Evolutionary analyses of intrinsically disordered regions reveal widespread signals of conservation.

Intrinsically disordered regions (IDRs) are segments of proteins without stable three-dimensional structures. As this flexibility allows them to interact with diverse binding partners, IDRs play key roles in cell signaling and gene expression. Despite the prevalence and importance of IDRs in eukaryotic proteomes and various biological processes, associating them with specific molecular functions remains a significant challenge due to their high rates of sequence evolution. However, by comparing the observed values of various IDR-associated properties against those generated under a simulated model of evolution, a recent study found most IDRs across the entire yeast proteome contain conserved features. Furthermore, it showed clusters of IDRs with common "evolutionary signatures," i.e. patterns of conserved features, were associated with specific biological functions. To determine if similar patterns of conservation are found in the IDRs of other systems, in this work we applied a series of phylogenetic models to over 7,500 orthologous IDRs identified in the Drosophila genome to dissect the forces driving their evolution. By comparing models of constrained and unconstrained continuous trait evolution using the Brownian motion and Ornstein-Uhlenbeck models, respectively, we identified signals of widespread constraint, indicating conservation of distributed features is mechanism of IDR evolution common to multiple biological systems. In contrast to the previous study in yeast, however, we observed limited evidence of IDR clusters with specific biological functions, which suggests a more complex relationship between evolutionary constraints and function in the IDRs of multicellular organisms.

The polarity protein Yurt associates with the plasma membrane via basic and hydrophobic motifs embedded in its FERM domain.

The subcellular distribution of the polarity protein Yurt (Yrt) is subjected to a spatio-temporal regulation in Drosophila melanogaster embryonic epithelia. After cellularization, Yrt binds to the lateral membrane of ectodermal cells and maintains this localization throughout embryogenesis. During terminal differentiation of the epidermis, Yrt accumulates at septate junctions and is also recruited to the apical domain. Although the mechanisms through which Yrt associates with septate junctions and the apical domain have been deciphered, how Yrt binds to the lateral membrane remains as an outstanding puzzle. Here, we show that the FERM domain of Yrt is necessary and sufficient for membrane localization. Our data also establish that the FERM domain of Yrt directly binds negatively charged phospholipids. Moreover, we demonstrate that positively charged amino acid motifs embedded within the FERM domain mediates Yrt membrane association. Finally, we provide evidence suggesting that Yrt membrane association is functionally important. Overall, our study highlights the molecular basis of how Yrt associates with the lateral membrane during the developmental time window where it is required for segregation of lateral and apical domains.

Key arginine residues in R2D2 dsRBD1 and dsRBD2 lead the siRNA recognition in Drosophila melanogaster RNAi pathway.

In Drosophila melanogaster, Dcr-2:R2D2 heterodimer binds to the 21 nucleotide siRNA duplex to form the R2D2/Dcr-2 Initiator (RDI) complex, which is critical for the initiation of siRNA-induced silencing complex (RISC) assembly. During RDI complex formation, R2D2, a protein that contains three dsRNA binding domains (dsRBD), senses two aspects of the siRNA: thermodynamically more stable end (asymmetry sensing) and the 5'-phosphate (5'-P) recognition. Despite several detailed studies to date, the molecular determinants arising from R2D2 for performing these two tasks remain elusive. In this study, we have performed structural, biophysical, and biochemical characterization of R2D2 dsRBDs. We found that the solution NMR-derived structure of R2D2 dsRBD1 yielded a canonical α1-ß1-ß2-ß3-α2 fold, wherein two arginine salt bridges provide additional stability to the R2D2 dsRBD1. Furthermore, we show that R2D2 dsRBD1 interacts with thermodynamically asymmetric siRNA duplex independent of its 5'-phosphorylation state, whereas R2D2 dsRBD2 prefers to interact with 5'-P siRNA duplex. The mutation of key arginine residues, R53 and R101, in concatenated dsRBDs of R2D2 results in a significant loss of siRNA duplex recognition. Our study deciphers the active roles of R2D2 dsRBDs by showing that dsRBD1 initiates siRNA recognition, whereas dsRBD2 senses 5'-phosphate as an authentic mark on functional siRNA.

Structural basis for sugar perception by Drosophila gustatory receptors.

