Philadelphia -- For the first time, scientists working in The Wistar Institute laboratory of Ronen Marmorstein, PhD, in collaboration with Shelley Berger, PhD, have determined the three-dimensional structure of a key enzyme involved in gene activation. In nature, this enzyme, which is called GCN5, functions to attach acetyl groups to histone proteins that are bound to DNA to facilitate gene activation. The Marmorstein group has obtained a structure of the histone acetyltransferase (HAT) domain of GCN5 bound to both its histone target and to its coenzyme-A cofactor. Details of this structure appear in the paper, "Crystal structure of Tetrahymena GCN5 with bound coenzyme-A and histone H3 peptide," in the September 2, 1999 issue of Nature.
Earlier this year, this same group of Wistar scientists solved the three-dimensional structure of two other HAT enzymes. The first structure, of PCAF, the human homologue of GCN5, bound to coenzyme-A, appeared in the July 1, 1999 issue of The EMBO Journal. The second structure, of yeast GCN5 in its unbound form, appeared along with editorial commentary in the August 3, 1999 issue of Proceedings of the National Academy of Sciences.
By comparing the structures of the unbound HAT, HAT bound to coenzyme-A and HAT bound to both coenzyme-A and histone, the Wistar group has gained valuable information about the mechanism of histone acetylation. In particular, they have identified the details of how the enzyme carries out its function, the structural adjustments the enzyme needs to make at each stage of its activities, and, most importantly, how it finds its histone target for proper regulation of gene activation.
Histones are proteins that bind to and restrict the activity of DNA in the chromosomes. Before the gene activation can occur, general transcription factor and coactivator proteins must bind to DNA and DNA and histones must be separated through a process that is facilitated by histone acetylation. Scientists have been divided in their approaches to the study of gene activation. While one group of researchers focused on chromatin, a complex of nucleic acids and histones, another group examined the activity of coactivator proteins.
These two approaches to the study of histone acetylation seemed unconnected until C. David Allis, PhD, then at the University of Rochester and co-author of the Nature paper, cloned the protein, HAT type A, now called tGCN5. When Dr. Allis sequenced the DNA encoding this enzyme, he found it to be strikingly similar to a yeast protein, GCN5, a previously identified transcriptional coactivator, as shown by Dr. Berger's laboratory, among others. Thus, Dr. Allis' findings demonstrated for the first time that histone acetylation is a targeted phenomenon directly linked to gene activation.
Other proteins with HAT activity were cloned shortly after Dr. Allis' discovery. Among them were hTAFII 250 and p300/CBP, two other previously identified transcriptional coactivators. The current evidence suggests that these different HAT enzymes have different histone specificities, and the current notion is that these divergent target specificities are important for gene activation.
Interestingly, p300/CBP proteins are found translocated in certain leukemia types and p300 gene alterations are found in colorectal and gastric cancers. This discovery suggests that there may be a direct link between alterations in histone acetylation and the development of cancer and other diseases associated with a loss of regulation of gene activation.
Conversely, histone deacetylases (HDAC) have been recently shown to help turn off transcription, and several HDAC proteins have also been implicated to play a role in particular leukemia types. Taken together, scientists believe that the histone acetylation balance is critical for normal cell function, and that a disruption of this balance leads to diseases such as cancer.
The Wistar Institute, established in 1892, was the first independent medical research facility in the country. For more than 100 years, Wistar scientists have been making history and improving world health through their development of vaccines for diseases that include rabies, German measles, infantile gastroenteritis (rotavirus), and cytomegalovirus; discovery of molecules like interleukin-12, which are helping the immune system fight bacteria, parasites, viruses and cancer; and location of genes that contribute to the development of diseases like breast, lung and prostate cancer. Wistar is a National Cancer Institute Cancer Center.
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