Molecular basis for GIGYF-Me31B complex assembly in 4EHP-mediated translational repression.

GIGYF (Grb10-interacting GYF [glycine-tyrosine-phenylalanine domain]) proteins coordinate with 4EHP (eIF4E [eukaryotic initiation factor 4E] homologous protein), the DEAD (Asp-Glu-Ala-Asp)-box helicase Me31B/DDX6, and mRNA-binding proteins to elicit transcript-specific repression. However, the underlying molecular mechanism remains unclear. Here, we report that GIGYF contains a motif necessary and sufficient for direct interaction with Me31B/DDX6. A 2.4 Å crystal structure of the GIGYF-Me31B complex reveals that this motif arranges into a coil connected to a ß hairpin on binding to conserved hydrophobic patches on the Me31B RecA2 domain. Structure-guided mutants indicate that 4EHP-GIGYF-DDX6 complex assembly is required for tristetraprolin-mediated down-regulation of an AU-rich mRNA, thus revealing the molecular principles of translational repression.

GIGYF1/2 proteins use auxiliary sequences to selectively bind to 4EHP and repress target mRNA expression.

The eIF4E homologous protein (4EHP) is thought to repress translation by competing with eIF4E for binding to the 5' cap structure of specific mRNAs to which it is recruited through interactions with various proteins, including the GRB10-interacting GYF (glycine-tyrosine-phenylalanine domain) proteins 1 and 2 (GIGYF1/2). Despite its similarity to eIF4E, 4EHP does not interact with eIF4G and therefore fails to initiate translation. In contrast to eIF4G, GIGYF1/2 bind selectively to 4EHP but not eIF4E. Here, we present crystal structures of the 4EHP-binding regions of GIGYF1 and GIGYF2 in complex with 4EHP, which reveal the molecular basis for the selectivity of the GIGYF1/2 proteins for 4EHP. Complementation assays in a GIGYF1/2-null cell line using structure-based mutants indicate that 4EHP requires interactions with GIGYF1/2 to down-regulate target mRNA expression. Our studies provide structural insights into the assembly of 4EHP-GIGYF1/2 repressor complexes and reveal that rather than merely facilitating 4EHP recruitment to transcripts, GIGYF1/2 proteins are required for repressive activity.

Direct role for the Drosophila GIGYF protein in 4EHP-mediated mRNA repression.

The eIF4E-homologous protein (4EHP) is a translational repressor that competes with eIF4E for binding to the 5'-cap structure of specific mRNAs, to which it is recruited by protein factors such as the GRB10-interacting GYF (glycine-tyrosine-phenylalanine domain) proteins (GIGYF). Several experimental evidences suggest that GIGYF proteins are not merely facilitating 4EHP recruitment to transcripts but are actually required for the repressor activity of the complex. However, the underlying molecular mechanism is unknown. Here, we investigated the role of the uncharacterized Drosophila melanogaster (Dm) GIGYF protein in post-transcriptional mRNA regulation. We show that, when in complex with 4EHP, Dm GIGYF not only elicits translational repression but also promotes target mRNA decay via the recruitment of additional effector proteins. We identified the RNA helicase Me31B/DDX6, the decapping activator HPat and the CCR4-NOT deadenylase complex as binding partners of GIGYF proteins. Recruitment of Me31B and HPat via discrete binding motifs conserved among metazoan GIGYF proteins is required for downregulation of mRNA expression by the 4EHP-GIGYF complex. Our findings are consistent with a model in which GIGYF proteins additionally recruit decapping and deadenylation complexes to 4EHP-containing RNPs to induce translational repression and degradation of mRNA targets.

Distinct modes of recruitment of the CCR4-NOT complex by Drosophila and vertebrate Nanos.

Nanos proteins repress the expression of target mRNAs by recruiting effector complexes through non-conserved N-terminal regions. In vertebrates, Nanos proteins interact with the NOT1 subunit of the CCR4-NOT effector complex through a NOT1 interacting motif (NIM), which is absent in Nanos orthologs from several invertebrate species. Therefore, it has remained unclear whether the Nanos repressive mechanism is conserved and whether it also involves direct interactions with the CCR4-NOT deadenylase complex in invertebrates. Here, we identify an effector domain (NED) that is necessary for the Drosophila melanogaster (Dm) Nanos to repress mRNA targets. The NED recruits the CCR4-NOT complex through multiple and redundant binding sites, including a central region that interacts with the NOT module, which comprises the C-terminal domains of NOT1-3. The crystal structure of the NED central region bound to the NOT module reveals an unanticipated bipartite binding interface that contacts NOT1 and NOT3 and is distinct from the NIM of vertebrate Nanos. Thus, despite the absence of sequence conservation, the N-terminal regions of Nanos proteins recruit CCR4-NOT to assemble analogous repressive complexes.

