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

Abstract

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 Dsubdomain 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.