Genetic dissection of the Drosophila Cubitus interruptus signaling complex

Engrailed, Suppressor of fused and Roadkill modulate the Drosophila GLI transcription factor Cubitus interruptus at multiple levels

Morphogen gradients need to be robust, but may also need to be tailored for specific tissues. Often this type of regulation is carried out by negative regulators and negative feedback loops. In the Hedgehog (Hh) pathway, activation of patched (ptc) in response to Hh is part of a negative feedback loop limiting the range of the Hh morphogen. Here, we show that in the Drosophila wing imaginal disc two other known Hh targets genes feed back to modulate Hh signaling. First, anterior expression of the transcriptional repressor Engrailed modifies the Hh gradient by attenuating the expression of the Hh pathway transcription factor cubitus interruptus (ci), leading to lower levels of ptc expression. Second, the E-3 ligase Roadkill shifts the competition between the full-length activator and truncated repressor forms of Ci by preferentially targeting full-length Ci for degradation. Finally, we provide evidence that Suppressor of fused, a negative regulator of Hh signaling, has an unexpected positive role, specifically protecting full-length Ci but not the Ci repressor from Roadkill.

Proteostasis in the Hedgehog signaling pathway

The Hedgehog (Hh) signaling pathway is crucial for the development of vertebrate and invertebrate animals alike. Hh ligand binds its receptor Patched (Ptc), allowing the activation of the obligate signal transducer Smoothened (Smo). The levels and localizations of both Ptc and Smo are regulated by ubiquitination, and Smo is under additional regulation by phosphorylation and SUMOylation. Downstream of Smo, the Ci/Gli family of transcription factors regulates the transcriptional responses to Hh. Phosphorylation, ubiquitination and SUMOylation are important for the stability and localization of Ci/Gli proteins and Hh signaling output. Finally, Suppressor of Fused directly regulates Ci/Gli proteins and itself is under proteolytic regulation that is critical for normal Hh signaling.

Contributions of Costal 2-Fused interactions to Hedgehog signaling in Drosophila

The Drosophila kinesin-family protein Costal 2 (Cos2) and its mammalian ortholog Kif7 play dual roles in Hedgehog (Hh) signaling. In the absence of Hh, Cos2 and Kif7 contribute to proteolytic processing and silencing of the Hh-regulated transcription factors, Drosophila Cubitus interruptus (Ci) and mammalian Gli proteins. Cos2 and Kif7 are also necessary for full activation of full-length Ci-155 and Gli transcription factors in response to Hh proteins. Here, we use classical fused alleles and transgenic Cos2 products deficient for Fused (Fu) association to show that Cos2 must bind to Fu to support efficient Ci-155 processing. Residual Ci-155 processing in the absence of Cos2-Fu interaction did not require Suppressor of Fused, which has been implicated in processing mammalian Gli proteins. We also provide evidence that Cos2 binding to the CORD domain of Ci-155 contributes to both Ci-155 processing and Ci-155 silencing in the absence of Hh. In the presence of Hh, Ci-155 processing is blocked and Cos2 now promotes activation of Ci-155, which requires Fu kinase activity. Here, we show that normal Ci-155 activation by Hh requires Cos2 binding to Fu, supporting the hypothesis that Cos2 mediates the apposition of Fu molecules suitable for cross-phosphorylation and consequent full activation of Fu kinase. We also find that phosphorylation of Cos2 by Fu at two previously mapped sites, S572 and S931, which is thought to mediate Ci-155 activation, is not required for normal activation of Ci-155 by Hh or by activated Fu.

Decoding Ci: from partial degradation to inhibition

Hedgehog is a morphogen, which is widely involved in the regulation of cell proliferation, differentiation and tissue patterning during development in both vertebrate and invertebrate, such as in coordination of eye, brain, gonad, gut and tracheal development. In invertebrate, Cubitus interruptus (Ci) modification process is the last identified step before transcriptional activation in the Hh signaling pathway. Ci can form a truncated repressor (Ci(R) /Ci75) or act as an activator (Ci(A) /Ci155) based on Hh gradient to regulate the expressions of target genes. The activity of Ci is mediated by different mechanisms, including processing, trafficking and degradation. While in vertebrate, Glioblastomas (Glis), homologs of Ci, play similar but more complex roles in the regulation of mammals Hh pathway. Hh signaling is responsible for a wide variety of processes during embryonic development and adult tissue homeostasis. Malfunction of Hh signaling could cause various diseases including birth defects and cancers. Enormous efforts were made in the past decades to explore the Hh pathway regulation and the results have provided us important insights into disease diagnosis and therapeutic treatment. In this review, we focus on a small branch of Hh pathway regulation based on studies in the Drosophila system, mainly about Ci degradation, aiming to explain how Ci is modified by different ubiquitin ligases due to the strong or moderate Hh signals and then been subjected to complete or partial degradation by proteasomes. Overall, we intend to offer an overview on how Ci responds to and relays Hh signals in a precise manner to control target genes expressions and ensures proper Hh signal transduction.