miRISC and the CCR4-NOT complex silence mRNA targets independently of 43S ribosomal scanning.

miRNAs associate with Argonaute (AGO) proteins to silence the expression of mRNA targets by inhibiting translation and promoting deadenylation, decapping, and mRNA degradation. A current model for silencing suggests that AGOs mediate these effects through the sequential recruitment of GW182 proteins, the CCR4-NOT deadenylase complex and the translational repressor and decapping activator DDX6. An alternative model posits that AGOs repress translation by interfering with eIF4A function during 43S ribosomal scanning and that this mechanism is independent of GW182 and the CCR4-NOT complex in Drosophila melanogaster Here, we show that miRNAs, AGOs, GW182, the CCR4-NOT complex, and DDX6/Me31B repress and degrade polyadenylated mRNA targets that are translated via scanning-independent mechanisms in both human and Dm cells. This and additional observations indicate a common mechanism used by these proteins and miRNAs to mediate silencing. This mechanism does not require eIF4A function during ribosomal scanning.

A C-terminal silencing domain in GW182 is essential for miRNA function.

Proteins of the GW182 family are essential for miRNA-mediated gene silencing in animal cells; they interact with Argonaute proteins (AGOs) and are required for both the translational repression and mRNA degradation mediated by miRNAs. To gain insight into the role of the GW182-AGO1 interaction in silencing, we generated protein mutants that do not interact and tested them in complementation assays. We show that silencing of miRNA targets requires the N-terminal domain of GW182, which interacts with AGO1 through multiple glycine-tryptophan (GW)-repeats. Indeed, a GW182 mutant that does not interact with AGO1 cannot rescue silencing in cells depleted of endogenous GW182. Conversely, silencing is impaired by mutations in AGO1 that strongly reduce the interaction with GW182 but not with miRNAs. We further show that a GW182 mutant that does not localize to P-bodies but interacts with AGO1 rescues silencing in GW182-depleted cells, even though in these cells, AGO1 also fails to localize to P-bodies. Finally, we show that in addition to the N-terminal AGO1-binding domain, the middle and C-terminal regions of GW182 (referred to as the bipartite silencing domain) are essential for silencing. Together our results indicate that miRNA silencing in animal cells is mediated by AGO1 in complex with GW182, and that P-body localization is not required for silencing.

The C-terminal region of Ge-1 presents conserved structural features required for P-body localization.

The removal of the 5' cap structure by the DCP1-DCP2 decapping complex irreversibly commits eukaryotic mRNAs to degradation. In human cells, the interaction between DCP1 and DCP2 is bridged by the Ge-1 protein. Ge-1 contains an N-terminal WD40-repeat domain connected by a low-complexity region to a conserved C-terminal domain. It was reported that the C-terminal domain interacts with DCP2 and mediates Ge-1 oligomerization and P-body localization. To understand the molecular basis for these functions, we determined the three-dimensional crystal structure of the most conserved region of the Drosophila melanogaster Ge-1 C-terminal domain. The region adopts an all alpha-helical fold related to ARM- and HEAT-repeat proteins. Using structure-based mutants we identified an invariant surface residue affecting P-body localization. The conservation of critical surface and structural residues suggests that the C-terminal region adopts a similar fold with conserved functions in all members of the Ge-1 protein family.

Similar modes of interaction enable Trailer Hitch and EDC3 to associate with DCP1 and Me31B in distinct protein complexes.

Trailer Hitch (Tral or LSm15) and enhancer of decapping-3 (EDC3 or LSm16) are conserved eukaryotic members of the (L)Sm (Sm and Like-Sm) protein family. They have a similar domain organization, characterized by an N-terminal LSm domain and a central FDF motif; however, in Tral, the FDF motif is flanked by regions rich in charged residues, whereas in EDC3 the FDF motif is followed by a YjeF_N domain. We show that in Drosophila cells, Tral and EDC3 specifically interact with the decapping activator DCP1 and the DEAD-box helicase Me31B. Nevertheless, only Tral associates with the translational repressor CUP, whereas EDC3 associates with the decapping enzyme DCP2. Like EDC3, Tral interacts with DCP1 and localizes to mRNA processing bodies (P bodies) via the LSm domain. This domain remains monomeric in solution and adopts a divergent Sm fold that lacks the characteristic N-terminal alpha-helix, as determined by nuclear magnetic resonance analyses. Mutational analysis revealed that the structural integrity of the LSm domain is required for Tral both to interact with DCP1 and CUP and to localize to P-bodies. Furthermore, both Tral and EDC3 interact with the C-terminal RecA-like domain of Me31B through their FDF motifs. Together with previous studies, our results show that Tral and EDC3 are structurally related and use a similar mode to associate with common partners in distinct protein complexes.

