Recruitment of Nanos to hunchback mRNA by Pumilio

Translational regulation of hunchback (hb) mRNA is essential for posterior patterning of the Drosophila embryo. This regulation is mediated by sequenc es in the 3'-untranslated region of hb mRNA (the Nanos response elements or NREs), as well as two transacting factors-Nanos and Pumilio. Pumilio recog nizes the NREs via a conserved binding motif. The mechanism of Nanos action has not been clear. In this report we use protein-protein and protein-RNA interaction assays in yeast and in vitro to show that Nanos forms a ternary complex with the RNA-binding domain of Pumilio and the NRE. Mutant forms o f the NRE, Nos, and Pum that do not regulate hb mRNA normally in embryos do not assemble normally into a ternary complex. In particular, recruitment o f Nos is dependent on bases in the center of the NRE, on the carboxy-termin al Cys/His domain of Nos, and on residues in the eighth repeat of the Pum R NA-binding domain. These residues differ in a closely related human protein that also binds to the NRE but cannot recruit Drosophila Nos. Taken togeth er, these findings suggest models for how Nos and Pum collaboratively targe t hb mRNA. More generally, they suggest that. Pum-like proteins from other species may also act by recruiting cofactors to regulate translation.