Evidence for Four Thylakoid-Located Protein Kinases

Reversible protein phosphorylation on serine and threonine residues plays a crucial role for the regulation of numerous cellular functions and signal transduction pathways. Control of this process is achieved by the modulation of the activities and substrate specificities of corresponding protein kinases and protein phosphatases. In thylakoids, a substantial number of proteins can be reversibly phosphorylated [I], and an intrinsic, redox-controlled kinase/phosphatase system is involved to adjust the energy distribution between the photosystems I and II [2]. Transient changes in light intensity and quality lead frequently to an imbalance of the excitation rates of the two photosystems which can result in a reduced photosynthetic efficiency. Plants harboring chlorophyll a/b antenna systems respond to this imbalance by modulating the association and energy transfer between the major light-harvesting antenna (LHCII) and the two photosystems via phosphorylation/dephosphorylation of LHCPs. This „state transition“ process is redox-controlled and activated by changes in the redox state of the thylakoid plastoquinone pool [3]. The exact mechanism of the phosphorylation-related reversible energy distribution of the LHCII antenna between the photosystems is not yet well understood. The signal transduction system appears to include a redox-controlled protein kinase of approx. 65 kDa molecular mass, the cytochrome b6f complex, induced conformational substrate (LHCII) changes (Zer et al., these Proceedings), and an immunophilin of the complex type (TLP40) located in the thylakoid lumen that appears to interact with the protein dephosphorylation signal transduction chain and to link it with protein degradation [4]. Attempts to isolate thylakoid-located kinase(s) and their cDNAs or genes have not been successful so far.