Cientistas próximos de decifrar o código das histonas

terça-feira, dezembro 22, 2009

Scientists Take a Step Towards Uncovering the Histone Code

ScienceDaily (Dec. 21, 2009) — Researchers at Emory University School of Medicine have determined the structures of two enzymes that customize histones, the spool-like proteins around which DNA coils inside the cell.

The structures provide insight into how DNA's packaging is just as important and intricate as the information in the DNA itself, and how these enzymes are part of a system of inspectors making sure the packaging is in order.

The results are published online this week in the journal Nature Structural and Molecular Biology.

A team of scientists led by Xiaodong Cheng, PhD, professor of biochemistry at Emory and a Georgia Research Alliance eminent scholar, used X-rays to probe the architecture of two enzymes, PHF8 and KIAA1718. The enzymes are known as histone demethylases because they remove methyl groups (chemical modifications of a protein) from histones.

Mutations in the gene encoding one of the enzymes, PHF8, cause a type of inherited mental retardation. Understanding how PHF8 works may help doctors better understand or even prevent mental retardation.

Many biologists believe the modifications on histones are a code, analogous to the genetic code. Depending on the histones' structure, access to DNA in the nucleus can be restricted or relatively free. The idea is: the modifications tell enzymes that act on DNA valuable information about getting to the DNA itself.

"This work represents a step toward uncovering the molecular basis for how demethylases handle multiple signals on histones," says Paula Flicker, PhD, who oversees cell signaling grants at the National Institutes of Health's National Institute of General Medical Sciences. "Knowledge of how these complex signals help govern patterns of gene activity will bring us closer to understanding how cells determine their identity during development."

To understand histone demethylases' role in the cell, Cheng says, think of the cell as a library with thousands of books in it.

"To find a particular book in a library, you need some signs telling you how the stacks are organized," he says. "Similarly, the machinery that reads DNA needs some guidance to get to the right place."

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Article abstract

Nature Structural & Molecular Biology
Published online: 20 December 2009 | doi:10.1038/nsmb.1753

Enzymatic and structural insights for substrate specificity of a family of jumonji histone lysine demethylases

John R Horton1,4, Anup K Upadhyay1,4, Hank H Qi2,3,4, Xing Zhang1, Yang Shi2,3 & Xiaodong Cheng1


Combinatorial readout of multiple covalent histone modifications is poorly understood. We provide insights into how an activating histone mark, in combination with linked repressive marks, is differentially 'read' by two related human demethylases, PHF8 and KIAA1718 (also known as JHDM1D). Both enzymes harbor a plant homeodomain (PHD) that binds Lys4-trimethylated histone 3 (H3K4me3) and a jumonji domain that demethylates either H3K9me2 or H3K27me2. The presence of H3K4me3 on the same peptide as H3K9me2 makes the doubly methylated peptide a markedly better substrate of PHF8, whereas the presence of H3K4me3 has the opposite effect, diminishing the H3K9me2 demethylase activity of KIAA1718 without adversely affecting its H3K27me2 activity. The difference in substrate specificity between the two is explained by PHF8 adopting a bent conformation, allowing each of its domains to engage its respective target, whereas KIAA1718 adopts an extended conformation, which prevents its access to H3K9me2 by its jumonji domain when its PHD engages H3K4me3.

1. Department of Biochemistry, Emory University School of Medicine, Atlanta, Georgia, USA.

2. Department of Pathology, Harvard Medical School, Boston, Massachusetts, USA.

3. Division of Newborn Medicine, Department of Medicine, Children's Hospital, Boston, Massachusetts, USA.

4. These authors contributed equally to this work.

Correspondence to: Xiaodong Cheng1 e-mail:


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A seleção natural (um processo cego, aleatório, que não tem como 'visualizar' o futuro das ações bióticas) pode explicar a origem da informação complexa especificada das histonas? Nem a pau, Juvenal! É 100% design inteligente!!! Um processo télico!!!