Human Upf Proteins in NMD

Abstract

he human Upf (hUpf) proteins work at the core of the nonsense-mediated mRNA decay (NMD) pathway. The three hUpf proteins, hUpf1, hUpf2 and hUpf3, form the hUpf complex, which is critical for the recognition and degradation of mRNAs con- taining premature termination codons (PTCs). The recognition of PTC-containing mRNAs by the hUpf complex in mammalian cells is promoted by the splicing dependent exon-junction complex (EJC), with which hUpf3 interacts. Following the recognition of PTCs, the hUpf complex is believed to disrupt mRNP structure to prevent further translation and trigger mRNA decay. Emerging evidence suggests that hSmg proteins involved in phosphorylation and de- phosphorylation of hUpf1 may play a key role in delivering PTC-containing mRNAs to the mRNA decay machinery. Introduction In recent years the cellular machinery that identifies mRNAs with premature termination codons (PTCs) and subjects them to NMD has been characterized in several eukaryotes. The NMD machinery has the capability to discriminate PTC-containing mRNAs from normal mRNAs, and to inhibit translation and activate decay of the NMD target mRNAs. The three Upf (Up-frameshift) proteins, Upf1, Upf2 and Upf3, work at the heart of this pathway in all organisms studied. The Upf proteins were first discovered in yeast, 1-3 and orthologs have sub- sequently been identified in other eukaryotes. 4 The conservation across species is highest for Upf1 and lower for Upf2 and Upf3. 4 However, despite this conservation, mechanistic differ- ences may exist between various organisms in how PTCs are recognized by the NMD machin- ery. 5 In this chapter, we discuss the current understanding of the role of Upf proteins in the recognition of mRNAs that are targeted for NMD in humans, and how these proteins may shunt the mRNA from the translational pool to the mRNA decay machinery. Evidence That Upf Proteins Are Involved in the Human NMD Pathway It is now well established that human Upf (referred to as hUpf hereafter) proteins play an essential role in NMD. Initial evidence came from identification of the human ortholog to yeast Upf1p based on sequence similarity. 6,7 Several residues of yeast Upf1p that are essential for NMD were found to be conserved in hUpf1. An arginine-to-cysteine mutation at residue 844 was shown to create a dominant-negative form of hUpf1 that impairs NMD in human cells. 8 A similar mutation was earlier shown to inhibit NMD in yeast. 9 Subsequently, hUpf2 and hUpf3, the human orthologs to the other two essential yeast NMD proteins, were identified. The demonstration that they interact with hUpf1 implicated these two proteins in the human