STRUCTURAL ASPECTS OF THE CYTOCHROME-B6F COMPLEX - STRUCTURE OF THE LUMEN-SIDE DOMAIN OF CYTOCHROME-F

The following findings concerning the structure of the cytochrome b6f complex and its component polypeptides, cyt b6, subunit IV and cytochr ome subunit are discussed: (1) Comparison of the amino acid sequences of 13 and 16 cytochrome b6 and subunit IV polypeptides, respectively, led to (a) reconsideration of the helix lengths and probable interface regions, (b) identification of two likely surface-seeking helices in cyt b6 and one in SU IV, and (c) documentation of a high degree of seq uence invariance compared to the mitochondrial cytochrome. The extent of identity is particularly high (88% for conserved and pseudo-conserv ed residues) in the segments of cyt b6 predicted to be extrinsic on th e n-side of the membrane. (2) The intramembrane attractive forces betw een trans-membrane helices that normally stabilize the packing of inte gral membrane proteins are relatively weak. (3) The complex isolated i n dimeric form has been visualized, along with isolated monomer, by el ectron microscopy. The isolated dimer is much more active than the mon omer, is the major form of the complex isolated and purified from chlo roplasts, and is inferred to be a functional form in the membrane. (4) The isolated cyt b6f complex contains one molecule of chlorophyll a. (5) The structure of the 252 residue lumen-side domain of cytochrome f isolated from turnip chloroplasts has been solved by X-ray diffractio n analysis to a resolution of 2.3 angstrom.