Li Shanshan, Shogren-Knaak Michael A
Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, Iowa 50011, USA.
J Biol Chem. 2009 Apr 3;284(14):9411-7. doi: 10.1074/jbc.M809617200. Epub 2009 Feb 13.
Bromodomains are acetyl lysine binding modules found in many complexes that regulate gene transcription. In budding yeast, the coactivator complex SAGA (Spt-Ada-Gcn5-acetyl-transferase) predominantly facilitates transcription of stress-activated genes and requires the bromodomain of the Gcn5 subunit for full activation of a number of these genes. This bromodomain has previously been shown to promote retention of the complex to H3 and H4 acetylated nucleosomes. Because the SAGA complex mediates histone H3 acetylation, we sought to determine to what extent the Gcn5 bromodomain directly modulates histone acetylation activity. Kinetic analysis of SAGA-mediated acetylation of nucleosomal substrates reveals that this bromodomain: 1) is required for the cooperative acetylation of nucleosomes, 2) enhances acetylation of an H3 histone tail when the other H3 tail within a nucleosome is already acetylated, and 3) augments the acetylation turnover of nucleosomes previously acetylated at lysine 16 of the histone H4 tails. These results indicate that the Gcn5 bromodomain promotes the establishment of nucleosome acetylation through multiple mechanisms and more generally show how chromatin recognition domains can modulate the enzymatic activity of chromatin modifying complexes.
溴结构域是在许多调控基因转录的复合物中发现的乙酰赖氨酸结合模块。在芽殖酵母中，共激活复合物SAGA（Spt-Ada-Gcn5-乙酰转移酶）主要促进应激激活基因的转录，并且许多这类基因的完全激活需要Gcn5亚基的溴结构域。此前已表明，这个溴结构域可促进该复合物与H3和H4乙酰化核小体的结合。由于SAGA复合物介导组蛋白H3乙酰化，我们试图确定Gcn5溴结构域在多大程度上直接调节组蛋白乙酰化活性。对SAGA介导的核小体底物乙酰化的动力学分析表明，这个溴结构域：1）是核小体协同乙酰化所必需的；2）当核小体内的另一条H3尾巴已经乙酰化时，可增强H3组蛋白尾巴的乙酰化；3）增加先前在组蛋白H4尾巴赖氨酸16位点乙酰化的核小体的乙酰化周转。这些结果表明，Gcn5溴结构域通过多种机制促进核小体乙酰化的建立，更广泛地显示了染色质识别结构域如何调节染色质修饰复合物的酶活性。
