Structural aspects of the cytochrome b6f complex; structure of the lumen-side domain of cytochrome f.

The following findings concerning the structure of the cytochromeb6f complex and its component polypeptides, cytb6, subunit IV and cytochromef subunit are discussed: (1) Comparison of the amino acid sequences of 13 and 16 cytochromeb6 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 cytb6 and one in SU IV, and (c) documentation of a high degree of sequence invariance compared to the mitochondrial cytochrome. The extent of identity is particularly high (88% for conserved and pseudoconserved residues) in the segments of cytb6 predicted to be extrinsic on then-side of the membrane. (2) The intramembrane attractive forces betweentrans-membrane helices that normally stabilize the packing of integral membrane proteins are relatively weak. (3) The complex isolated in dimeric form has been visualized, along with isolated monomer, by electron microscopy. The isolated dimer is much more active than the monomer, is the major form of the complex isolated and purified from chloroplasts, and is inferred to be a functional form in the membrane. (4) The isolated cytb6f complex contains one molecule of chlorophylla. (5) The structure of the 252 residue lumen-side domain of cytochromef isolated from turnip chloroplasts has been solved by X-ray diffraction analysis to a resolution of 2.3 A.