Musashi Protein-directed Translational Activation of Target mRNAs Is Mediated by the Poly( A) Polymerase, Germ Line Development Defective-2

Background: Although Musashi mediates target mRNA polyadenylation, the underlying molecular mechanism has not been elucidated. Results: Germ line development defective-2, a poly(A) polymerase, associates with Musashi and is necessary and sufficient for Musashi-directed polyadenylation. Conclusion: Germ line development defective-2 mediates Musashi-dependent mRNA translation. Significance: Germ line development defective-2 couples Musashi to polyadenylation and translational activation of target mRNAs. The mRNA-binding protein, Musashi, has been shown to regulate translation of select mRNAs and to control cellular identity in both stem cells and cancer cells. Within the mammalian cells, Musashi has traditionally been characterized as a repressor of translation. However, we have demonstrated that Musashi is an activator of translation in progesterone-stimulated oocytes of the frog Xenopus laevis, and recent evidence has revealed Musashi's capability to function as an activator of translation in mammalian systems. The molecular mechanism by which Musashi directs activation of target mRNAs has not been elucidated. Here, we report a specific association of Musashi with the noncanonical poly(A) polymerase germ line development defective-2 (GLD2) and map the association domain to 31 amino acids within the C-terminal domain of Musashi. We show that loss of GLD2 interaction through deletion of the binding domain or treatment with antisense oligonucleotides compromises Musashi function. Additionally, we demonstrate that overexpression of both Musashi and GLD2 significantly enhances Musashi function. Finally, we report a similar co-association also occurs between murine Musashi and GLD2 orthologs, suggesting that coupling of Musashi to the polyadenylation apparatus is a conserved mechanism to promote target mRNA translation.