A balancing act: Interactions within NuA4/TIP60 regulate picNuA4 function in Saccharomyces cerevisiae and humans

The NuA4 lysine acetyltransferase complex acetylates histone and non-histone proteins and functions in transcription regulation, cell cycle progression, and DNA repair. NuA4 harbors an interesting duality in that its catalytic module can function independently and distinctly as picNuA4. At the molecular level, picNuA4 anchors to its bigger brother via physical interactions between the C-terminus of Epl1 and the HSA domain of Eaf1, the NuA4 central scaffolding subunit. This is reflected at the regulatory level, as picNuA4 can be liberated genetically from NuA4 by disrupting the Epl1-Eaf1 interaction. As such, removal of either Eaf1 or the Epl1 C-terminus offers a unique opportunity to elucidate the contributions of Eaf1 and Epl1 to NuA4 biology and in turn their roles in balancing picNuA4 and NuA4 activities. Using high-throughput genetic and gene expression profiling, and targeted functional assays to compare eaf1∆ and epl1-C∆ mutants, we found that EAF1 and EPL1 had both overlapping and distinct roles. Strikingly, loss of EAF1 or its HSA domain led to a significant decrease in the amount of picNuA4, while loss of the Epl1 C-terminus increased picNuA4 levels, suggesting starkly opposing effects on picNuA4 regulation. The eaf1∆ epl1-C∆ double mutants resembled the epl1-C∆ single mutants, indicating that Eaf1’s role in picNuA4 regulation depended on the Epl1 C-terminus. Key aspects of this regulation were evolutionary conserved, as truncating an Epl1 homolog in human cells increased the levels of other picNuA4 subunits. Our findings suggested a model in which distinct aspects of the Epl1-Eaf1 interaction regulated picNuA4 amount and activity.NuA4赖氨酸乙酰转移酶复合物（NuA4 lysine acetyltransferase complex）可催化组蛋白与非组蛋白的乙酰化修饰，并参与转录调控、细胞周期进程及DNA修复等生物学过程。NuA4具有一种有趣的双重特性：其催化模块可作为独立的picNuA4复合体，独立且独特地发挥功能。在分子层面，picNuA4通过Epl1的C端与NuA4中央支架亚基Eaf1的HSA结构域（HSA domain）之间的物理相互作用，锚定至完整的NuA4复合物。这一特征在调控层面同样有所体现：通过破坏Epl1与Eaf1的相互作用，可在遗传层面将picNuA4从NuA4中分离出来。因此，敲除Eaf1或截除Epl1的C端，为解析Eaf1与Epl1在NuA4生物学功能中的作用，以及二者在平衡picNuA4与NuA4活性中的功能，提供了独特的研究契机。本研究通过高通量遗传与基因表达谱分析，结合靶向功能实验，对eaf1Δ（Eaf1缺失）突变体与epl1-CΔ（Epl1 C端截短）突变体进行比较分析，结果显示EAF1与EPL1既存在功能重叠，又具有各自独特的生物学功能。值得注意的是，敲除EAF1或其HSA结构域会导致picNuA4的含量显著降低，而截除Epl1的C端则会提升picNuA4的水平，这表明二者对picNuA4的调控存在截然相反的作用。eaf1Δ epl1-CΔ双突变体的表型与epl1-CΔ单突变体相似，这表明Eaf1对picNuA4的调控作用依赖于Epl1的C端。该调控机制的关键特征在进化过程中具有保守性：在人类细胞中截短Epl1的同源蛋白，会提升其他picNuA4亚基的表达水平。本研究结果提出了一个模型，即Epl1与Eaf1相互作用的不同层面，分别调控picNuA4的含量与活性。