Insects rely on a family of seven transmembrane proteins called gustatory receptors (GRs) to encode different taste modalities, such as sweet and bitter. We report structures of Drosophila sweet taste receptors GR43a and GR64a in the apo and sugar-bound states. Both GRs form tetrameric sugar-gated cation channels composed of one central pore domain (PD) and four peripheral ligand-binding domains (LBDs). Whereas GR43a is specifically activated by the monosaccharide fructose that binds to a narrow pocket in LBDs, disaccharides sucrose and maltose selectively activate GR64a by binding to a larger and flatter pocket in LBDs. Sugar binding to LBDs induces local conformational changes, which are subsequently transferred to the PD to cause channel opening. Our studies reveal a structural basis for sugar recognition and activation of GRs.

The origin recognition complex requires chromatin tethering by a hypervariable intrinsically disordered region that is functionally conserved from sponge to man.

The first step toward eukaryotic genome duplication is loading of the replicative helicase onto chromatin. This 'licensing' step initiates with the recruitment of the origin recognition complex (ORC) to chromatin, which is thought to occur via ORC's ATP-dependent DNA binding and encirclement activity. However, we have previously shown that ATP binding is dispensable for the chromatin recruitment of fly ORC, raising the question of how metazoan ORC binds chromosomes. We show here that the intrinsically disordered region (IDR) of fly Orc1 is both necessary and sufficient for recruitment of ORC to chromosomes in vivo and demonstrate that this is regulated by IDR phosphorylation. Consistently, we find that the IDR confers the ORC holocomplex with ATP-independent DNA binding activity in vitro. Using phylogenetic analysis, we make the surprising observation that metazoan Orc1 IDRs have diverged so markedly that they are unrecognizable as orthologs and yet we find that these compositionally homologous sequences are functionally conserved. Altogether, these data suggest that chromatin is recalcitrant to ORC's ATP-dependent DNA binding activity, necessitating IDR-dependent chromatin tethering, which we propose poises ORC to opportunistically encircle nucleosome-free regions as they become available.

Random, de novo, and conserved proteins: How structure and disorder predictors perform differently.

Understanding the emergence and structural characteristics of de novo and random proteins is crucial for unraveling protein evolution and designing novel enzymes. However, experimental determination of their structures remains challenging. Recent advancements in protein structure prediction, particularly with AlphaFold2 (AF2), have expanded our knowledge of protein structures, but their applicability to de novo and random proteins is unclear. In this study, we investigate the structural predictions and confidence scores of AF2 and protein language model-based predictor ESMFold for de novo and conserved proteins from Drosophila and a dataset of comparable random proteins. We find that the structural predictions for de novo and random proteins differ significantly from conserved proteins. Interestingly, a positive correlation between disorder and confidence scores (pLDDT) is observed for de novo and random proteins, in contrast to the negative correlation observed for conserved proteins. Furthermore, the performance of structure predictors for de novo and random proteins is hampered by the lack of sequence identity. We also observe fluctuating median predicted disorder among different sequence length quartiles for random proteins, suggesting an influence of sequence length on disorder predictions. In conclusion, while structure predictors provide initial insights into the structural composition of de novo and random proteins, their accuracy and applicability to such proteins remain limited. Experimental determination of their structures is necessary for a comprehensive understanding. The positive correlation between disorder and pLDDT could imply a potential for conditional folding and transient binding interactions of de novo and random proteins.

Chemical shift assignments of dsRBD1 and linker region of R2D2, a siRNA binding protein in the Drosophila RNAi pathway.

In the model organism Drosophila melanogaster, one of the Dicer homologs, Dcr-2, initiates the RNA interference pathway by cleaving long double-stranded RNA into small interfering RNA (siRNA). The Dcr-2:R2D2 heterodimer subsequently binds to the 21-nucleotide siRNA to form the R2D2:Dcr-2 Initiator (RDI) complex, which is critical for initiating the assembly of the RNA-induced silencing complex containing guide siRNA strand. During RDI complex formation, R2D2 senses the stability of the 5' end of the siRNA and a 5'-phosphate group, although the underlying mechanism of siRNA asymmetry sensing and 5'-phosphate recognition by R2D2 is elusive. In this study, we present nearly complete chemical shift assignments of the backbone and the side chain of a construct that comprises the N-terminus dsRBD1 and linker of R2D2 (~ 10.3 kDa; henceforth: R2D2D1L). Our study would further aid in the structural and functional characterization of R2D2.

