The Xenopus ELAV protein ElrB represses Vg1 mRNA translation during oogenesis

Xenopus laevis Vg1 mRNA undergoes both localization and translational control during oogenesis. We previously characterized a 250-nucleotide AU-rich element, the Vg1 translation element (VTE), in the 3'-untranslated region (UTR) of this mRNA that is responsible for the translational repression. UV-cross-linking and immunoprecipitation experiments, described here, revealed that the known AU-rich element binding proteins, EIrA and EIrB, and TIA-1 and TIAR interact with the VTE. The levels of these proteins during oogenesis are most consistent with a possible role for EIrB in the translational control of Vg1 mRNA, and EIrB, in contrast to TIA-1 and TIAR, is present in large RNP complexes. Immunodepletion of TIA-1 and TIAR from Xenopus translation extract confirmed that these proteins are not involved in the translational repression. Mutagenesis of a potential EIrB binding site destroyed the ability of the VTE to bind EIrB and also abolished translational repression. Moreover, multiple copies of the consensus motif both bind EIrB and support translational control. Therefore, there is a direct correlation between EIrB binding and translational repression by the Vg1 3'-UTR. In agreement with the reporter data, injection of a monoclonal antibody against EIrB into Xenopus oocytes resulted in the production of Vg1 protein, arguing for a role for the ELAV proteins in the translational repression of Vg1 mRNA during early oogenesis.