Red light makes PSII run faster than PSI, but within minutes phosphate is added to a fraction of the LHCII molecules attached to PSII, and the transition to state 2, associated with the migration of modified LHCII to PSI, ensues. "We previously identified the enzyme that attaches phosphate to LHCII as STN7", says Leister, "and showed that STN7 is activated when the carriers that relay electrons to PSI are overloaded." When the modified LHCII proteins bind to PSI, they permit it to utilize more light and accept electrons from PSII, relieving carrier overload and balancing the activities of the two photosystems.
The reverse transition (2-to-1) requires the removal of phosphate from LHCII. In their latest publication, the researchers report how they found the phosphatase enzyme that performs this task. "First we individually inactivated the genes for the nine phosphatases known to reside in the chloroplast, but none of the mutations affected state transitions", explains Leister. However, the team then hit upon another phosphatase, At4g27800, among chloroplast proteins that had been identified by mass spectroscopy.
It proved to be an inspired choice. "We confirmed that this protein, which we renamed TAP38, is associated with thylakoids, and we identified mutant strains that lacked it. These mutants remain locked in state 2, irrespective of lighting conditions, as one would expect if TAP38 is required for removal of the phosphate." And indeed, addition of purified TAP38 to the modified LHCII was found to lead directly to loss of the phosphate group.
|Contact: Prof. Dr. Dario Leister|