ELAVL1 and ELAVL4 Are Required for Musashi-Dependent Translational Activation.

The RNA-binding proteins Musashi1 and Musashi2 (MSI1 and MSI2) regulate stem cell function and tissue plasticity by modulating mRNA translation. While typically known as translational repressors, the MSI1 and MSI2 proteins can also act as context-dependent activators of mRNA translation, although the mechanism of MSI-mediated translational activation is unknown. Here, we identify Embryonic Lethal Abnormal Vision-like (ELAVL) proteins as essential co-regulators of MSI1-dependent translational activation. In Xenopus laevis oocytes, antisense oligonucleotide knockdown of Elavl4 inhibited progesterone-stimulated maturation and blocked polyadenylation and translation of key MSI target mRNAs, including the Mos and Cyclin B5 mRNAs. Exogenous expression of ELAVL4 rescued these defects, confirming its necessity for maturation and cell cycle progression. Mechanistically, we determined that the ELAVL4 C-terminal domain interacts with the N-terminal RNA recognition motifs of MSI1 in an RNA-independent manner. Mass spectrometry and functional assays revealed this interaction is evolutionarily conserved: mouse ELAVL1 interacts with MSI1 in the pituitary, and human ELAVL1 rescues Elavl4-depleted Xenopus oocytes. Furthermore, knockdown of Elavl1 in a mammalian cell line abrogated MSI-dependent translational activation of a pituitary Prop1 3' UTR mRNA reporter. Our results establish a conserved mechanism where ELAVL family members interact with MSI to promote MSI-dependent mRNA translational activation.