A Red-shifted Antenna Protein Associated with Photosystem II in Physcomitrella patens

Antenna systems of plants and green algae are made up of pigment-proteins belonging to the light-harvesting complex (LHC) multigene family. LHCs increase the light harvesting cross section of photosystem I and II and catalyze photoprotective reactions which prevent light induced damages in oxygenic environment. The genome of the moss Physcomitrella patens contains two genes encoding LHCb9, a new antenna protein which bears an overall sequence similarity to Photosystem II antenna proteins but carries a specific motif typical of Photosystem I antenna proteins. This consists in the presence of an asparagine residue as a ligand for Chl 603 (A5) chromophore rather than a histidine, the common ligand in all other LHCbs. Asparagine as Chl 603 (A5) ligand generate red-shifted spectral forms associated to Photosystem I rather than to Photosystem II, suggesting that in Physcomitrella patens the energy landscape of Photosystem II might be different with respect to that of most green algae and plants. In this work we show that the in vitro refolded LHCb9-pigment complexes carry a red-shifted fluorescence emission peak, different from all other known Photosystem II antenna proteins. By using a specific antibody, we localized LHCb9 within PSII supercomplexes in the thylakoid membranes. This is the first report of red-shifted spectral forms in a PSII antenna system, suggesting that this biophysical feature might have a special role either in optimization of light use efficiency or in photoprotection in the specific environmental conditions experienced by this moss.