Widespread regulatory specificities between transcriptional co-repressors and enhancers in Drosophila.

Gene expression is controlled by the precise activation and repression of transcription. Repression is mediated by specialized transcription factors (TFs) that recruit co-repressors (CoRs) to silence transcription, even in the presence of activating cues. However, whether CoRs can dominantly silence all enhancers or display distinct specificities is unclear. In this work, we report that most enhancers in Drosophila can be repressed by only a subset of CoRs, and enhancers classified by CoR sensitivity show distinct chromatin features, function, TF motifs, and binding. Distinct TF motifs render enhancers more resistant or sensitive to specific CoRs, as we demonstrate by motif mutagenesis and addition. These CoR-enhancer compatibilities constitute an additional layer of regulatory specificity that allows differential regulation at close genomic distances and is indicative of distinct mechanisms of transcriptional repression.

Drosophila class-I myosins that can impact left-right asymmetry have distinct ATPase kinetics.

Myosin-1D (myo1D) is important for Drosophila left-right asymmetry, and its effects are modulated by myosin-1C (myo1C). De novo expression of these myosins in nonchiral Drosophila tissues promotes cell and tissue chirality, with handedness depending on the paralog expressed. Remarkably, the identity of the motor domain determines the direction of organ chirality, rather than the regulatory or tail domains. Myo1D, but not myo1C, propels actin filaments in leftward circles in in vitro experiments, but it is not known if this property contributes to establishing cell and organ chirality. To further explore if there are differences in the mechanochemistry of these motors, we determined the ATPase mechanisms of myo1C and myo1D. We found that myo1D has a 12.5-fold higher actin-activated steady-state ATPase rate, and transient kinetic experiments revealed myo1D has an 8-fold higher MgADP release rate compared to myo1C. Actin-activated phosphate release is rate limiting for myo1C, whereas MgADP release is the rate-limiting step for myo1D. Notably, both myosins have among the tightest MgADP affinities measured for any myosin. Consistent with ATPase kinetics, myo1D propels actin filaments at higher speeds compared to myo1C in in vitro gliding assays. Finally, we tested the ability of both paralogs to transport 50 nm unilamellar vesicles along immobilized actin filaments and found robust transport by myo1D and actin binding but no transport by myo1C. Our findings support a model where myo1C is a slow transporter with long-lived actin attachments, whereas myo1D has kinetic properties associated with a transport motor.

Paracatalytic induction: Subverting specificity in hedgehog protein autoprocessing with small molecules.

Paracatalytic inducers are antagonists that shift the specificity of biological catalysts, resulting in non-native transformations. In this Chapter we describe methods to discover paracatalytic inducers of Hedgehog (Hh) protein autoprocessing. Native autoprocessing uses cholesterol as a substrate nucleophile to assist in cleaving an internal peptide bond within a precursor form of Hh. This unusual reaction is brought about by HhC, an enzymatic domain that resides within the C-terminal region of Hh precursor proteins. Recently, we reported paracatalytic inducers as a novel class of Hh autoprocessing antagonists. These small molecules bind HhC and tilt the substrate specificity away from cholesterol in favor of solvent water. The resulting cholesterol-independent autoproteolysis of the Hh precursor generates a non-native Hh side product with substantially reduced biological signaling activity. Protocols are provided for in vitro FRET-based and in-cell bioluminescence assays to discover and characterize paracatalytic inducers of Drosophila and human hedgehog protein autoprocessing, respectively.

Cryptochrome-Timeless structure reveals circadian clock timing mechanisms.

Circadian rhythms influence many behaviours and diseases1,2. They arise from oscillations in gene expression caused by repressor proteins that directly inhibit transcription of their own genes. The fly circadian clock offers a valuable model for studying these processes, wherein Timeless (Tim) plays a critical role in mediating nuclear entry of the transcriptional repressor Period (Per) and the photoreceptor Cryptochrome (Cry) entrains the clock by triggering Tim degradation in light2,3. Here, through cryogenic electron microscopy of the Cry-Tim complex, we show how a light-sensing cryptochrome recognizes its target. Cry engages a continuous core of amino-terminal Tim armadillo repeats, resembling how photolyases recognize damaged DNA, and binds a C-terminal Tim helix, reminiscent of the interactions between light-insensitive cryptochromes and their partners in mammals. The structure highlights how the Cry flavin cofactor undergoes conformational changes that couple to large-scale rearrangements at the molecular interface, and how a phosphorylated segment in Tim may impact clock period by regulating the binding of Importin-α and the nuclear import of Tim-Per4,5. Moreover, the structure reveals that the N terminus of Tim inserts into the restructured Cry pocket to replace the autoinhibitory C-terminal tail released by light, thereby providing a possible explanation for how the long-short Tim polymorphism adapts flies to different climates6,7.

