Translation of mRNAs containing premature termination codons (PTCs) can result in truncated protein products with deleterious effects. Nonsense-mediated decay (NMD) is a surveillance path-way responsible for detecting and degrading PTC containing transcripts. While the molecular mechanisms governing mRNA degradation have been extensively studied, the fate of the nascent protein product remains largely uncharacterized. Here, we use a fluorescent reporter system in mammalian cells to reveal a

more »

... ctive degradation pathway specifically targeting the protein product of an NMD mRNA. We show that this process is post-translational, and dependent on an intact ubiquitin proteasome system. To systematically uncover factors involved in NMD-linked protein quality control, we conducted genome-wide flow cytometry-based screens. Our screens recovered known NMD factors, and suggested a lack of dependence on the canonical ribosome-quality control (RQC) pathway. Finally, one of the strongest hits in our screens was the E3 ubiquitin ligase CNOT4, a member of the CCR4-NOT complex, which is involved in initiating mRNA degradation. We show that CNOT4 is involved in NMD coupled protein degradation, and its role depends on a functional RING ubiquitin ligase domain. Our results demonstrate the existence of a targeted pathway for nascent protein degradation from PTC containing mRNAs, and provide a framework for identifying and characterizing factors involved in this process.