Optimizing Crystal Size of Photosystem II by Macroseeding: Toward Neutron Protein Crystallography.

Photosystem II (PSII) catalyzes the photo-oxidation of water to molecular oxygen and protons. The water splitting reaction occurs inside the oxygen-evolving complex (OEC) via a Mn4 CaO5 cluster. To elucidate the reaction mechanism, detailed structural information for each intermediate state of the OEC is required. Despite the current high-resolution crystal structure of PSII at 1.85 Å and other efforts to follow the structural changes of the Mn4 CaO5 cluster using X-ray free electron laser (XFEL) crystallography in addition to spectroscopic methods, many details about the reaction mechanism and conformational changes in the catalytic site during water oxidation still remain elusive. In this study, we present a rarely found successful application of the conventional macroseeding method to a large membrane protein like the dimeric PSII core complex (dPSIIcc). Combining microseeding with macroseeding crystallization techniques allowed us to reproducibly grow large dPSIIcc crystals with a size of ~3 mm. These large crystals will help improve the data collected from spectroscopic methods like polarized extended X-ray absorption fine structure (EXAFS) and single crystal electron paramagnetic resonance (EPR) techniques and are a prerequisite for determining a three-dimensional structure using neutron diffraction.