Novel function of the eukaryotic polypeptide-chain releasing factor 3 (eRF3/GSPT) in the mRNA degradation pathway.

The mammalian GTP-binding protein GSPT, whose carboxy-terminal sequence is homologous to the eukaryotic elongation factor EF1alpha, binds to the polypeptide chain releasing factor eRF1 to function as eRF3 in translation termination. However, the amino-terminal domain of GSPT, which contains a prion-like sequence, is not required for the binding. Instead, the amino-terminal domain is capable of binding to the carboxy-terminal domain of polyadenylate-binding protein (PABP), whose amino terminus is associating with the poly(A) tail of mRNAs, presumably for their stabilization. Interestingly, multimerization of PABP with poly(A), which is ascribed to the action of its carboxy-terminal domain, was completely inhibited by the interaction with the amino-terminal domain of GSPT. This may facilitate shortening of the poly(A) tail of mRNAs by an RNase. Thus, GSPT/eRF3 appears to function not only as a stimulator of eRF1 in the translation termination but also as an initiator of the mRNA degradation machinery. Further physiological and cell biological approaches will be necessary to show whether our current in vitro findings on GSPT/eRF3 indeed reflect its bifunctional properties in living cells.