Wing tips: The wing disc as a platform for studying Hedgehog signaling

Hedgehog (Hh) signal transduction is necessary for the development of most mammalian tissues and can go awry and cause birth defects or cancer. Hh signaling was initially described in Drosophila, and much of what we know today about mammalian Hh signaling was directly guided by discoveries in the fly. Indeed, Hh signaling is a wonderful example of the use of non-vertebrate model organisms to make basic discoveries that lead to new disease treatment. The first pharmaceutical to treat hyperactive Hh signaling in Basal Cell Carcinoma was released in 2012, approximately 30 years after the isolation of Hh mutants in Drosophila. The study of Hh signaling has been greatly facilitated by the imaginal wing disc, a tissue with terrific experimental advantages. Studies using the wing disc have led to an understanding of Hh ligand processing, packaging into particles for transmission, secretion, reception, signal transduction, target gene activation, and tissue patterning. Here we describe the imaginal wing disc, how Hh patterns this tissue, and provide methods to use wing discs to study Hh signaling in Drosophila. The tools and approaches we highlight form the cornerstone of research efforts in many laboratories that use Drosophila to study Hh signaling, and are essential for ongoing discoveries.

Extramacrochaetae imposes order on the Drosophila eye by refining the activity of the Hedgehog signaling gradient

The compound eye of Drosophila melanogaster is configured by a differentiating wave, the morphogenetic furrow, that sweeps across the eye imaginal disc and transforms thousands of undifferentiated cells into a precisely ordered repetitive array of 800 ommatidia. The initiation of the furrow at the posterior margin of the epithelium and its subsequent movement across the eye field is controlled by the activity of the Hedgehog (Hh) signaling pathway. Differentiating photoreceptors that lie behind the furrow produce and secrete the Hh morphogen, which is captured by cells within the furrow itself. This leads to the stabilization of the full-length form of the zinc-finger transcription factor Cubitus interruptus (Ci(155)), the main effector of Hh signaling. Ci(155) functions as a transcriptional activator of a number of downstream targets, including decapentaplegic (dpp), a TGFβ homolog. In this report, we describe a mechanism that is in place within the fly retina to limit Hh pathway activity within and ahead of the furrow. We demonstrate that the helix-loop-helix (HLH) protein Extramacrochaetae (Emc) regulates Ci(155) levels. Loss of emc leads to an increase in Ci(155) levels, nuclear migration, apical cell constriction and an acceleration of the furrow. We find that these roles are distinct from the bHLH protein Hairy (H), which we show restricts atonal (ato) expression ahead of the furrow. Secondary furrow initiation along the dorsal and ventral margins is blocked by the activity of the Wingless (Wg) pathway. We also show that Emc regulates and cooperates with Wg signaling to inhibit lateral furrow initiation.

The Hedgehog signal transduction network

Hedgehog (Hh) proteins regulate the development of a wide range of metazoan embryonic and adult structures, and disruption of Hh signaling pathways results in various human diseases. Here, we provide a comprehensive review of the signaling pathways regulated by Hh, consolidating data from a diverse array of organisms in a variety of scientific disciplines. Similar to the elucidation of many other signaling pathways, our knowledge of Hh signaling developed in a sequential manner centered on its earliest discoveries. Thus, our knowledge of Hh signaling has for the most part focused on elucidating the mechanism by which Hh regulates the Gli family of transcription factors, the so-called "canonical" Hh signaling pathway. However, in the past few years, numerous studies have shown that Hh proteins can also signal through Gli-independent mechanisms collectively referred to as "noncanonical" signaling pathways. Noncanonical Hh signaling is itself subdivided into two distinct signaling modules: (i) those not requiring Smoothened (Smo) and (ii) those downstream of Smo that do not require Gli transcription factors. Thus, Hh signaling is now proposed to occur through a variety of distinct context-dependent signaling modules that have the ability to crosstalk with one another to form an interacting, dynamic Hh signaling network.

