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Questioning the Histone Code

Stable incorporation of HP1 into heterochromatin occurs in late S-phase.

At one time histones were regarded as dull proteins, involved only in DNA compaction in the nucleus. Lately, however, the tails of histones have been recognized as sites of multiple modifications such as methylation, acetylation, and phosphorylation. It has been proposed that permutations of such modifications can function as an epigenetic "histone code" to regulate gene transcription. Indeed, this has become the "standard model" in the field. However, this Paper of the Week presents strong evidence that the situation may be more complex than thought and that in some cases a "replication timing" mechanism may be operative as well.

George K. Dialynas and colleagues examined the chromatin-associated protein HP1 and its interactions with histones in a variety of molecular contexts in vitro and in vivo. They found that H3/H4 particles and subparticles that bound HP1 possessed a complex pattern of post-translational modifications but were not particularly methylated at lysine 9, the widely believed signal for HP1 targeting. As well, transient transfection assays with synchronized cells showed that stable incorporation of HP1 into heterochromatin requires passage through S-phase. These data reveal evidence to support replication timing as an alternative or adjunct to code reading in dictating HP1 binding.

FOOTNOTES

See referenced article, J. Biol. Chem. 2006, 281, 14350-14360

Electron Transfer from One Protein to Another

Model of the complex between cytochrome c552 and the ba3 oxidase.

The conduits by which electrons are transferred between two metalloproteins have been extensively studied by both theoretical and experimental approaches for the last decade. Despite this effort, there are very few cases where the electron transfer pathways are established. In this Paper of the Week, Lucia Muresanu and colleagues utilize the power of high field NMR spectroscopy to establish a highly plausible route through which electron transfer occurs from the heme center of a c-type cytochrome to its partner copper center of a terminal oxidase. The work includes the use of thermostable protein partners, advanced NMR spectroscopic methods, and sophisticated analysis to establish a plausible model for the electron transfer partner complex.

Using NMR-based chemical shift perturbation mapping, the researchers identified the contact regions between the redox complex of the soluble cytochrome c₅₅₂ and the membrane-integral cytochrome ba₃ oxidase of Thermus thermophilus. Chemical shift analysis provided a topographical description of the contact surface on each partner molecule. A protein-protein docking calculation was then preformed to produce a structure ensemble of 10 closely related conformers representing the complex between the cytochromes. Based on these structures, Muresanu et al. predicted the electron transfer pathway from the heme of cytochrome c₅₅₂ to the Cu_A center of the ba₃ oxidase.

FOOTNOTES

See referenced article, J. Biol. Chem. 2006, 281, 14503-14513

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