Abstract
Iron-stress induced protein A (IsiA) is a chlorophyll-binding membrane-spanning protein in photosynthetic prokaryote cyanobacteria, and is associated with photosystem I (PSI) trimer cores, but its structural and functional significance in light harvesting remains unclear. Here we report a 2.7-Å resolution cryo-electron microscopic structure of a supercomplex between PSI core trimer and IsiA from a thermophilic cyanobacterium Thermosynechococcus vulcanus. The structure showed that 18 IsiA subunits form a closed ring surrounding a PSI trimer core. Detailed arrangement of pigments within the supercomplex, as well as molecular interactions between PSI and IsiA and among IsiAs, were resolved. Time-resolved fluorescence spectra of the PSI–IsiA supercomplex showed clear excitation-energy transfer from IsiA to PSI, strongly indicating that IsiA functions as an energy donor, but not an energy quencher, in the supercomplex. These structural and spectroscopic findings provide important insights into the excitation-energy-transfer and subunit assembly mechanisms in the PSI–IsiA supercomplex.
Highlights
Iron-stress induced protein A (IsiA) is a chlorophyll-binding membrane-spanning protein in photosynthetic prokaryote cyanobacteria, and is associated with photosystem I (PSI) trimer cores, but its structural and functional significance in light harvesting remains unclear
In most plants and algae, Light-harvesting complexes (LHCs) are membraneembedded protein complexes associated with the two photosystems, and the structures of various LHC-photosystem supercomplexes have been determined from a number of different species: PSI–LHCI from pea[2,3], PSI–LHCI from a red alga[4,5], PSI–LHCI from green algae[6,7,8], C2S2-type PSII–LHCII from spinach[9], C2S2M2-type PSII–LHCII from Pea[10], and C2S2M2N2 (C2S2M2L2)-type PSII–LHCII from a green alga[11,12]
To examine the structure and function of IsiA in a greater detail, we solved the structure of a PSI–IsiA supercomplex from a thermophilic cyanobacterium Thermosynechococcus vulcanus (T. vulcanus) by single-particle cryoelectron microscopy (cryo-EM) analysis at a much improved resolution of 2.7 Å, which revealed the excitation energy transfer (EET) pathway and detailed protein–protein and pigment–pigment interactions between PSI and IsiAs
Summary
Iron-stress induced protein A (IsiA) is a chlorophyll-binding membrane-spanning protein in photosynthetic prokaryote cyanobacteria, and is associated with photosystem I (PSI) trimer cores, but its structural and functional significance in light harvesting remains unclear. Time-resolved fluorescence spectra of the PSI–IsiA supercomplex showed clear excitation-energy transfer from IsiA to PSI, strongly indicating that IsiA functions as an energy donor, but not an energy quencher, in the supercomplex These structural and spectroscopic findings provide important insights into the excitation-energy-transfer and subunit assembly mechanisms in the PSI–IsiA supercomplex. To examine the structure and function of IsiA in a greater detail, we solved the structure of a PSI–IsiA supercomplex from a thermophilic cyanobacterium Thermosynechococcus vulcanus (T. vulcanus) by single-particle cryo-EM analysis at a much improved resolution of 2.7 Å, which revealed the excitation energy transfer (EET) pathway and detailed protein–protein and pigment–pigment interactions between PSI and IsiAs. We performed time-resolved fluorescence (TRF) analysis of the PSI–IsiA supercomplex to examine its EET dynamics. Together with the spectroscopic observations, the PSI–IsiA structure from T. vulcanus (T_PSI–IsiA) provides important functional insights into how IsiA contributes to the EET events in the PSI–IsiA supercomplex
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