Smaug protein is involved with RNA-binding and translation inhibition. Specifically, Smaug is involved in anterior-posterior segmentation of the embryo during Drosophila melanogaster embryonic development. Smaug also plays a role in the midblastula transition of D. melanogaster development. 1 It is localized throughout the cytoplasm and pole plasm at the syncitial blastoderm stage. Later, during cellularization, it is most concentrated at the posterior pole. 2.
Function
Most eukaryotes rely on maternal mRNA in early development because they synthesize the proteins that control the primary events of embryogenesis (cell fate decisions, axis determination, etc.). Because maternal mRNAs are already present at the time of fertilization, their subsequent expression must be restricted post-transcriptionally 3.
Smaug protein functions to regulate the posterior pole of the embryo by inhibiting translation of maternal nanos mRNA 1. Smaug effectively performs this task by binding to the 3'UTR region of the nanos mRNA via its sterile alpha motif (SAM) domain 2 This prevents nanos from passively diffusing to the posterior cortex of the embryo and undergoing translation. Smaug protein's mechanism of action involves the recruitment of another protein called CUP up binding to the 3'UTR region of nanos mRNA. Together, they prevent the association with a ribosome that would trigger translation. The nanos-Smaug-CUP complex can be undone, however, if the complex reaches the posterior cortex. In this case, the complex will be disassembled, which allows the mRNA to prepare its message for translation. In addition, Smaug is responsible for marking maternal mRNA for destruction in the mid-blastula transition. During this time, the embryo should begin a concomitant process called the maternal-to-zygotic transition. This process involves the degradation of maternal mRNA in favor of an increase in production of mRNA by the zygote 1.
Disease
Smaug is coded for by maternal mRNA, and its levels peak around the same time that the zygote can efficiently transcribe its genome. For maternal Smaug mutants, the mid-blastula and maternal-to-zygotic transitions will be hindered, and the zygotic genome transcription will ultimately be thwarted 1.
Localization of nanos mRNA to the posterior cortex of the embryo is essential for proper development. Nanos protein is responsible for specifying the formation of the abdomen. If it is allowed to build up in the anterior cortex of the embryo, not only will it begin to specify abdominal precursors, but it will also suppress the development of the thorax and head of the embryo 3.
About this Structure
Smaug protein is relatively large, weighing in at 999 amino acids. However, only about 180 amino acids within the structure are responsible for its RNA-binding activity. This domain is known as the SAM domain, which is mentioned above. The SAM domain is found in conjunction with a pseudo-HEAT repeat analogous topology (PHAT) domain. Studies show that the SAM domain is the primary interactive portion of the protein that interacts specifically with the binding region (TCE) of maternal nanos mRNA. Thus, SAM domains are pivotal in RNA binding.
Smaug protein’s secondary structure consists primarily of alpha helices and turns. The ribosome binding bdomain (RBD) has an upper (SAM; helices α1-α4 and h2, residues 596–657) and lower (PHAT; helices α7-α13, residues 658–764). The two are associated through helices α5 and α6, respectively.
the globular Smg SAM domain houses seven lysines and two arginines, several of which are condensed over one face of the domain. This makes up a “patch” of electropositive potential. The patch is seen over helices α1, h2, and α5, and expanses the PHAT domain interface to the N-terminus of α5. The electropositive nature of the Smaug SAM domain makes it unique among other proteins studied thus far. The PHAT domain is made up of of three layered parallel helices (α6, α9, and α13) in conjunction with two layered antiparallel helices (α7 and α11) into five-helix cylinder. This contrasts classical four-helix bundles in that it does not contain the usual alternating parallel and antiparallel helices. Unlike the SAM domain, most of the PHAT surface domain is electronegative.
4
1oxj is a 1 chain structure with sequence from D. melanogaster. Full crystallographic information is available from OCA.
