Peptidyl-prolyl cis-trans Isomerases in the Chloroplast Thylakoid Lumen
Abstract: The Sun is the ultimate energy source on Earth. Photosynthetic organisms are able to catalyze the conversion of solar energy to chemical energy by a reaction called photosynthesis. In plants, this process occurs inside a green organelle called the chloroplast. The protein complexes involved in the photosynthetic light reactions are situated in the thylakoid membrane, which encloses a tiny space called lumen. The Peptidyl-Prolyl cis-trans Isomerase (PPIase) family is the most abundant protein family in the thylakoid lumen. The three PPIase subfamilies, cyclophilins, FKBPs (FK506 binding proteins) and parvulins form a group by their enzymatic activity despite lack of sequence similarity between the subfamilies. Cyclophilins and FKBPs, collectively called immunophilins, were originally discovered as the targets of the immunosuppressive drugs cyclosporine A and FK506, respectively. By suppressing the immune response in humans, these immunophilin-drug complexes revolutionized the field of organ transplantation by preventing graft rejection. Cis-trans isomerization of peptide bonds preceding the amino acid proline is the rate-limiting step of protein folding and several immunophilins have been shown to be important for catalysis of protein folding in vivo. PPIases have been found to be part of large protein complexes as well as in functions such as signalling, protein secretion, RNA processing and cell cycle control. A picture is therefore emerging in which the actual interaction between the PPIase and its target is perhaps more important than the PPIase activity.In the present work, PPIases have been characterized in the chloroplast thylakoid lumen of Spinacia oleracea (spinach) and Arabidopsis thaliana (Arabidopsis). The most active PPIase in the spinach lumen was identified as the cyclophilin TLP20. AtCYP20-2, the Arabidopsis homologue of TLP20, was found to be upregulated at high light and attached to the thylakoid membrane, more precisely to the outer regions of photosystem II supercomplexes. In Arabidopsis, up to 5 cyclophilins and 11 FKBPs were predicted to reside in the lumen. Of these 16 immunophilins, only 2 were identified as active PPIases and significant differences were observed between the two plant species. AtCYP20-2, like TLP20, is an active isomerase although AtFKBP13 is the most active PPIase in the lumen of Arabidopsis. Mutant Arabidopsis plants deficient in AtCYP20-2 displayed no phenothypical changes or decrease in total lumenal PPIase activity. Being the only active PPIase in the mutants, the redox sensitive AtFKBP13 is proposed to compensate for the lack of AtCYP20-2 by oxidative activation. In agreement with the experimental data, the sequence analyses of catalytic domains of lumenal immunophilins demonstrate that only AtCYP20-2 and AtFKBP13 possess the amino acids found essential for PPIase activity in earlier studies of human cyclophilin A and FKBP12. It is concluded that with the exception of AtCYP20-2 and AtFKBP13 most immunophilins in the lumen of Arabidopsis lost their PPIase activity on peptide substrates and developed other specialized functions.
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