Formation and Characterization of Two-Dimensional Crystals of Photosystem II

Abstract

Photosystem II (PS II), a eukaryotic photosynthetic reaction center which converts solar energy to chemical energy, is also capable of evolving oxygen, making it uniquely important for our biosphere. We report the formation of two-dimensional crystals of the PS II complex. The crystals were tubular, approximately 0.2 by 1-2 μm. Characterization of the crystals by gel electrophoresis, immunoblotting, and absorption spectroscopy suggested that the crystals contain PS II exclusively, with no other protein complexes. Atomic force microscopy revealed that the complexes were closely packed, suggesting that the process of crystallization involves partial removal of lipid from the membrane. The structure of the complex was investigated using low-dose electron microscopy and image analysis. A projection map at 1.7 nm resolution was produced. The unit cell was 11.5 × 16.1 nm and consisted of two monomeric units arranged around a central cavity to form a dimer. Volume calculations suggested that each dimer consisted of two PS II complexes. The monomeric unit, which appears to be a single PS II complex, had four areas of density. The gap between the two PS II complexes was quite small, indicating that there may be a functional connection between the two halves of the dimer.