Mammalian homologues of Drosophila fused kinase

Sonic Hedgehog (Shh) signaling pathway is implicated in various developmental and postnatal processes. Much of the current knowledge about the mechanisms of Shh signal transduction in vertebrates comes from the investigations of the respective pathway in fruit fly Drosophila melanogaster. In Drosophila, serine/threonine kinase fused is involved in all aspects of regulation of the Hh-dependent transcription factor cubitus interruptus possessing both catalytic and regulatory functions. Two proteins, Stk36 and Ulk3, share similarity with fu and have been suggested as mammalian fu homologues. However, in vivo data clarify that Stk36 is not required for embryonic development in mice and participates in Shh-independent genesis of motile cilia. Even if Stk36 is associated with any pathological or physiological aspect of postnatal Shh signaling in mammals, it has perhaps only regulatory functions since its catalytic activity seems to be lost during evolution. In contrast to Stk36, Ulk3 is an active kinase. In non-stimulated cells, Ulk3 catalytic activity is blocked, and it is involved in negative control of Gli proteins, mediators of Shh signaling. In response to Shh, Ulk3 positively regulates Gli proteins by directly phosphorylating them. Thus, Ulk3 is able to recapitulate both positive and negative roles of fu in vitro. However, Ulk3 functioning in vivo remains to be investigated.

The Hedgehog-induced Smoothened conformational switch assembles a signaling complex that activates Fused by promoting its dimerization and phosphorylation

Hedgehog (Hh) transduces signal by regulating the subcellular localization and conformational state of the GPCR-like protein Smoothened (Smo) but how Smo relays the signal to cytoplasmic signaling components remains poorly understood. Here, we show that Hh-induced Smo conformational change recruits Costal2 (Cos2)/Fused (Fu) and promotes Fu kinase domain dimerization. We find that induced dimerization through the Fu kinase domain activates Fu by inducing multi-site phosphorylation of its activation loop (AL) and phospho-mimetic mutations of AL activate the Hh pathway. Interestingly, we observe that graded Hh signals progressively increase Fu kinase domain dimerization and AL phosphorylation, suggesting that Hh activates Fu in a dose-dependent manner. Moreover, we find that activated Fu regulates Cubitus interruptus (Ci) by both promoting its transcriptional activator activity and inhibiting its proteolysis into a repressor form. We provide evidence that activated Fu exerts these regulations by interfering with the formation of Ci-Sufu and Ci-Cos2-kinase complexes that normally inhibit Ci activity and promote its processing. Taken together, our results suggest that Hh-induced Smo conformational change facilitates the assembly of active Smo-Cos2-Fu signaling complexes that promote Fu kinase domain dimerization, phosphorylation and activation, and that Fu regulates both the activator and repressor forms of Ci.

Dual function of UNC-51-like kinase 3 (Ulk3) in the Sonic hedgehog signaling pathway

The Sonic hedgehog (Shh) signaling pathway controls a variety of developmental processes and is implicated in tissue homeostasis maintenance and neurogenesis in adults. Recently, we identified Ulk3 as an active kinase able to positively regulate Gli proteins, mediators of the Shh signaling in mammals. Here, we provide several lines of evidence that Ulk3 participates in the transduction of the Shh signal also independently of its kinase activity. We demonstrate that Ulk3 through its kinase domain interacts with Suppressor of Fused (Sufu), a protein required for negative regulation of Gli proteins. Sufu blocks Ulk3 autophosphorylation and abolishes its ability to phosphorylate and positively regulate Gli proteins. We show that Shh signaling destabilizes the Sufu-Ulk3 complex and induces the release of Ulk3. We demonstrate that the Sufu-Ulk3 complex, when co-expressed with Gli2, promotes generation of the Gli2 repressor form, and that reduction of the Ulk3 mRNA level in Shh-responsive cells results in higher potency of the cells to transmit the Shh signal. Our data suggests a dual function of Ulk3 in the Shh signal transduction pathway and propose an additional way of regulating Gli proteins by Sufu, through binding to and suppression of Ulk3.

