The Structure of Chloroplast Cytochrome c6at 1.9 Å Resolution: Evidence for Functional Oligomerization

Abstract

The molecular structure of cytochrome c6from the green alga Chlamydomonas reinhardtiihas been determined from two crystal forms and refined to 1.9 Å resolution. The two crystal forms are likely the result of different levels of post-translational modification of the protein. This is the first report of a high-resolution structure of a chloroplast-derived class I c-type cytochrome. The overall fold is similar to that of other class I c-type cytochromes, consisting of a series of α-helices and turns that envelop the heme prosthetic group. There is also a short two-stranded anti-parallel β-sheet in the vicinity of the methionine axial ligand to the heme; this region of the molecule is formed by the most highly conserved residues in c6-type cytochromes. Although class I c-type cytochromes are assumed to function as monomers, both crystal forms of cytochrome c6exhibit oligomerization about the heme crevice that is, in part, mediated by the short anto-parallel β-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. f2 f2 Abbreviations used: cyt, cytochrome; PSI, photosystem I; SIR, single isomorphous replacement; MIR, multiple isomorphous replacement; SEC, size-exclusion chromatography; HiPIP, high potential iron-sulfur protein.