Protein Kinases in Metabolic Adaptation of Plants

This project is a continuation of the previous FWF-project "Targets of calcium-dependent protein kinases", and it is focused on the posttranslational modification of the group C Arabidopsis bZIP transcription factor bZIP63 and its dimerization partners by a novel complex. This kinase complex has been identified in our group as the major phosphorylating activity for bZIP63 in plants and a similar complex has also been discovered independently by another group. We will now join biochemical approaches to verify that this new kinase complex does serve as major regulator of the group C bZIP factor and key metabolic enzymes in planta. The impact of these processes on plant development and metabolic adaptation will be analyzed in a systematic manner using different approaches to study the specificity, stoichiometry, and localization of the protein kinase complex in response to different stimuli. We have a close collaboration with partners studying bZIP transcription factors in Tübingen (K. Harter), Würzburg (W. Dröge-Laser), Utrecht (S. Smeekens), and Madrid (J. Vicente-Carbajosa). This will enable us to study the mechanistic consequences of bZIP phosphorylation in planta using mutant and transgenic plant lines.

All organisms need to adjust their metabolism in response to changing environmental conditions to ensure that they balance their energy intake with the demands of growth and reproduction. In plants, an enzyme called SnRK1 plays a crucial role in responses to starvation in two ways: by altering the activities of enzymes involved in metabolism and by regulating the expression of genes. To perform the second job, SnRK1 is thought to control the activity of proteins called transcription factors which alter the expression of genes by binding to DNA but it is not known which ones.SnRK1 has kinase activity, that is, it can alter the activities of other proteins by adding small molecules called phosphates to them. It has been suggested that a group of transcription factors called the bZIP proteins may be regulated by SnRK1. Two bZIP proteins work together to switch on a gene, and the combination of bZIP proteins that interact can influence which genes are switched on. In this project we studied the role of a bZIP protein called bZIP63 during starvation in the plant Arabidopsis. The experiments show that bZIP63 is involved in controlling responses to starvation. Furthermore, its activity is regulated by SnRK1, which adds phosphates to three specific locations on the protein. These phosphates alter the ability of bZIP63 to interact with other bZIP proteins, leading to changes in gene expression during starvation. Plants lacking bZIP63 showed an abnormal response to starvation: When starvation was triggered by keeping the plants in darkness for several days, they remained green while normal (wild-type) plants showed yellowing of the leaves. In contrast, plants containing higher amounts of this bZIP protein showed the opposite effect and became much faster yellow than the wild-type. This effect seems to be caused by altered regulation of metabolism in response to starvation, mediated by SnRK1. The bZIP63se mutant plants could be rescued by the normal bZIP63 protein, but not by another version of the protein that SnRK1 is unable to add phosphates to.We propose a model in which SnRK1 adds phosphates to bZIP63 when the plants are starved. This increases its ability to bind to other bZIP proteins and leads to changes in gene expression. As these bZIP proteins are also found in animals a future challenge remains to find out whether these proteins are also regulated in a similar way.