Mechanism and evolution of cytosolic Hedgehog signal transduction

Hedgehog (Hh) signaling is required for embryonic patterning and postnatal physiology in invertebrates and vertebrates. With the revelation that the primary cilium is crucial for mammalian Hh signaling, the prevailing view that Hh signal transduction mechanisms are conserved across species has been challenged. However, more recent progress on elucidating the function of core Hh pathway cytosolic regulators in Drosophila, zebrafish and mice has confirmed that the essential logic of Hh transduction is similar between species. Here, we review Hh signaling events at the membrane and in the cytosol, and focus on parallel and divergent functions of cytosolic Hh regulators in Drosophila and mammals.

Dynamic phosphorylation of the kinesin Costal-2 in vivo reveals requirement of fused kinase activity for all levels of hedgehog signalling

Hedgehog (Hh) proteins are secreted molecules that play an essential role in development and tumorigenesis. In Drosophila cultured cells, phosphorylation of the kinesin-like Costal2 (Cos2) protein at Ser572 is triggered by the kinase fused (Fu) upon Hh pathway activation. Here, we validate the first phospho-antibody for one of the Hh pathway components, Cos2, as a universal in situ readout of Hh signal transduction. For the first time, this tool allows the visualisation of a gradient of signalling activity and therefore the range of the activating Hh ligand in different tissues. We also show that, in vivo, Fu kinase is activated by and necessary to transduce all levels of intracellular Hh signalling. Our study fills a gap in the understanding of the Hh pathway by showing that the molecular cascade leading to Cos2 phosphorylation is conserved in all cells activated by Hh. Therefore, we propose that the extracellular Hh information is conveyed to an intracellular signal through graded Fu kinase activity.

The primary cilium: a signalling centre during vertebrate development

The primary cilium has recently stepped into the spotlight, as a flood of data show that this organelle has crucial roles in vertebrate development and human genetic diseases. Cilia are required for the response to developmental signals, and evidence is accumulating that the primary cilium is specialized for hedgehog signal transduction. The formation of cilia, in turn, is regulated by other signalling pathways, possibly including the planar cell polarity pathway. The cilium therefore represents a nexus for signalling pathways during development. The connections between cilia and developmental signalling have begun to clarify the basis of human diseases associated with ciliary dysfunction.

Patched regulates Smoothened trafficking using lipoprotein-derived lipids

Hedgehog (Hh) is a lipoprotein-borne ligand that regulates both patterning and proliferation in a wide variety of vertebrate and invertebrate tissues. When Hh is absent, its receptor Patched (Ptc) represses Smoothened (Smo) signaling by an unknown catalytic mechanism that correlates with reduced Smo levels on the basolateral membrane. Ptc contains a sterol-sensing domain and is similar to the Niemann-Pick type C-1 protein, suggesting that Ptc might regulate lipid trafficking to repress Smo. However, no endogenous lipid regulators of Smo have yet been identified, nor has it ever been shown that Ptc actually controls lipid trafficking. Here, we show that Drosophila Ptc recruits internalized lipoproteins to Ptc-positive endosomes and that its sterol-sensing domain regulates trafficking of both lipids and Smo from this compartment. Ptc utilizes lipids derived from lipoproteins to destabilize Smo on the basolateral membrane. We propose that Ptc normally regulates Smo degradation by changing the lipid composition of endosomes through which Smo passes, and that the presence of Hh on lipoproteins inhibits utilization of their lipids by Ptc.

Cilium-independent regulation of Gli protein function by Sufu in Hedgehog signaling is evolutionarily conserved

A central question in Hedgehog (Hh) signaling is how evolutionarily conserved components of the pathway might use the primary cilium in mammals but not fly. We focus on Suppressor of fused (Sufu), a major Hh regulator in mammals, and reveal that Sufu controls protein levels of full-length Gli transcription factors, thus affecting the production of Gli activators and repressors essential for graded Hh responses. Surprisingly, despite ciliary localization of most Hh pathway components, regulation of Gli protein levels by Sufu is cilium-independent. We propose that Sufu-dependent processes in Hh signaling are evolutionarily conserved. Consistent with this, Sufu regulates Gli protein levels by antagonizing the activity of Spop, a conserved Gli-degrading factor. Furthermore, addition of zebrafish or fly Sufu restores Gli protein function in Sufu-deficient mammalian cells. In contrast, fly Smo is unable to translocate to the primary cilium and activate the mammalian Hh pathway. We also uncover a novel positive role of Sufu in regulating Hh signaling, resulting from its control of both Gli activator and repressor function. Taken together, these studies delineate important aspects of cilium-dependent and cilium-independent Hh signal transduction and provide significant mechanistic insight into Hh signaling in diverse species.

