The physiological roles of immunophilins are unclear, but many possess peptidyl-prolyl isomerase (PPIase) activity, and they have been found in all organisms examined to date, implying that they are involved in fundamental, protein-folding processes. The chloroplast thylakoid lumen of the higher plant Arabidopsis thaliana contains up to 16 immunophilins (five cyclophilins and 11 FKBPs), but only two of them, AtCYP20-2 and AtFKBP13, have been found to be active PPIases, indicating that the other immunophilins in this cellular compartment may have lost their putative PPIase activities. To assess this possibility, we characterized two independent Arabidopsis knockout lines lacking AtCYP20-2 in enzymological and quantitative proteomic analyses. The PPIase activity in thylakoid lumen preparations of both mutants was equal to that of corresponding wild-type preparations, and comparative two-dimensional difference gel electrophoresis analyses of the lumenal proteins of the mutants and wild type showed that none of the potential PPIases was more abundant in the AtCYP20-2 deficient plants. Enzymatic analyses established that all PPIase activity in the mutant thylakoid lumen was attributable to AtFKBP13, and oxidative activation of this enzyme compensated for the lack of AtCYP20-2. Accordingly, sequence analyses of the potential catalytic domains of lumenal cyclophilins and FKBPs demonstrated that only AtCYP20-2 and AtFKBP13 possess all of the amino acid residues found to be essential for PPIase activity in earlier studies of human cyclophilin A and FKBP12. Thus, none of the immunophilins in the chloroplast thylakoid lumen of Arabidopsis except AtCYP20-2 and AtFKBP13 appear to possess prolyl isomerase activity toward peptide substrates.
In higher plants, chloroplasts are the site for the photosynthetic reactions, converting solar energy to chemical energy. Within the chloroplast the thylakoid membrane network encloses the soluble lumen compartment. Until recently the knowledge of the lumen composition and function was limited, but a more profound understanding of the thylakoid lumen content is gradually emerging. The discovery that the thylakoid lumen contains numerous enzymes, besides the already known proteins directly involved or associated with the photosynthetic reactions, have changed the view on this compartment.
The first part of the thesis the lumen proteome maps of Arabidopsis and spinach were resolved. These two proteome maps showed good correlation and the same protein groups were represented in the two proteomes. Thirty eight proteins were identified and in combination with an in silico prediction for the proteome it was estimated that at least 80 different proteins are lumen located.
The second part was to further investigate the functions of two lumen localized proteins, the cyclophilin AtCYP20-2 and peroxiredoxin Q (PrxQ). AtCYP20-2 is suggested to be responsible for the major peptidyl prolyl cis/trans isomerases (PPIase) activity in the lumen. In AtCYP20-2 knockout mutants the total PPIase activity is not altered, instead AtFKBP13 may be oxidatively activated to compensate for the loss of AtCYP20-2. With respect to the PrxQ protein we were able to show that it is a lumenal protein that seems to exist as a soluble pool in the lumen as no interaction with the thylakoid membrane could be detected.
In the last part of this thesis the dynamics of the thylakoid membrane and lumen proteome were studied during oxidative stress. Important changes to the proteome were observed in response to the stress treatment, including abundance changes of proteins related to photosynthesis, as well as proteins not previously identified in relation to stress.
Peroxiredoxins; have been discovered in many organisms ranging from eubacteria to mammals, and their known biological functions include both oxidant defense and signal transduction. The genome of Arabidopsis thaliana encodes for ten individual peroxiredoxins, of which four are located in the chloroplast. The best-characterized member of the chloroplast peroxiredoxins is 2-Cys Prx that is associated with the stroma side of the thylakoid membrane and is considered to participate in antioxidant defense and protection of photosynthesis. This study addressed the chloroplast peroxiredoxin Prx Q and showed that its subcellular location is the lumen of the thylakoid membrane. To get insight in the biological function of the Prx Q protein of Arabidopsis, the protein levels of the Prx Q protein in thylakoid membranes were studied under different light conditions and oxidative stress. A T-DNA knockout mutant of Prx Q did not show any visible phenotype and had normal photosynthetic performance with a slightly increased oxygen evolving activity.
The thylakoid membrane of the chloroplast is the center of oxygenic photosynthesis. To better understand the function of the luminal compartment within the thylakoid network, we have carried out a systematic characterization of the luminal thylakoid proteins from the model organism Arabidopsis thaliana. Our data show that the thylakoid lumen has its own specific proteome, of which 36 proteins were identified. Besides a large group of peptidyl-prolyl cis-trans isomerases and pro. teases, a family of novel PsbP domain proteins was found. An analysis of the luminal signal peptides showed that 19 of 36 luminal precursors were marked by a twin-arginine motif for import via the Tat pathway. To compare the model organism Arabidopsis with another typical higher plant, we investigated the proteome from the thylakoid lumen of spinach and found that the luminal proteins from both plants corresponded well. As a complement to our experimental investigation, we made a theoretical prediction of the luminal proteins from the whole Arabidopsis genome and estimated that the thylakoid lumen of the chloroplast contains similar to80 proteins.