Datta Shrimati, Osborne Timothy F
Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697-3900, USA.
J Biol Chem. 2005 Feb 4;280(5):3338-45. doi: 10.1074/jbc.M411222200. Epub 2004 Nov 17.
Sterol regulatory element-binding proteins (SREBPs) are basic helix-loop-helix leucine zipper proteins that act as dimers to activate genes in lipid metabolism. Three SREBP isoforms, 1a, 1c, and 2, are expressed at varying levels in different tissues. Thus, homo- and heterodimers probably contribute to overall SREBP activity. No studies have directly evaluated the formation or activation properties of SREBP homo- and heterodimers. Studies with overexpressed SREBP monomers are inconclusive regarding the function of a particular SREBP dimer because of potential dimerization with endogenous proteins. To assess activation by a particular SREBP dimer, we fused DNA encoding individual monomers together via a predicted flexible polypeptide tether. Tethered SREBP dimers bound DNA equivalently to the monomeric proteins and were resistant to dominant negative SREBP-1 inhibition, confirming preferential formation of intramolecular dimers. Tethered SREBP-1a and -2 homodimers, similar to the monomeric forms, activated target genes more robustly than tethered SREBP-1c homodimers. A forced SREBP-1a/2 heterodimer had similar activity to the respective homodimers. However, SREBP-1c in a heterodimer with either SREBP-1a or -2 attenuated the activity relative to the SREBP-1a or -2 homodimers. These experiments provide some of the first data showing that the integrity of both activation domains in a dimeric transcription factor is required for maximal activity. In addition, the results support a model where changes in SREBP-1c protein expression that occur in response to insulin signaling and liver X receptor signaling would be predicted to increase or decrease overall SREBP activity in a tissue-specific fashion depending on the initial fractional contribution of SREBP-1c to total cellular levels of SREBP.
固醇调节元件结合蛋白（SREBPs）是碱性螺旋-环-螺旋亮氨酸拉链蛋白，作为二聚体激活脂质代谢中的基因。三种SREBP亚型，即1a、1c和2，在不同组织中以不同水平表达。因此，同二聚体和异二聚体可能共同决定了SREBP的整体活性。尚无研究直接评估SREBP同二聚体和异二聚体的形成或激活特性。由于过表达的SREBP单体可能与内源性蛋白质发生二聚化，因此关于特定SREBP二聚体功能的研究尚无定论。为了评估特定SREBP二聚体的激活作用，我们通过预测的柔性多肽连接子将编码单个单体的DNA融合在一起。连接的SREBP二聚体与单体蛋白等效地结合DNA，并且对显性负性SREBP-1抑制具有抗性，证实了分子内二聚体的优先形成。与单体形式类似，连接的SREBP-1a和-2同二聚体比连接的SREBP-1c同二聚体更有力地激活靶基因。强制形成的SREBP-1a/2异二聚体具有与各自同二聚体相似的活性。然而，与SREBP-1a或-2形成异二聚体的SREBP-1c相对于SREBP-1a或-2同二聚体减弱了活性。这些实验提供了一些初步数据，表明二聚体转录因子中两个激活结构域的完整性对于最大活性是必需的。此外，结果支持这样一种模型，即响应胰岛素信号和肝脏X受体信号而发生的SREBP-1c蛋白表达变化，预计将根据SREBP-1c对细胞总SREBP水平的初始分数贡献，以组织特异性方式增加或降低整体SREBP活性。
