The Use of Contact Mode Atomic Force Microscopy in Aqueous Medium for Structural Analysis of Spinach Photosynthetic Complexes

To investigate the dynamics of photosynthetic pigment-protein complexes in vascular plants at high resolution in an aqueous environment, membrane-protruding oxygen-evolving complexes (OECs) associated with photosystem II (PSII) on spinach (Spinacia oleracea) grana membranes were examined using contact mode atomic force microscopy. This study represents, to our knowledge, the first use of atomic force microscopy to distinguish the putative large extrinsic loop of Photosystem II CP47 reaction center protein (CP47) from the putative oxygen-evolving enhancer proteins 1, 2, and 3 (PsbO, PsbP, and PsbQ) and large extrinsic loop of Photosystem II CP43 reaction center protein (CP43) in the PSII-OEC extrinsic domains of grana membranes under conditions resulting in the disordered arrangement of PSII-OEC particles. Moreover, we observed uncharacterized membrane particles that, based on their physical characteristics and electrophoretic analysis of the polypeptides associated with the grana samples, are hypothesized to be a domain of photosystem I that protrudes from the stromal face of single thylakoid bilayers. Our results are interpreted in the context of the results of others that were obtained using cryo-electron microscopy (and single particle analysis), negative staining and freeze-fracture electron microscopy, as well as previous atomic force microscopy studies.