein size, structure and complexity. FKBPs perform a variety of cellular functions, from protein folding and trafficking to signal transduction and stress response. The higher plant genome encodes around twenty FKBPs and half of these are targeted to the chloroplast thylakoid lumen (TL), comprising the largest group of proteins in the lumen, although a role for the TL FKBPs remains largely undefined. A cleavable N-terminal signal peptide serves as the chaperone domain for one lumenal FKBP in Arabidopsis, which interacts with a subunit of the thylakoid electron transfer complex, while several other TL FKBPs have also been linked with assembly and stability of the photosynthetic apparatus. Despite their potential for photosynthesis regulation, the cereal TL FKBPs remain unexplored. We present the first characterisation of cereal thylakoid FKBPs with the isolation of the genes encoding FKBP13, FKBP16-1 and FKBP16-3 in Triticum aestivum. The translated coding sequences of these genes share 50-60% identity with their Arabidopsis orthologues, including conservation of cleavable target peptides that indicate thylakoid translocation and provide potential chaperone domains. The features that confer PPIase activity and redox sensitivity to the Arabidopsis lumenal FKBPs are also well conserved in the wheat orthologues. Isolation and analysis of promoter regions of the wheat FKBP genes has revealed elements that indicate light-regulated, stress-regulated and endosperm-specific expression of these genes. We propose that the wheat FKBP13, FKBP16-1 and FKBP16-3 operate as molecular chaperones in light- and redox-regulated assembly of the photosynthetic apparatus