Protein phosphorylation and proteolysis - regulation and adaptive responses in photosynthesis

Just as all other organisms, plants adapt to various environmental conditions. This work focuses on the role of chloroplast light harvesting complex II, LHCII, in adaptation of photosynthesis to changed light conditions.

The two photosystems of oxygenic photosynthesis, photosystem I and II, use light of slightly different wavelengths for photochemistry. Since the photosystems are coupled in series, a light favouring one photosystem over the other leads to unbalanced electron transport. This work describes how plants adapt to such unbalanced light by "state transitions". State transitions involve changing the distribution of the light harvesting antenna complex, LHCII, between the two photosystems. Using antibodies directed towards phosphotyrosine and phosphothreonine, we found that phosphorylation of LHCII on both tyrosine and threonine residues were inhibited by the tyrosine kinase inhibitor, genistein, at concentrations not expected to cause any direct effect on threonine kinase activity. Results from spectroscopic methods for monitoring state transitions in combination with genistein suggested that tyrosine kinase activity is required for the state 2 transition.

Phosphorylation of LHCII and expression of genes encoding certain proteins involved in photosynthesis have been shown to be controlled by the redox-state of the plastoquinone pool. It is however not known how the signals emanate from the plastoquinone pool. One hypothesis is that two-component systems are involved. We have identified nuclear genes encoding elements of two-component systems, whose gene products are predicted to be targeted to the chloroplast. Thus genomic evidence suggests that chloroplasts have retained this system for signal transduction. We also describe a phosphoprotein associated with photosystem II which becomes dissociated from it upon phosphorylation. This phosphoprotein might also be involved in signalling events.

Two novel chloroplast proteolytic activities are described. One of the activities is found in the stroma and the other one is associated with the thylakoid membranes. Both proteases responsible are capable of cleaving a recombinant LHCII N-terminal peptide. The stromal protease, which cleaves preferentially on the carboxy-side of glutamic acid, has been partially purified. The protease associated with the thylakoid membrane seems to be under redox-control and may be involved in the degradation of LHCII as plants adapt to high-light conditions.