Interactions of CCCH zinc finger proteins with mRNA - Tristetraprolin-mediated Au-rich element-dependent mRNA degradation can occur in the absence ofa poly(A) tail

The CCCH family of tandem zinc finger proteins has recently been shown to p romote the turnover of certain mRNAs containing class II AU-rich elements ( AREs). In the case of one member of this family, tristetraprolin (TTP), abs ence of the protein in knockout mice leads to stabilization of two mRNAs co ntaining AREs of this type, those encoding tumor necrosis factor alpha (TNF alpha) and granulocyte-macrophage colony-stimulating factor. To bean to de cipher the mechanism by which these zinc finger proteins stimulate the brea kdown of this class of mRNAs, we co-transfected TTP and its related CCCH pr oteins into 293 cells with vectors encoding full-length TNF alpha, granuloc yte-macrophage colony-stimulating factor, and interleukin-3 mRNAs. Go-expre ssion of the CCCH proteins caused the rapid turnover of these ARE-containin g mRNAs and also promoted the accumulation of stable breakdown intermediate s that were truncated at the 3'-end of the mRNA, even further 5' than the 5 '-end of the poly(A) tail. To determine whether an intact poly(A) tail was necessary for TTP to promote this type of mRNA degradation, we inserted the TNF alpha ARE into a nonpolyadenylated histone mRNA and also attached a hi stone 3'-end-processing sequence to the 3'-end of nonpolyadenylated interle ukin-3 and TNF alpha mRNAs. In all three cases, TTP stimulated the turnover of the ARE-containing mRNAs, despite the demonstrated absence of a poly(A) tail. These studies indicate that members of this class of CCCH proteins c an promote class II ARE containing mRNA turnover even in the absence of a p oly(A) tail, suggesting that the processive removal of the poly(A) tail may not be required for this type of CCCH protein-stimulated mRNA turnover.