Supercomplexes of photosystem I and antenna proteins in cyanobacteria and green plants

Photosystem I (PSI) and Photosystem II (PSII) can form supercomplexes with various types of antenna proteins. One such protein is Light-harvesting complex II (LHCII). In plants it is bound in its trimeric form to dimeric PSII. In spinach and Arabidopsis thaliana most dimers bind 2-4 LHCII trimers. We have characterized the PSI-LHCII complex from Arabidopsis membranes in state two by single particle electron microscopy at about 16 Å. PSI binds one trimer at the site of the PsaL and PsaK subunits. Cyanobacteria express large quantities of the Iron Stress Inducible protein IsiA under iron deficiency. IsiA can assemble into numerous types of single or double rings surrounding PS I. These supercomplexes are functional in light-harvesting, but empty IsiA rings are effective energy dissipators. Electron microscopy studies on over 130 000 particle projections of these supercomplexes show that photosystem I trimers find 18 IsiA copies in a single ring; whereas monomers may bind up to 35 copies in two rings. The double rings are formed by inner rings of 12, 13 or 14 copies and the outer ones of 19, 20 and 21 copies, respectively. This shows that IsiA can form a remarkable large variety of ring-like structures. Work on particles purified from mutants indicates that the PsaF and PsaL subunits facilitate the formation of closed rings around PSI monomers. But these subunits are not obligatory components in the formation of PSI-IsiA supercomplexes.