Chloroplast-targeted J-domain proteins-Expression and role of AtJ8, AtJ11 and AtJ20 in Arabidopsis thaliana
【摘要】：DnaJ type of chaperones is a large family of proteins found in all life kingdoms. Our database searches revealed that Arabidopsis chloroplasts may contain as many as 26 different J-domain proteins and these chloroplast-targeted J-domain proteins can be roughly divided into two distinct groups according to the gene expression profiles from a number of experimental conditions. Both informations from clustering of gene express profiles and asignment of domain compositions showed that the J-domain proteins may have diverse roles both in chloroplast activity and in stress responses, possiblely functioning as chaperones or/and co-chaperones. The expression of one of the chloroplast-targeted J-domain protein AT1G80920 (AtJ8) that harbors only the J-domain, is negatively regulated both at transcript and protein levels by light and signals derived from photosynthesis such as sucrose and glucose. Further studies showed that the AtJ8 protein is absent in light due to transcriptional downregulation, and although present in darkness the protein undergoes rapid turnover. The responsiveness of the expression of AtJ8 to sugars and light/dark treatments suggests that the expression of this protein, is regulated by photosynthesis-derived signals but is also tightly connected to the presence of FTSH proteases. For getting further insight of J-domain proteins in the chloroplast, the role of three small chloroplast-targeted J-domain proteins, AtJ8 (At1g80920), AtJ11 (At4g36040) and AtJ20 (At4g13830), was investigated using knock-out mutants of Arabidopsis thaliana. Photochemical efficiency, capacity of CO2 assimilation, stabilization of Photosystem (PS) II dimers and supercomplexes under high light illumination, energy distribution between PSI and PSII and phosphorylation of PSII-LHCII proteins, global gene expression profiles and oxidative stress responses of these J-domain protein mutants were analyzed. The results obtained strongly indicated that knockout of one of these proteins caused a series of events including a decrease in photosynthetic efficiency, destabilization of PSII complexes and loss of control for balancing the redox reactions in chloroplasts. Data obtained with DNA microarray analysis demonstrated that the lack of one of these J-domain proteins triggers a global stress response and therefore confers the plants greater tolerance to oxidative stress induced by high light or methyl viologen treatments. Expression of a set of genes encoding enzymes that detoxify reactive oxygen species (ROS) as well as a number of stress-related transcription factors behaved in the mutants at growth light similarly to that when WT plants were transferred to high light. Also a set of genes related to redox regulation were upregulated in the mutants. On the other hand, although the three J-domain proteins reside in chloroplasts, the expression of most genes encoding thylakoid membrane proteins was not changed in the mutants. These results suggest that the functions of the three small chloroplast-targeted J-domain proteins AtJ8, AtJ11 and AtJ20 seem to be, at least partially, redundant in Arabidopsis , yet it is evident that also specific functions exist for each protein. Knockout of the AtJ8 protein had less drastic consequences than the knockout of either the AtJ11 or the AtJ20 protein. Because of the small sizes and lacking the client protein-interaction domains, the multiple, yet subtle, effects on photosynthesis performance induced by the lack of any one of the small DnaJ proteins, AtJ8, AtJ11 or AtJ20, makes it hard to distinguish the individual roles of these DnaJ proteins in co-chaperone/chaperone cohort. Nevertheless, in general, these small chloroplast DnaJ proteins participate in optimization of CO2 fixation, in stabilization of PSII complexes and balancing the electron transfer reactions. As a whole, it is conceivable that the tolerance of the J-domain protein knockout plants to oxidative stress results from an unbalance of the redox reactions in chloroplasts, thereby modifying the chloroplast retrograde signaling mechanisms and inducing the up-or down-regulation of stress responsive genes in the nucleus. Further studies with double and triple mutants are expected to provide stronger phenotypes and also to give deeper insights into the functions of the J-domain proteins in chloroplasts.