PK1 from Drosophila obscurin is an inactive pseudokinase with scaffolding properties.

Obscurins are large filamentous proteins with crucial roles in the assembly, stability and regulation of muscle. Characteristic of these proteins is a tandem of two C-terminal kinase domains, PK1 and PK2, that are separated by a long intrinsically disordered sequence. The significance of this conserved domain arrangement is unknown. Our study of PK1 from Drosophila obscurin shows that this is a pseudokinase with features typical of the CAM-kinase family, but which carries a minimalistic regulatory tail that no longer binds calmodulin or has mechanosensory properties typical of other sarcomeric kinases. PK1 binds ATP with high affinity, but in the absence of magnesium and lacks detectable phosphotransfer activity. It also has a highly diverged active site, strictly conserved across arthropods, that might have evolved to accommodate an unconventional binder. We find that PK1 interacts with PK2, suggesting a functional relation to the latter. These findings lead us to speculate that PK1/PK2 form a pseudokinase/kinase dual system, where PK1 might act as an allosteric regulator of PK2 and where mechanosensing properties, akin to those described for regulatory tails in titin-like kinases, might now reside on the unstructured interkinase segment. We propose that the PK1-interkinase-PK2 region constitutes an integrated functional unit in obscurin proteins.

Disordered proteins mitigate the temperature dependence of site-specific binding free energies.

Biophysical characterization of protein-protein interactions involving disordered proteins is challenging. A common simplification is to measure the thermodynamics and kinetics of disordered site binding using peptides containing only the minimum residues necessary. We should not assume, however, that these few residues tell the whole story. Son of sevenless, a multidomain signaling protein from Drosophila melanogaster, is critical to the mitogen-activated protein kinase pathway, passing an external signal to Ras, which leads to cellular responses. The disordered 55 kDa C-terminal domain of Son of sevenless is an autoinhibitor that blocks guanidine exchange factor activity. Activation requires another protein, Downstream of receptor kinase (Drk), which contains two Src homology 3 domains. Here, we utilized NMR spectroscopy and isothermal titration calorimetry to quantify the thermodynamics and kinetics of the N-terminal Src homology 3 domain binding to the strongest sites incorporated into the flanking disordered sequences. Comparing these results to those for isolated peptides provides information about how the larger domain affects binding. The affinities of sites on the disordered domain are like those of the peptides at low temperatures but less sensitive to temperature. Our results, combined with observations showing that intrinsically disordered proteins become more compact with increasing temperature, suggest a mechanism for this effect.

Micro-electron diffraction structure of the aggregation-driving N terminus of Drosophila neuronal protein Orb2A reveals amyloid-like ß-sheets.

Amyloid protein aggregation is commonly associated with progressive neurodegenerative diseases, however not all amyloid fibrils are pathogenic. The neuronal cytoplasmic polyadenylation element binding protein is a regulator of synaptic mRNA translation and has been shown to form functional amyloid aggregates that stabilize long-term memory. In adult Drosophila neurons, the cytoplasmic polyadenylation element binding homolog Orb2 is expressed as 2 isoforms, of which the Orb2B isoform is far more abundant, but the rarer Orb2A isoform is required to initiate Orb2 aggregation. The N terminus is a distinctive feature of the Orb2A isoform and is critical for its aggregation. Intriguingly, replacement of phenylalanine in the fifth position of Orb2A with tyrosine (F5Y) in Drosophila impairs stabilization of long-term memory. The structure of endogenous Orb2B fibers was recently determined by cryo-EM, but the structure adopted by fibrillar Orb2A is less certain. Here we use micro-electron diffraction to determine the structure of the first 9 N-terminal residues of Orb2A, at a resolution of 1.05 Å. We find that this segment (which we term M9I) forms an amyloid-like array of parallel in-register ß-sheets, which interact through side chain interdigitation of aromatic and hydrophobic residues. Our structure provides an explanation for the decreased aggregation observed for the F5Y mutant and offers a hypothesis for how the addition of a single atom (the tyrosyl oxygen) affects long-term memory. We also propose a structural model of Orb2A that integrates our structure of the M9I segment with the published Orb2B cryo-EM structure.