Role and regulation of human tumor suppressor SUFU in Hedgehog signaling

Originally identified as factors affecting Drosophila embryogenesis, the Hedgehog (Hh) pathway is one of the primary signaling systems that specify patterns of cell growth and differentiation during vertebrate development. Mutations in various components of this pathway frequently occur in tumors originated from the skin, cerebellum, and skeletal muscle, and abnormal pathway activity is associated with a subset of lung, digestive tract, pancreatic, and prostate cancers. Because of these potent biological activities, this pathway is negatively regulated at multiple levels to ensure appropriate signaling responses. Suppressor of fused (Sufu) is one such negative regulator of Hh signaling. Although not essential in Drosophila, Sufu is absolutely required for mouse embryonic development. Mutations of Sufu are associated with a childhood brain tumor in human and an increased susceptibility to the same type of cancer in the TP53 null background in mice, and RNAi-mediated silencing of Sufu is sufficient to activate the Hh signaling in cultured fibroblasts. All these data point to a central role of Sufu in controlling the vertebrate Hh signaling pathway; however, for years what exactly Sufu does in the Hh pathway and what controls its activity remains a deep mystery. This chapter will go over all studies curated in the PubMed database with Sufu as a main subject during the past 17 years, and attempt to provide a balanced view on Sufu gene and protein structure, activities in Drosophila as well as mammalian development, and its involvement in cancer.

Hedgehog: functions and mechanisms

The Hedgehog (Hh) family of proteins control cell growth, survival, and fate, and pattern almost every aspect of the vertebrate body plan. The use of a single morphogen for such a wide variety of functions is possible because cellular responses to Hh depend on the type of responding cell, the dose of Hh received, and the time cells are exposed to Hh. The Hh gradient is shaped by several proteins that are specifically required for Hh processing, secretion, and transport through tissues. The mechanism of cellular response, in turn, incorporates multiple feedback loops that fine-tune the level of signal sensed by the responding cells. Germline mutations that subtly affect Hh pathway activity are associated with developmental disorders, whereas somatic mutations activating the pathway have been linked to multiple forms of human cancer. This review focuses broadly on our current understanding of Hh signaling, from mechanisms of action to cellular and developmental functions. In addition, we review the role of Hh in the pathogenesis of human disease and the possibilities for therapeutic intervention.

The role of kinases in the Hedgehog signalling pathway

The Hedgehog (Hh) signalling pathway has a crucial role in several developmental processes and is aberrantly activated in a variety of cancers. In Drosophila, many of the canonical Hh pathway components are phosphorylated, yet the precise role of these phosphorylation events in the regulation of Hh signal transduction is unclear. Furthermore, the Hh pathway receives input from several kinases that have well-described roles in other cellular functions, some of which have both positive and negative effects on Hh signalling. Several recent studies have characterized the role of specific phosphorylation events in the Hh pathway, and have begun to shed light on how phosphorylation of Hh signalling components affects their subcellular location, stability and activity to mediate the transcriptional response to the Hh gradient.

Costal2 functions as a kinesin-like protein in the hedgehog signal transduction pathway

The Hedgehog (Hh) signaling pathway initiates an evolutionarily conserved developmental program required for the proper patterning of many tissues [1]. Although Costal2 (Cos2) is a requisite component of the Hh pathway, its mechanistic role is not well understood. Because of its primary sequence, Cos2 was initially predicted to function as a kinesin-like protein [2]. However, evidence showing that Cos2 function might require kinesin-like properties has been lacking [2-6]. Thus, the prevailing dogma in the field is that Cos2 functions solely as a scaffolding protein [7, 8]. Here, we show that Cos2 motility is required for its biological function and that this motility may be Hh regulated. We show that Cos2 motility requires an active motor domain, ATP, and microtubules. Additionally, Cos2 recruits and transports other components of the Hh signaling pathway, including the transcription factor Cubitus interruptus (Ci). Drosophila expressing cos2 mutations that encode proteins that lack motility are attenuated in their ability to regulate Ci activity and exhibit phenotypes consistent with attenuated Cos2 function [9]. Combined, these results demonstrate that Cos2 motility plays an important role in its function, regulating the amounts and activity of Ci that ultimately interpret the level of Hh to which cells are exposed.

