%0 Journal Article %A Jana Kubikova %A Gabriele Ubartaite %A Jutta Metz %A Mandy Jeske %T Structural basis for binding of Smaug to the GPCR Smoothened and to the germline inducer Oskar %D 2023 %R 10.1101/2023.02.19.529116 %J bioRxiv %P 2023.02.19.529116 %X Drosophila Smaug and its orthologs comprise a family of mRNA repressor proteins that exhibit various functions during animal development. Smaug proteins contain a characteristic RNA-binding sterile-a motif (SAM) domain and a conserved but uncharacterized N-terminal domain (NTD). Here, we resolved the crystal structure of the NTD of the human SAM domain-containing protein 4A (SAMD4A, a.k.a. Smaug1) to 2.0 Å resolution, which revealed its composition of a homodimerization D-subdomain and a subdomain with similarity to a PHAT domain. Furthermore, we show that Drosophila Smaug directly interacts with the Drosophila germline inducer Oskar and with the Hedgehog signaling transducer Smoothened through its D-PHAT domain. We determined the crystal structure of the D-PHAT domain of Smaug in complex with a Smoothened a-helical peptide to 1.61 Å resolution. The peptide binds within a groove that is formed by both the D- and PHAT subdomains. Structural modeling supported by experimental data suggested that an a-helix within the disordered region of Oskar binds to the D-PHAT domain in a mode similar to Smoothened. Together, our data uncover the N-terminal D-PHAT domain of Smaug as peptide-binding domain.Competing Interest StatementThe authors have declared no competing interest. %U https://www.biorxiv.org/content/biorxiv/early/2023/02/19/2023.02.19.529116.full.pdf