A conserved domain of Drosophila RNA-binding protein Pumilio interacts with multiple CCR4-NOT deadenylase complex subunits to repress target mRNAs.

Pumilio is a sequence-specific RNA-binding protein that controls development, stem cell fate, and neurological functions in Drosophila. Pumilio represses protein expression by destabilizing target mRNAs in a manner dependent on the CCR4-NOT deadenylase complex. Three unique repression domains in the N-terminal region of Pumilio were previously shown to recruit CCR4-NOT, but how they do so was not well understood. In this study, we identified the motifs that are necessary and sufficient for the activity of the third repression domain of Pumilio, designated RD3, which is present in all isoforms and has conserved regulatory function. We identified multiple conserved regions of RD3 that are important for repression activity in cell-based reporter gene assays. Using yeast two-hybrid assays, we show that RD3 contacts specific regions of the Not1, Not2, and Not3 subunits of the CCR4-NOT complex. Our results indicate that RD3 makes multivalent interactions with CCR4-NOT mediated by conserved short linear interaction motifs. Specifically, two phenylalanine residues in RD3 make crucial contacts with Not1 that are essential for its repression activity. Using reporter gene assays, we also identify three new target mRNAs that are repressed by Pumilio and show that RD3 contributes to their regulation. Together, these results provide important insights into the mechanism by which Pumilio recruits CCR4-NOT to regulate the expression of target mRNAs.

Computational Assessment of Protein-Protein Binding Specificity within a Family of Synaptic Surface Receptors.

Atomic-level information is essential to explain the formation of specific protein complexes in terms of structure and dynamics. The set of Dpr and DIP proteins, which play a key role in the neuromorphogenesis in the nervous system of Drosophila melanogaster, offer a rich paradigm to learn about protein-protein recognition. Many members of the DIP subfamily cross-react with several members of the Dpr family and vice versa. While there exists a total of 231 possible Dpr-DIP heterodimer complexes from the 21 Dpr and 11 DIP proteins, only 57 "cognate" pairs have been detected by surface plasmon resonance (SPR) experiments, suggesting that the remaining 174 pairs have low or unreliable binding affinity. Our goal is to assess the performance of computational approaches to characterize the global set of interactions between Dpr and DIP proteins and identify the specificity of binding between each DIP with their corresponding Dpr binding partners. In addition, we aim to characterize how mutations influence the specificity of the binding interaction. In this work, a wide range of knowledge-based and physics-based approaches are utilized, including mutual information, linear discriminant analysis, homology modeling, molecular dynamics simulations, Poisson-Boltzmann continuum electrostatics calculations, and alchemical free energy perturbation to decipher the origin of binding specificity of the Dpr-DIP complexes examined. Ultimately, the results show that those two broad strategies are complementary, with different strengths and limitations. Biological inter-relations are more clearly revealed through knowledge-based approaches combining evolutionary and structural features, the molecular determinants controlling binding specificity can be predicted accurately with physics-based approaches based on atomic models.

Structure of the Dicer-2-R2D2 heterodimer bound to a small RNA duplex.

In flies, Argonaute2 (Ago2) and small interfering RNA (siRNA) form an RNA-induced silencing complex to repress viral transcripts1. The RNase III enzyme Dicer-2 associates with its partner protein R2D2 and cleaves long double-stranded RNAs to produce 21-nucleotide siRNA duplexes, which are then loaded into Ago2 in a defined orientation2-5. Here we report cryo-electron microscopy structures of the Dicer-2-R2D2 and Dicer-2-R2D2-siRNA complexes. R2D2 interacts with the helicase domain and the central linker of Dicer-2 to inhibit the promiscuous processing of microRNA precursors by Dicer-2. Notably, our structure represents the strand-selection state in the siRNA-loading process, and reveals that R2D2 asymmetrically recognizes the end of the siRNA duplex with the higher base-pairing stability, and the other end is exposed to the solvent and is accessible by Ago2. Our findings explain how R2D2 senses the thermodynamic asymmetry of the siRNA and facilitates the siRNA loading into Ago2 in a defined orientation, thereby determining which strand of the siRNA duplex is used by Ago2 as the guide strand for target silencing.

