Photosystem II and the Evolution of the Light-harvesting Antennae of Vascular Plants: A New Concept

Abstract

Cryo-electron crystallography of grana membranes has led toa new concept with regards to the positioning of photosystem II (PSII) relative to its peripheral light-harvesting complex (LHCII). In projection, the structural data reveals small domains surrounding the central PSII core which are compatible with the size and expected stoichiometry of the LHCII proteins. When viewed in 3D, however, these small domains are shown to occupy a membrane separate from the membrane plane that houses the PSII core region. This observation fits with the known morphology of the grana membrane preparation which consists of paired, tightly appressed membranes. The structural data has been confirmed by separate biochemical experiments where LHCII-enriched and core PSII-enriched membrane fractions have been isolated. A vertical segregation of LHCII and PSII within the grana will be of wide significance: (a) Optimisation of light harvesting capacity (packing one membrane with LHCII, whilst at the same time maintaining efficient diffusion of plastoquinone in an adjacent membrane loosely packed with PSII core complexes). (b) Rapid adaptation to changes in light quality and intensity (via physical separation/appression of membrane pairs). (c) Cooperativity (PSII core complexes can tap into a large LHCII antenna located in an adjacent membrane). (d) New understanding of the evolution of light harvesting in plants (cyanobacteria also move light excitation energy vertically from the phycobilisome to PSII core via linker proteins).