Fused-Costal2 protein complex regulates Hedgehog-induced Smo phosphorylation and cell-surface accumulation

The seven-transmembrane protein Smoothened (Smo) acts as a signal transducer in the Hedgehog (Hh) pathway that mediates many key developmental processes. In Drosophila, Hh-induced phosphorylation promotes Smo cell-surface accumulation and signaling activity; however, the mechanisms controlling Smo phosphorylation and cell-surface accumulation are still unknown. The intracellular signaling complex containing Fused (Fu) and Costal2 (Cos2) is thought to transduce the Hh signal downstream from Smo. Here, we identify a novel feedback mechanism that regulates Smo through the Fu-Cos2 complex. We found that Hh-induced Smo accumulation is inhibited in fu mutant clones or by expressing a dominant-negative form of Fu, and such inhibition is alleviated by removal of Cos2. Conversely, overexpressing Cos2 blocks Smo accumulation, which is reversed by coexpressing Fu. Cos2 blocks Smo accumulation through its C-terminal Smo-interacting domain, and Fu antagonizes Cos2 by phosphorylating Cos2 at Ser572. Furthermore, we found that Ser572 phosphorylation attenuates the Cos2-Smo interaction and promotes Cos2 instability. Finally, we provided evidence that Fu and Cos2 control Smo cell-surface accumulation by regulating Smo phosphorylation. Our data suggest that Cos2-Smo interaction blocks Hh-induced Smo phosphorylation, and that Fu promotes Smo phosphorylation by antagonizing Cos2.

Clonal analysis of Hedgehog signaling in Drosophila somatic tissues

To fully understand how animals develop, it is often necessary to remove the function of a particular gene in a specific cell type or subset of cells. In Drosophila melanogaster, mosaic animals have been widely utilized to study cell fate, growth and patterning, and restriction of cell fate. This chapter describes using FLP recombinase to generate mosaic Drosophila, discussing the chromosomes and cross scheme, how to induce the clones, how to properly identify the appropriate progeny, and how to prepare and analyze the tissues, clones, and phenotypes. It then presents three examples, applying this technique to study Hedgehog signaling. The first example describes moderate-sized costal clones in imaginal discs, using green fluorescent protein (GFP) as a marker and dppLacZ and Engrailed expression as phenotypic reporters. The second describes filling the adult eye with roadkill mutant clones, using white as a marker and scoring morphology. The third describes clonal misexpression of a truncated form of Smoothened, using GFP and yellow as markers.

Genomics and expression profiles of the Hedgehog and Notch signaling pathways in sea urchin development

The Hedgehog (Hh) and Notch signal transduction pathways control a variety of developmental processes including cell fate choice, differentiation, proliferation, patterning and boundary formation. Because many components of these pathways are conserved, it was predicted and confirmed that pathway components are largely intact in the sea urchin genome. Spatial and temporal location of these pathways in the embryo, and their function in development offer added insight into their mechanistic contributions. Accordingly, all major components of both pathways were identified and annotated in the sea urchin Strongylocentrotus purpuratus genome and the embryonic expression of key components was explored. Relationships of the pathway components, and modifiers predicted from the annotation of S. purpuratus, were compared against cnidarians, arthropods, urochordates, and vertebrates. These analyses support the prediction that the pathways are highly conserved through metazoan evolution. Further, the location of these two pathways appears to be conserved among deuterostomes, and in the case of Notch at least, display similar capacities in endomesoderm gene regulatory networks. RNA expression profiles by quantitative PCR and RNA in situ hybridization reveal that Hedgehog is produced by the endoderm beginning just prior to invagination, and signals to the secondary mesenchyme-derived tissues at least until the pluteus larva stage. RNA in situ hybridization of Notch pathway members confirms that Notch functions sequentially in the vegetal-most secondary mesenchyme cells and later in the endoderm. Functional analyses in future studies will embed these pathways into the growing knowledge of gene regulatory networks that govern early specification and morphogenesis.

