2-DIMENSIONAL CRYSTALS OF PHOTOSYSTEM-I IN HIGHER-PLANT GRANA MARGINS

In this report, we present new structural data on the size, shape, and oligomeric form of higher plant photosystem I (PSI) formed within the thylakoid grana margins. We show that PSI complexes can be assembled into ordered molecular monolayers (two-dimensional crystals) using thy lakoid membranes from a variety of higher plant sources. Digital image analysis of negatively stained two-dimensional crystals (a = 26.9 nm, b = 28.0 nm, gamma = 90 degrees, p22(1)2(1) plane group) resulted in a projection map consisting of 4 monomers/unit cell. Higher plant PSI is slightly larger than its cyanobacterial equivalent but shows many s imilar features. Structural changes after urea and salt washing of the crystals supported the biochemical characterization and were mainly a ssigned to the stromal side of the complex where the psaC, psaD, and p saE gene products are known to be bound, Labeling with ferredoxin coll oidal gold complexes provided direct evidence for a segregated PSI pop ulation, with 5 nm diameter ferredoxin-gold particles enriched in the thylakoid grana margins and the two-dimensional crystals. This lateral segregation of photosynthetic complexes is important for the understa nding of the kinetics of electron transfer between photosystem II and PSI in higher plants.