Three-dimensional electron microscopy of the photosystem II core complex

A three-dimensional image of the spinach photosystem II core complex compos ed of CP47, D1, D2, cytochrome b-559, and psbI gene product was reconstruct ed at 20-Angstrom resolution from the two-dimensional crystals negatively s tained with phosphotungstate. Confirming the previous proposal, the crystal had a p22(1)2(1) symmetry. One PSII core complex was measured to be 80 x 8 0 Angstrom in the membrane plane and 88 Angstrom normal to it. The mass dis tribution was asymmetric about the lipid bilayer, consistent with predictio ns from the amino acid sequences. The lumenal mass consisted of three domai ns forming a characteristic triangular platform with another domain on top of it. Three stromal domains were smaller and linearly arranged. Due to str ong stain exclusion in the hydrophobic core part of the lipid bilayer, the transmembrane region appeared to be imaged with a reversed contrast. Invert ing the contrast resulted in a reasonable density distribution for that par t. Thus, though the information on the transmembrane region is limited, the domain structure of the PSII core complex was revealed and allowed us to p ropose a model for the arrangement of subunits in the PSII core complex. (C ) 1998 Academic Press.