Green plant photosystem I binds light-harvesting complex I on one side of the complex

We report a structural characterization by electron microscopy of green pla nt photosystem I solubilized by the mild detergent n-dodecyl-alpha -D-malto side. It is shown by immunoblotting that the isolated complexes contain all photosystem I core proteins and all peripheral light-havesting proteins. T he electron microscopic analysis is based on a large data set of 14 000 neg atively stained single-particle projections and reveals that most of the co mplexes are oval-shaped monomers. The monomers have a tendency to associate into artificial dimers, trimers, and tetramers in which the monomers are o ppositely oriented. Classification of the dimeric complexes suggests that s ome of the monomers lack a part of the peripheral antenna. On the basis of a comparison with projections from trimeric photosystem I complexes from cy anobacteria, we conclude that light-harvesting complex I only binds to the core complex at the side of the photosystem I F/J subunits and does not cau se structural hindrances for the type of trimerization observed in cyanobac terial photosystem I.