Structural role of photosystem II supercomplexes in thylakoid membrane stacking

Lead Research Organisation: University of Sheffield
Department Name: Molecular Biology and Biotechnology


Life on earth depends on photosynthesis, the source of all of our food, oxygen and most of our energy. The early steps of photosynthesis involve trapping of solar energy by electron transfer reactions in the photosynthetic membrane. Our recent studies have revealed an unexpected role for the membrane protein photosystem II (PSII) supercomplexes in mediating the stacking of chloroplast thylakoid membranes. Membrane stacking instigates the spatial segregation of the slow excitation energy trap PSII from the faster trap photosystem I (PSI), and promotes energy transfer among PSII units both of which are crucial for the efficiency of photosynthesis. For the first time we have biochemically-isolated a unique stacked form of the PSII supercomplex which will allow us to investigate this critical feature of the process. This project will make use of the latest advances in structural and functional microscopies to characterise this PSII supercomplex and understand how and why thylakoid membranes stack in molecular detail.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
BB/M011151/1 01/10/2015 30/09/2023
1945068 Studentship BB/M011151/1 01/10/2017 31/12/2021 Lorna Malone
Description The cytochrome b6 f (cytb6 f ) complex has a central role in oxygenic photosynthesis, linking electron transfer between photosystems I and II and converting solar energy into a transmembrane proton gradient for ATP synthesis1,2,3. Electron transfer within cytb6 f occurs via the quinol (Q) cycle, which catalyses the oxidation of plastoquinol (PQH2) and the reduction of both plastocyanin (PC) and plastoquinone (PQ) at two separate sites via electron bifurcation2. In higher plants, cytb6 f also acts as a redox-sensing hub, pivotal to the regulation of light harvesting and cyclic electron transfer that protect against metabolic and environmental stresses3. Here we present a 3.6 Å resolution cryo-electron microscopy (cryo-EM) structure of the dimeric cytb6 f complex from spinach, which reveals the structural basis for operation of the Q cycle and its redox-sensing function. The complex contains up to three natively bound PQ molecules. The first, PQ1, is located in one cytb6 f monomer near the PQ oxidation site (Qp) adjacent to haem bp and chlorophyll a. Two conformations of the chlorophyll a phytyl tail were resolved, one that prevents access to the Qp site and another that permits it, supporting a gating function for the chlorophyll a involved in redox sensing. PQ2 straddles the intermonomer cavity, partially obstructing the PQ reduction site (Qn) on the PQ1 side and committing the electron transfer network to turnover at the occupied Qn site in the neighbouring monomer. A conformational switch involving the haem cn propionate promotes two-electron, two-proton reduction at the Qn site and avoids formation of the reactive intermediate semiquinone. The location of a tentatively assigned third PQ molecule is consistent with a transition between the Qp and Qn sites in opposite monomers during the Q cycle. The spinach cytb6 f structure therefore provides new insights into how the complex fulfils its catalytic and regulatory roles in photosynthesis.
Exploitation Route Genetic manipulation of photosynthetic regulation is now recognized as being key to increasing crop yields to feed a global population projected to approach 10 billion by 205018. Indeed, overproduction of the Rieske iron-sulfur protein (ISP) of cytb6 f in Arabidopsis thaliana led to a 51% increase in yield19. Further progress in understanding the regulatory roles of cytb6 f and potentially manipulating them for crop improvement requires knowledge of the structure of the higher plant complex. Here, using a gentle purification procedure to obtain a highly active dimeric complex (Extended Data Fig. 1) and single-particle cryo-EM, we resolve the cytb6 f complex from Spinacia oleracea (spinach) at 3.6 Å resolution (Extended Data Fig. 2, Extended Data Table 1).
Sectors Agriculture, Food and Drink,Energy,Environment

First Year Of Impact 2020
Sector Agriculture, Food and Drink