Sep. 4, 2003 CHAPEL HILL -- A cellular protein identified by scientists at the University of North Carolina at Chapel Hill may be the crucial molecular element for gene silencing.
The research findings, published Aug. 29 in the science journal Molecular Cell, add important knowledge to the understanding of epigenetic signals. These chemical signals affect the modulation of gene expression - activation or repression - throughout the genome.
Studies at UNC and elsewhere have shown that epigenetic phenomena underpin the shutting down of one copy of the X chromosome occurring in female mammals, and parental "imprinting" - in which a gene's activity depends on whether it's inherited from the mother or father. During development, the expression of whole sets of genes must be repressed, or silenced, after their proteins set the body pattern.
One such epigenetic event is histone methylation, the addition of one or more methyl groups to lysine, one of the amino acids that make up the "tail" domain of histone proteins. Within the cell nucleus, spiraling strands of DNA are wrapped tightly around four core histone proteins and then fold to form a densely packed structure called chromatin. This complex of nucleic acids and proteins packages DNA into higher order structures, ultimately forming a chromosome.
The chemical modification of histone tails can alter chromatin structure, loosening or tightening it, which in turn influences the expression of adjacent genes. In the journal article, a study team led by Dr. Yi Zhang, assistant professor of biochemistry and biophysics in UNC's School of Medicine and a member of UNC's Lineberger Comprehensive Cancer Center, reported having identified for the first time a protein that directly regulates lysine methylation on the core histone protein, H3, in a way that represses gene activity.
"We have found the first molecule, the first gene product, that can regulate methylation," Zhang said.
In earlier research, Zhang identified a catalytic subunit associated with lysine methylation. This is the murine (mouse) enzyme ESET and its human homologue SETDB1. However, subsequent studies showed that such methylation might not be enough by itself to trigger gene silencing.
The newly discovered murine regulatory protein is called "mAM." Its human equivalent, or homologue, is "hAM." Stimulated by this protein, the state of methylation of lysine-9 on H3 that's produced by the enzymatic subunit is made more complex - moving from dimethylation, the addition of two methyl groups, to trimethylation, the addition of three. In this new state, lysine-9 methylation becomes the signal for gene repression.
While the catalytic subunit alone can methylate a particular lysine residue on H3, in this case lysine-9, gene silencing occurs only when the lysine is methylated to the trimethyl state, Zhang said.
"The catalytic subunit by itself can have enzymatic activity, but not enough potency to repress gene expression," Zhang said. "Now we have demonstrated both in vitro and in vivo that gene repression is dependent on trimethylation." Zhang and his team are studying the biological significance of their discovery. "We have some indications that it's important for apoptosis, programmed cell death. We're also studying chromatin epigenetics with a view toward determining if they play a role in the ability of stem cells to commit to a specific lineage."
Along with Zhang, UNC co-authors of the report include Drs. Hengbin Wang and Li Xia and doctoral student Ru Cao. Other co-authors are Woojin An and Robert G. Roeder of Rockefeller University; Hediye Erdjument-Bromage and Paul Tempst of Memorial Sloan-Kettering Cancer Center in New York; and Bruno Chatton of CNRS-INSERM, in Strasbourg, France.
The research was supported by a grant from the National Institute of General Medicine, a component of the National Institutes of Health.
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