Characterization of the Chloroplast Cytochrome b6f Complex as a Structural and Functional Dimer?

ABSTRACT: Size analysis of the cytochrome bdcomplex by FPLC Superose-12 chromatography and Blue Native PAGE indicated a predominantly dimeric component with M, = (1.9-2.5) X lo5. The true dimer molecular weight including bound lipid, but not detergent, was estimated to be 2.3 X lo5. Size and shape analysis by negative-stain single-particle electron microscopy indicated that the preparation of dimeric complexes contains a major population that has a protein cross section 40 % larger than the monomer, binds more negative stain, and has a geometry with a distinct 2-fold axis of symmetry compared to the monomeric complex. The dimeric species is more stable at higher ionic strength with respect to conversion to the monomeric species. SDS-PAGE of monomer and dimer preparations indicated that both contain the four major polypeptides in approximately equal stoichiometry and also contain thepetG M, 4000 subunit. One bound chlorophyll a per monomer, part of the bound lipid, is present in monomer and dimer. The in vitro electron-transport activity (decyl-PQHz- PC-ferricyanide) of the separated dimer was comparable to that of the isolated bdcomplex and was 4-5-fold greater than that of the monomer preparation, whose activity could be attributed to residual dimer. No difference in the properties of the dimer and monomer was detected by SDS-PAGE or redox difference spectrophotometry that could account for the difference in activities. However, the concentration of the Rieske [2Fe-2S] center was found by EPR analysis of th