Splicing and 3′ end formation in the definition of nonsense-mediated decay-competent human β-globin mRNPs

Premature translation termination codons are common causes of genetic disorders. mRNAs with such mutations are degraded by a surveillance mechanism termed nonsense-mediated decay (NMD), which represents a phylogenetically widely conserved post-transcriptional mechanism for the quality control of gene expression. How NMD-competent mRNPs are formed and specified remains a central question. Here, we have used human β-globin mRNA as a model system to address the role of splicing and polyadenylation for human NMD. We show that (i) splicing is an indispensable component of the human β-globin NMD pathway, which cannot be compensated for by exonic β-globin ‘failsafe’ sequences; (ii) the spatial requirements of human β-globin NMD, as signified by the maximal distance of the nonsense mutation to the final exon–exon junction, are less constrained than in yeast; and (iii) non-polyadenylated mRNAs with a histone 3′ end are NMD competent. Thus, the formation of NMD-competent mRNP particles critically depends on splicing but does not require the presence of a poly(A) tail.