Structural insights into dsRNA processing by Drosophila Dicer-2-Loqs-PD.

Small interfering RNAs (siRNAs) are the key components for RNA interference (RNAi), a conserved RNA-silencing mechanism in many eukaryotes1,2. In Drosophila, an RNase III enzyme Dicer-2 (Dcr-2), aided by its cofactor Loquacious-PD (Loqs-PD), has an important role in generating 21 bp siRNA duplexes from long double-stranded RNAs (dsRNAs)3,4. ATP hydrolysis by the helicase domain of Dcr-2 is critical to the successful processing of a long dsRNA into consecutive siRNA duplexes5,6. Here we report the cryo-electron microscopy structures of Dcr-2-Loqs-PD in the apo state and in multiple states in which it is processing a 50 bp dsRNA substrate. The structures elucidated interactions between Dcr-2 and Loqs-PD, and substantial conformational changes of Dcr-2 during a dsRNA-processing cycle. The N-terminal helicase and domain of unknown function 283 (DUF283) domains undergo conformational changes after initial dsRNA binding, forming an ATP-binding pocket and a 5'-phosphate-binding pocket. The overall conformation of Dcr-2-Loqs-PD is relatively rigid during translocating along the dsRNA in the presence of ATP, whereas the interactions between the DUF283 and RIIIDb domains prevent non-specific cleavage during translocation by blocking the access of dsRNA to the RNase active centre. Additional ATP-dependent conformational changes are required to form an active dicing state and precisely cleave the dsRNA into a 21 bp siRNA duplex as confirmed by the structure in the post-dicing state. Collectively, this study revealed the molecular mechanism for the full cycle of ATP-dependent dsRNA processing by Dcr-2-Loqs-PD.

Multifaceted roles of YEATS domain-containing proteins and novel links to neurological diseases.

The so-called Yaf9, ENL, AF9, Taf14, and Sas5 (YEATS) domain-containing proteins, hereafter referred to as YD proteins, take control over the transcription by multiple steps of regulation either involving epigenetic remodelling of chromatin or guiding the processivity of RNA polymerase II to facilitate elongation-coupled mRNA 3' processing. Interestingly, an increasing amount of evidence suggest a wider repertoire of YD protein's functions spanning from non-coding RNA regulation, RNA-binding proteins networking, post-translational regulation of a few signalling transduction proteins and the spindle pole formation. However, such a large set of non-canonical roles is still poorly characterized. Notably, four paralogous of human YEATS domain family members, namely eleven-nineteen-leukaemia (ENL), ALL1-fused gene from chromosome 9 protein (AF9), YEATS2 and glioma amplified sequence 41 (GAS41), have a strong link to cancer yet new findings also highlight a potential novel role in neurological diseases. Here, in an attempt to more comprehensively understand the complexity of four YD proteins and to gain more insight into the novel functions they may accomplish in the neurons, we summarized the YD protein's networks, systematically searched and reviewed the YD genetic variants associated with neurodevelopmental disorders and finally interrogated the model organism Drosophila melanogaster.

Panoramix SUMOylation on chromatin connects the piRNA pathway to the cellular heterochromatin machinery.

Nuclear Argonaute proteins, guided by small RNAs, mediate sequence-specific heterochromatin formation. The molecular principles that link Argonaute-small RNA complexes to cellular heterochromatin effectors on binding to nascent target RNAs are poorly understood. Here, we explain the mechanism by which the PIWI-interacting RNA (piRNA) pathway connects to the heterochromatin machinery in Drosophila. We find that Panoramix, a corepressor required for piRNA-guided heterochromatin formation, is SUMOylated on chromatin in a Piwi-dependent manner. SUMOylation, together with an amphipathic LxxLL motif in Panoramix's intrinsically disordered repressor domain, are necessary and sufficient to recruit Small ovary (Sov), a multi-zinc-finger protein essential for general heterochromatin formation and viability. Structure-guided mutations that eliminate the Panoramix-Sov interaction or that prevent SUMOylation of Panoramix uncouple Sov from the piRNA pathway, resulting in viable but sterile flies in which Piwi-targeted transposons are derepressed. Thus, Piwi engages the heterochromatin machinery specifically at transposon loci by coupling recruitment of a corepressor to nascent transcripts with its SUMOylation.