The last 59 amino acids of Smoothened cytoplasmic tail directly bind the protein kinase Fused and negatively regulate the Hedgehog pathway

The Hedgehog (HH) signaling pathway is crucial for the development of many organisms and its inappropriate activation is involved in numerous cancers. HH signal controls the traffic and activity of the seven-pass transmembrane protein Smoothened (SMO), leading to the transcriptional regulation of HH-responsive genes. In Drosophila, the intracellular transduction events following SMO activation depend on cytoplasmic multimeric complexes that include the Fused (FU) protein kinase. Here we show that the regulatory domain of FU physically interacts with the last 52 amino acids of SMO and that the two proteins colocalize in vivo to vesicles. The deletion of this region of SMO leads to a constitutive activation of SMO, promoting the ectopic transcription of HH target genes. This activation is partially dependent of FU activity. Thus, we identify a novel link between SMO and the cytoplasmic complex(es) and reveal a negative role of the SMO C-terminal region that interacts with FU. We propose that FU could act as a switch, activator in presence of HH signal or inhibitor in absence of HH.

Alternative routes and mutational robustness in complex regulatory networks

Alternative pathways through a gene regulation network connect a regulatory molecule to its (indirect) regulatory target via different intermediate regulators. We here show for two large transcriptional regulation networks, and for 15 different signal transduction networks, that multiple alternative pathways between regulator and target pairs are the rule rather than the exception. We find that in the yeast transcriptional regulation network intermediate regulators that are part of many alternative pathways between a regulator and target pair evolve at faster rates. This variation is not solely explicable by higher expression levels of such regulators, nor is it solely explicable by their variable usage in different physiological or environmental conditions, as indicated by their variable expression. This suggests that such pathways can continue to function despite amino acid changes that may impair one intermediate regulator. Our results underscore the importance of systems biology approaches to understand functional and evolutionary constraints on genes and proteins.

Regulation of Ci and Su(fu) nuclear import in Drosophila

The Hedgehog (Hh) signal transduction pathway plays a central role in the development of invertebrates and vertebrates. While much is known about the pathway, the role of Suppressor of fused (Su(fu)), a component of the pathway's signaling complex has remained enigmatic. Previous studies have linked Su(fu) to the cytoplasmic sequestration of the zinc finger transcription factor, Cubitus interruptus (Ci), while other studies suggest a role in modulating target gene expression. In examining the cell biology of the pathway, we have found that like its vertebrate homologue, Drosophila Su(fu) enters the nucleus. Furthermore, we find that the nuclear import of Su(fu) occurs in concert with that of Ci in response to Hh signaling. Here, we examine the mechanism by which Su(fu) regulates Ci import by investigating the importance of the Ci nuclear localization signal (NLS) and the effect of adding an additional NLS. Finally, we demonstrate that Ci can bring Su(fu) with it to a multimerized Ci DNA binding site. These results provide a basis for understanding the dual roles played by Su(fu) in the regulation of Ci.

Regulation of the Drosophila transcription factor, Cubitus interruptus, by two conserved domains

Hedgehog signaling is required for the development of many organisms, including Drosophila. In flies, Hh patterns the embryonic epidermis and larval imaginal discs by regulating the transcription factor, Cubitus interruptus (Ci). To date, three levels of regulation have been identified: proteolytic processing into a repressor, nuclear import, and activation. In this report, we characterize the function of two Ci domains that are conserved in the vertebrate homologues, GLI1, GLI2, and GLI3. One domain includes the first two of five C(2)-H(2) zinc-fingers. While conserved in all members of the GLI/Ci family, the first two fingers do not appear to make significant contacts with the DNA target sequence. Ci protein lacking this region is still able to interact with the cytoplasmic complex and activate transcription in embryos and wing imaginal discs, but it is no longer processed into the repressor form. The second domain, termed NR for "N-terminal Regulatory", binds Suppressor of Fused. Deletion of this region has little effect on embryonic patterning, but compromises cytoplasmic retention of Ci. Analysis of the amino acid sequence of this domain identifies 11 perfectly conserved serines and one tyrosine. We propose that this region may be modified, possibly by phosphorylation, to regulate Ci nuclear import.

