Deadenylation is the major step triggering mammalian mRNA decay. One consequence 
of deadenylation is the formation of nontranslatable messenger RNA (mRNA) 
protein complexes (messenger ribonucleoproteins [mRNPs]). Nontranslatable mRNPs 
may accumulate in P-bodies, which contain factors involved in translation 
repression, decapping, and 5'-to-3' degradation. We demonstrate that 
deadenylation is required for mammalian P-body formation and mRNA decay. We 
identify Pan2, Pan3, and Caf1 deadenylases as new P-body components and show 
that Pan3 helps recruit Pan2, Ccr4, and Caf1 to P-bodies. Pan3 knockdown causes 
a reduction of P-bodies and has differential effects on mRNA decay. Knocking 
down Caf1 or overexpressing a Caf1 catalytically inactive mutant impairs 
deadenylation and mRNA decay. P-bodies are not detected when deadenylation is 
blocked and are restored when the blockage is released. When deadenylation is 
impaired, P-body formation is not restorable, even when mRNAs exit the 
translating pool. These results support a dynamic interplay among deadenylation, 
mRNP remodeling, and P-body formation in selective decay of mammalian mRNA.