ISOLATION AND BIOCHEMICAL-CHARACTERIZATION OF MONOMERIC AND DIMERIC PHOTOSYSTEM-II COMPLEXES FROM SPINACH AND THEIR RELEVANCE TO THE ORGANIZATION OF PHOTOSYSTEM-II IN-VIVO

Membranes enriched in photosystem II were isolated from spinach and fu rther solubilised using n-octyl beta-D-glucopyranoside (OctGlc) and n- dodecyl beta-D-maltoside (DodGlc(2)). The OctGlc preparation had high rates of oxygen evolution and when subjected to size-exclusion HPLC an d sucrose density gradient centrifugation, in the presence of DodGlc(2 ), separated into dimeric (430 kDa), monomeric (236 kDa) photosystem I I cores and a fraction containing photosystem II light-harvesting comp lex (Lhcb) proteins. The dimeric core fraction was more stable, contai ned higher levels of chlorophyll, beta-carotene and plastoquinone per photosystem II reaction centre and had a higher oxygen-evolving activi ty than the monomeric cores. Their subunit composition was similar (CP 43, CP47, D1, D2, cytochrome b 559 and several lower-molecular-mass co mponents) except that the level of 33-kDa extrinsic protein was lower in the monomeric fraction. Direct solubilisation of photosystem-II-enr iched membranes with DodGlc(2), followed by sucrose density gradient c entrifugation, yielded a super complex (700 kDa) containing the dimeri c form of the photosystem II core and Lhcb proteins: Lhcb1, Lhcb2, Lhc b4 (CP29), and Lhcb5 (CP26). Like the dimeric and monomeric photosyste m II core complexes, the photosystem II-LHCII complex had lost the 23- kDa and 17-kDa extrinsic proteins, but maintained the 33-kDa protein a nd the ability to evolve oxygen. It is suggested, with a proposed mode l, that the isolated photosystem II-LHCII super complex represents an in vivo organisation that can sometimes form a lattice in granal membr anes of the type detected by freeze-etch electron microscopy [Seibert, M., DeWit, M. & Staehelin, L. A. (1987) J. Cell Biol. 105, 2257-2265] .