Biochemistry Center, Heidelberg University, Heidelberg 69120, Germany.
Proc Natl Acad Sci U S A. 2023 Aug 8;120(32):e2304385120. doi: 10.1073/pnas.2304385120. Epub 2023 Jul 31.
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-α 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 1.6 Å resolution, which revealed its composition of a homodimerization D subdomain and a subdomain with similarity to a pseudo-HEAT-repeat analogous topology (PHAT) domain. Furthermore, we show that Smaug directly interacts with the germline inducer Oskar and with the Hedgehog signaling transducer Smoothened through its NTD. We determined the crystal structure of the NTD of Smaug in complex with a Smoothened α-helical peptide to 2.0 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 α-helix within the disordered region of Oskar binds to the NTD of Smaug in a mode similar to Smoothened. Together, our data uncover the NTD of Smaug as a peptide-binding domain.
希有蛋白及其同源物组成了一类 mRNA 抑制剂蛋白家族，在动物发育过程中具有多种功能。希有蛋白包含一个特征性的 RNA 结合的无活性α基序（SAM）结构域和一个保守但未被鉴定的 N 端结构域（NTD）。在此，我们解析了人 SAM 结构域蛋白 4A（SAMD4A，也称为 Smaug1）的 NTD 的晶体结构，分辨率为 1.6Å，揭示了其由同源二聚体 D 结构域和与假 HEAT 重复类似拓扑（PHAT）结构域相似的结构域组成。此外，我们表明希有蛋白通过其 NTD 直接与生殖系诱导因子 Oskar 和 Hedgehog 信号转导 Smoothened 相互作用。我们确定了 Smaug 的 NTD 与 Smoothened α-螺旋肽复合物的晶体结构，分辨率为 2.0Å。该肽结合在由 D 结构域和 PHAT 结构域形成的凹槽中。基于实验数据的结构建模表明，Oskar 无序区中的一个α-螺旋以类似于 Smoothened 的模式结合到 Smaug 的 NTD。总的来说，我们的数据揭示了 Smaug 的 NTD 是一个肽结合结构域。