A divergent Sm fold in EDC3 proteins mediates DCP1 binding and P-body targeting.

Members of the (L)Sm (Sm and Sm-like) protein family are found across all kingdoms of life and play crucial roles in RNA metabolism. The P-body component EDC3 (enhancer of decapping 3) is a divergent member of this family that functions in mRNA decapping. EDC3 is composed of a N-terminal LSm domain, a central FDF domain, and a C-terminal YjeF-N domain. We show that this modular architecture enables EDC3 to interact with multiple components of the decapping machinery, including DCP1, DCP2, and Me31B. The LSm domain mediates DCP1 binding and P-body localization. We determined the three-dimensional structures of the LSm domains of Drosophila melanogaster and human EDC3 and show that the domain adopts a divergent Sm fold that lacks the characteristic N-terminal alpha-helix and has a disrupted beta4-strand. This domain remains monomeric in solution and lacks several features that canonical (L)Sm domains require for binding RNA. The structures also revealed a conserved patch of surface residues that are required for the interaction with DCP1 but not for P-body localization. The conservation of surface and of critical structural residues indicates that LSm domains in EDC3 proteins adopt a similar fold that has separable novel functions that are absent in canonical (L)Sm proteins.

Hormonal regulation of mummy is needed for apical extracellular matrix formation and epithelial morphogenesis in Drosophila.

Many epithelia produce apical extracellular matrices (aECM) that are crucial for organ morphogenesis or physiology. Apical ECM formation relies on coordinated synthesis and modification of constituting components, to enable their subcellular targeting and extracellular assembly into functional matrices. The exoskeleton of Drosophila, the cuticle, is a stratified aECM containing ordered chitin polysaccharide lamellae and proteinaceous layers, and is suited for studies of molecular functions needed for aECM assembly. Here, we show that Drosophila mummy (mmy) mutants display defects in epithelial organisation in conjunction with aberrant deposition of the cuticle and an apical matrix needed for tracheal tubulogenesis. We find that mmy encodes the UDP-N-acetylglucosamine pyrophosphorylase, which catalyses the production of UDP-N-acetylglucosamine, an obligate substrate for chitin synthases as well as for protein glycosylation and GPI-anchor formation. Consequently, in mmy mutants GlcNAc-groups including chitin are severely reduced and modification and subcellular localisation of proteins designated for extracellular space is defective. Moreover, mmy expression is selectively upregulated in epithelia at the time they actively deposit aECM, and is altered by the moulting hormone 20-Hydroxyecdysone, suggesting that mmy is part of a developmental genetic programme to promote aECM formation.

Drosophila Knickkopf and Retroactive are needed for epithelial tube growth and cuticle differentiation through their specific requirement for chitin filament organization.

Precise epithelial tube diameters rely on coordinated cell shape changes and apical membrane enlargement during tube growth. Uniform tube expansion in the developing Drosophila trachea requires the assembly of a transient intraluminal chitin matrix, where chitin forms a broad cable that expands in accordance with lumen diameter growth. Like the chitinous procuticle, the tracheal luminal chitin cable displays a filamentous structure that presumably is important for matrix function. Here, we show that knickkopf (knk) and retroactive (rtv) are two new tube expansion mutants that fail to form filamentous chitin structures, both in the tracheal and cuticular chitin matrices. Mutations in knk and rtv are known to disrupt the embryonic cuticle, and our combined genetic analysis and chemical chitin inhibition experiments support the argument that Knk and Rtv specifically assist in chitin function. We show that Knk is an apical GPI-linked protein that acts at the plasma membrane. Subcellular mislocalization of Knk in previously identified tube expansion mutants that disrupt septate junction (SJ) proteins, further suggest that SJs promote chitinous matrix organization and uniform tube expansion by supporting polarized epithelial protein localization. We propose a model in which Knk and the predicted chitin-binding protein Rtv form membrane complexes essential for epithelial tubulogenesis and cuticle formation through their specific role in directing chitin filament assembly.