Structure of Pumilio reveals similarity between RNA and peptide binding motifs

Translation regulation plays an essential role in the differentiation and d evelopment of animal cells. One well-studied case is the control of hunchba ck mRNA during early Drosophila embryogenesis by the transacting factors Pu milio, Nanos, and Brain Tumor. We report here a crystal structure of the cr itical region of Pumilio, the Puf domain, that organizes a multivalent repr ession complex on the 3' untranslated region of hunchback mRNA. The structu re reveals an extended, rainbow shaped molecule, with tandem helical repeat s that bear unexpected resemblance to the armadillo repeats in beta -cateni n and the HEAT repeats in protein phosphatase 2A. Based on the structure an d genetic experiments, we identify putative interaction surfaces for hunchb ack mRNA and the cofactors Nanos and Brain Tumor. This analysis suggests th at similar features in helical repeat proteins are used to bind extended pe ptides and RNA.