Structural basis for binding of Smaug to the GPCR Smoothened and to the germline inducer Oskar

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- 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 [A] 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 -helical peptide to 1.61 [A] 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 -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.