eIF4B mRNA Translation Contributes to Cleavage Dynamics in Early Sea Urchin Embryos.

During the first steps of sea urchin development, fertilization elicits a marked increase in protein synthesis essential for subsequent cell divisions. While the translation of mitotic cyclin mRNAs is crucial, we hypothesized that additional mRNAs must be translated to finely regulate the onset into mitosis. One of the maternal mRNAs recruited onto active polysomes at this stage codes for the initiation factor eIF4B. Here, we show that the sea urchin eIF4B orthologs present the four specific domains essential for eIF4B function and that Paracentrotus lividus eIF4B copurifies with eIF4E in a heterologous system. In addition, we investigated the role of eIF4B mRNA de novo translation during the two first embryonic divisions of two species, P. lividus and Sphaerechinus granularis . Our results show that injection of a morpholino directed against eIF4B mRNA results in a downregulation of translational activity and delays cell division in these two echinoids. Conversely, injection of an mRNA encoding for P. lividus eIF4B stimulates translation and significantly accelerates cleavage rates. Taken together, our findings suggest that eIF4B mRNA de novo translation participates in a conserved regulatory loop that contributes to orchestrating protein synthesis and modulates cell division rhythm during early sea urchin development.