Localization of AU-rich element-containing mRNA in cytoplasmic granules containing exosome subunits.

Eukaryotic mRNAs can be degraded in either decapping/5'-to-3' or 3'-to-5' direction after deadenylation. In yeast and mammalian cells, decay factors involved in the 5'-to-3' decay pathway are concentrated in cytoplasmic processing bodies (P bodies). The mechanistic steps and localization of mammalian mRNA decay are still not completely understood. Here, we investigate functions of human mRNA decay enzymes in AU-rich element (ARE)-mediated mRNA decay (AMD) and find that the deadenylase, poly(A) ribonuclease PARN, and enzymes involved in the 5'-to-3' and 3'-to-5' decay pathways are required for AMD. The ARE-containing reporter mRNA accumulates in discrete cytoplasmic granular structures, which are distinct from P bodies and stress granules. These granules consist of poly(A)-specific ribonuclease, exosome subunits, and decay-promoting ARE-binding proteins. Inhibition of AMD increases accumulation of ARE-mRNA in these granules. We refer to these structures as cytoplasmic exosome granules and suggest that some AMD may occur in these granules.