The paralogous human proteins UPF3A and UPF3B are involved in recognizing mRNAs targeted by nonsense-mediated mRNA decay (NMD). UPF3B has been demonstrated to support NMD, presumably by bridging an exon junction complex (EJC) to the NMD factor UPF2. The role of UPF3A has been described either as a weak NMD activator or an NMD inhibitor. Here, we present a comprehensive functional analysis of UPF3A and UPF3B in human cells using combinatory experimental approaches. Overexpression or knockout of UPF3A as well as knockout of UPF3B did not substantially change global NMD activity. In contrast, the co-depletion of UPF3A and UPF3B resulted in a marked NMD inhibition and a transcriptome-wide upregulation of NMD substrates, demonstrating a functional redundancy between both NMD factors. In rescue experiments, UPF2 or EJC binding-deficient UPF3B largely retained NMD activity. However, combinations of different mutants, including deletion of the middle domain, showed additive or synergistic effects and therefore failed to maintain NMD. Collectively, UPF3A and UPF3B emerge as fault-tolerant, functionally redundant NMD activators in human cells.

中文翻译：

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人类 UPF3A 和 UPF3B 能够容错激活无意义介导的 mRNA 衰变。

旁系同源人类蛋白质 UPF3A 和 UPF3B 参与识别无义介导的 mRNA 衰变 (NMD) 靶向的 mRNA。UPF3B 已被证明支持 NMD，大概是通过将外显子连接复合物 (EJC) 桥接到 NMD 因子 UPF2。UPF3A 的作用被描述为弱 NMD 激活剂或 NMD 抑制剂。在这里，我们使用组合实验方法对人体细胞中的 UPF3A 和 UPF3B 进行了全面的功能分析。UPF3A 的过度表达或敲除以及 UPF3B 的敲除并没有显着改变全球 NMD 活动。相比之下，UPF3A 和 UPF3B 的共同消耗导致显着的 NMD 抑制和 NMD 底物的转录组范围上调，证明了两个 NMD 因子之间的功能冗余。在救援实验中，UPF2 或 EJC 结合缺陷的 UPF3B 很大程度上保留了 NMD 活性。然而，不同突变体的组合，包括中间结构域的缺失，显示出累加或协同效应，因此无法维持 NMD。总的来说，UPF3A 和 UPF3B 作为人类细胞中的容错、功能冗余的 NMD 激活剂出现。