THE STRUCTURE OF CHLOROPLAST CYTOCHROME-C6 AT 1.9 ANGSTROM RESOLUTION- EVIDENCE FOR FUNCTIONAL OLIGOMERIZATION

The molecular structure of cytochrome c6 from the green alga Chlamydom onas reinhardtii has been determined from two crystal forms and refine d to 1.9 Angstrom resolution. The two crystal forms are likely the res ult of different levels of post-translational modification of the prot ein. This is the first report of a high-resolution structure of a chlo roplast-derived class I c-type cytochrome. The overall fold is similar to that of other class I c-type cytochromes, consisting of a series o f alpha-helices and turns that envelop the heme prosthetic group. Ther e is also a short two-stranded anti-parallel beta-sheet in the vicinit y of the methionine axial ligand to the heme; this region of the molec ule is formed by the most highly conserved residues in c6-type cytochr omes. Although class I c-type cytochromes are assumed to function as m onomers, both crystal forms of cytochrome c6 exhibit oligomerization a bout the heme crevice that is, in part, mediated by the short anti-par allel beta-sheet. The functional significance of this oligomerization is supported by the appearance of similar interfaces in other electron transfer couples, HPLC and light-scattering data, and is furthermore consistent with kinetic data on electron transfer reactions of c6-type cytochromes. (C) 1995 Academic Press Limited