Conformational changes in photosystem II supercomplexes upon removal of extrinsic subunits

Photosystem II is a multisubunit pigment-protein complex embedded in the th ylakoid membranes of chloroplasts, It consists of a large number of intrins ic membrane proteins involved in light-harvesting and electron-transfer pro cesses and of a number of extrinsic proteins required to stabilize photosyn thetic oxygen evolution. We studied the structure of dimeric supercomplexes of photosystem LI and its associated light-harvesting antenna by electron microscopy and single-particle image analysis. Comparison of averaged proje ctions from native complexes and complexes without extrinsic polypeptides i ndicates that the removal of 17 and 23 kDa extrinsic subunits induces a shi ft of about 1.2 nm in the position of the monomeric peripheral antenna prot ein CP29 toward the central part of the supercomplex. Removal of the 33 kDa extrinsic protein induces an inward shift of the strongly bound trimeric l ight-harvesting complex II (S-LHCII) of about 0.9 nm, and in addition desta bilizes the monomer-monomer interactions in the central core dimer, leading to structural rearrangements of the core monomers. It is concluded that th e extrinsic subunits keep the S-LHCII and CP29 subunits in proper positions at some distance from the central part of the photosystem II core dimer to ensure a directed transfer of excitation energy through the monomeric peri pheral antenna proteins CP26 and CP29 and/or to maintain sequestered domain s of inorganic cofactors required for oxygen evolution.