Nonsense-mediated mRNA Decay: Insights into Mechanism from the Cellular Abundance of Human Upf1, Upf2, Upf3, and Upf3X Proteins

Nonsense-mediated mRNA decay (NMD), also calledmRNA surveillance, is one of several posttranscriptionalmechanisms that eukaryotic cells employ to control thequality of mRNA function (for review, see Maquat andCarmichael 2001). NMD degrades aberrant transcriptsthat prematurely terminate translation in order to eliminate the production of incomplete proteins that couldfunction in dominant-negative or other deleterious ways(for review, see Maquat 1995, 2000; Jacobson and Peltz1996, 2000; Li and Wilkinson 1998; Culbertson 1999;Frischmeyer and Dietz 1999; Hentze and Kulozik 1999;Hilleren and Parker 1999). Insights into the mechanismof NMD will derive, in part, from an analysis of requiredfactors. Here, we report the cellular abundance of the fourhuman (h) Upf proteins known to function in NMD, andwe use this information to extend our understanding ofhow the premature termination of mRNA translation elicits mRNA decay...