Effects of high light and elevated temperature on the structure of photosystem II isolated from Arabidopsis thaliana.

Photosystem II (PS II) is a large membrane-bound pigment-protein complex responsible for extracting electrons and protons from water during photosynthesis and releasing molecular oxygen into the atmosphere. While light is required to drive the photochemical reactions, high light and high temperature can be damaging to PS II. To gain insight into potential structural consequences of such conditions, we performed an exploratory study where isolated PS II complexes (C2S2) from Arabidopsis thaliana were exposed to high light and elevated temperature and employed cryo-electron microscopy for structural determination. This resulted in a 'high light' structure (2.74 Å global resolution) which is compared to the published 2.44 Å structure of intact Arabidopsis-PSII vitrified under dim light conditions (control). The high light structure is missing several of the subunits present in the control, including the strongly bound light-harvesting antenna complexes as well as several small transmembrane and extrinsic subunits. Beyond the loss of subunits, the clearest evidence of specific photodamage was found on the acceptor side of the D1 and D2 subunits near the non-heme iron, including the loss of the bicarbonate ligand of the non-heme iron. Finally, we were able to observe that high light/elevated temperature conditions result in the movement of the D1:His252 residue, potentially forming a serine protease triad that may trigger auto-proteolysis of the D1 subunit.