Abstract
In higher plants and algae, the transduction of captured light energy is highly regulated as excess excitation of photosystem II (PSII) reaction centers can be redirected to photosystem I (PSI) reaction centers. Models that attempt to explain this phenomenon involve light-harvesting chlorophyll-protein complexes (LHCII) that capture light energy and migrate between PSII and PSI. This report shows that in pea chloroplasts, the major protein component of LHCII, light-harvesting chlorophyll-binding protein (LHCP), can indeed migrate within the thylakoid membrane. We show, however, that although newly imported LHCP inserts into both stacked and unstacked thylakoid membranes, it then moves only from the unstacked, PSI-rich membranes to the stacked, PSII-rich membranes. The observed migration is not affected by light treatment that induces a redistribution of captured light energy (state I-state II transition) that previously was thought to induce LHCP to migrate in the opposite direction, from stacked to unstacked membranes. A mutation that removes the site of LHCP phosphorylation, the proposed trigger of state transitions, also has no effect on the integration and movement of LHCP, but does render LHCP more susceptible to proteolytic degradation. These results are not consistent with current models that deal with the short-term change in the distribution of light energy.
Highlights
In higher plants and algae, the transduction of captured light energy is highly regulated as excess excitation of photosystem II (PSII) reaction centers can be redirected to photosystem I (PSI) reaction centers
The observed migration is not affected by light treatment that induces a redistribution of captured light energy that previously was thought to induce lightharvesting chlorophyll-binding protein (LHCP) to migrate in the opposite direction, from stacked to unstacked membranes
Polypeptide of LHCII, the light-harvesting chlorophyll-binding protein (LHCP) [3]. Both PSI1 and LHCII are enriched in the stacked array of thylakoids, and the phosphorylation of LHCP would induce a migration of LHCP to PSI, which resides in the unstacked thylakoids [2]
Summary
The in vitro expression and import of pea pLHCP into pea chloroplasts and the analysis of incorporated proteins by gel electrophoresis were as previously described (8-lo), except that all incubations were in 1.5-ml Sardstedt tubes. Import reactions contained chloroplasts at a final concentration of 1 mg/ml chlorophyll and were agitated every few minutes. Migration assays contained chloroplasts in import buffer with a chloroplast concentration of 1 mg/ml; these assays were carried out in 1.5-ml tubes immersed in 4 liters of water; and the temperature was maintained at 25 + 3 “C. A triple-branched light guide delivered both red modulated (100 Hz, 650 nm, 11 prnol photons mm s-‘) and far-red nonmodulated -6 pmol photons m-’ s-‘) light to the sample and directed part of the emitted fluorescence (>700 nm) to the photodiode. Inc., 15530480) providing red light with an intensity of 40 pmol photons rn-? Inc., 15530480) providing red light with an intensity of 40 pmol photons rn-? s-l
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