Abstract
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 thylakoid 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, p22121 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 similar features. Structural changes after urea and salt washing of the crystals supported the biochemical characterization and were mainly assigned to the stromal side of the complex where the psaC, psaD, and psaE gene products are known to be bound. Labeling with ferredoxin-colloidal gold complexes provided direct evidence for a segregated PSI population, 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 understanding of the kinetics of electron transfer between photosystem II and PSI in higher plants.
Highlights
The thylakoid membranes of higher plants house the main constituents for the harvesting of solar energy and the lightdriven electron transport of photosynthesis
We show that photosystem I (PSI) complexes can be assembled into ordered molecular monolayers using thylakoid membranes from a variety of higher plant sources
It is accepted that both the PSI and PSII populations are heterogeneous, with two subtypes identified, PSIa and PSIIa and PSIb and PSIIb. These sub-types are differentiated by the amounts of the respective light harvesting antennae (LHCI and LHCII) complexes associated with each photosystem core [13]
Summary
Spinach (Spinacia oleracea) PSII-enriched grana membranes (BBYs) [16] were prepared as described previously [17]. Resuspended membranes (2 mg ml Chl) were incubated with Triton X-100 at a detergent/chlorophyll ratio of 5:1 (w/w), in the dark, at (20 °C) for 20 min. The sample was centrifuged (4 °C) at 13,000 3 g for 5 min, and the pellet was resuspended in cold Buffer A (4 °C). A buffer containing 62 mM Tris, 10% glycerol (v/v), 2% SDS (w/v), and 0.05% bromphenol blue (w/v) was added to the solubilized protein at a Chl: SDS ratio of 1:5 (w/w) and immediately analyzed by gel electrophoresis at 4 °C (2 h at 60 V). The ferredoxin-gold conjugate at 4 3 1014 particles/ml was incubated with PSI-containing grana membranes at 50 mg/ml Chl for 20 min on ice prior to specimen preparation and examination by EM as described below. For each data set (control, Tris-washed, and urea-washed), amplitudes and phases were individually refined with a resolution cut-off of 2.5 nm, and individual data sets were merged for calculation of the final projection map
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