Tectonic, a novel regulator of the Hedgehog pathway required for both activation and inhibition

We report the identification of a novel protein that participates in Hedgehog-mediated patterning of the neural tube. This protein, named Tectonic, is the founding member of a previously undescribed family of evolutionarily conserved secreted and transmembrane proteins. During neural tube development, mouse Tectonic is required for formation of the most ventral cell types and for full Hedgehog (Hh) pathway activation. Epistasis analyses reveal that Tectonic modulates Hh signal transduction downstream of Smoothened (Smo) and Rab23. Interestingly, characterization of Tectonic Shh and Tectonic Smo double mutants indicates that Tectonic plays an additional role in repressing Hh pathway activity.

Mice deficient in the fused homolog do not exhibit phenotypes indicative of perturbed hedgehog signaling during embryonic development

Hedgehog (Hh) signaling plays a major role in multiple aspects of embryonic development. To understand how a single Hh signal is capable of generating distinct readouts in Hh-responsive cells requires elucidation of the signal transduction cascade at the molecular level. Key components that mediate Hh signal transduction downstream of the receptor include Fused (Fu), Suppressor of fused (Sufu), and Costal-2 (Cos2) or the vertebrate homologs Kif27/Kif7. Studies with both invertebrates and vertebrates have led to a model in which a protein complex composed of Fu, Sufu, and Cos2 controls the processing, activity, and subcellular distribution of the Ci/Gli transcription factors responsible for Hh target gene activation. These converging results obtained with different species reaffirm the prevailing view of pathway conservation during evolution. Genetic studies of Fu, Sufu, and Kif27/Kif7 in mice are required to provide further verification of Hh pathway conservation. To this end, we generated a gene-targeted allele of Fu in mice. Surprisingly, our analysis indicates that Fu-deficient mice do not exhibit any embryonic phenotypes indicative of perturbed Hh signaling. This could be due to either functional redundancy or Hh pathway divergence and clearly indicates greater complexity of Hh signaling in vertebrates.

Mouse intraflagellar transport proteins regulate both the activator and repressor functions of Gli transcription factors

Intraflagellar transport (IFT) is an active event in which cargo is transported along microtubules by motor proteins such as kinesin and dynein. IFT proteins are required for the formation and maintenance of flagella and cilia. We have previously shown that mouse mutants for two IFT proteins, IFT88 and IFT172, as well as Kif3a, a subunit of mouse kinesin 2, exhibit ventral spinal cord patterning defects that appear to result from reduced hedgehog (Hh) signaling. Although genetic epistasis experiments place IFT proteins downstream of the Hh receptor and upstream of the Gli transcription factors, the mechanism by which IFT regulates Gli function is unknown. The developing limb provides an excellent system to study Hh signaling, in particular as it allows a biological and molecular readout of both Gli activator and repressor function. Here we report that homozygous mutants for flexo (Fxo), a hypomorphic allele of mouse IFT88 generated in our ENU mutagenesis screen, exhibit polydactyly in all four limbs. Molecular analysis indicates that expression domains of multiple posteriorly restricted genes are expanded anteriorly in the mutant limbs, similar to loss of Gli3 transcriptional repressor function. Sonic hedgehog (Shh) expression is normal, yet Ptch1 and Gli1, two known targets of Hh signaling, are greatly reduced, consistent with loss of Shh signaling. Expression of Gli3 and Hand2 in the mutant limb indicates that the limb prepattern is abnormal. In addition, we show that partial loss-of-function mutations in another mouse IFT gene, Ift52 (Ngd5), result in similar phenotypes and abnormal Hh signaling as Fxo, indicating a general requirement for IFT proteins in Hh signaling and patterning of multiple organs. Analysis of Ift88 and Shh double mutants indicates that, in mouse, IFT proteins are required for both Gli activator and repressor functions, and Gli proteins are insensitive to Hh ligand in the absence of IFT proteins. Finally, our biochemical studies demonstrate that IFT proteins are required for proteolytic processing of Gli3 in mouse embryos. In summary, our results indicate that IFT function is crucial in the control of both the positive and negative transcriptional activities of Gli proteins, and essential for Hh ligand-induced signaling cascade.

Differential regulation of Hedgehog target gene transcription by Costal2 and Suppressor of Fused

The mechanism by which the secreted signaling molecule Hedgehog (Hh) elicits concentration-dependent transcriptional responses from cells is not well understood. In the Drosophila wing imaginal disc, Hh signaling differentially regulates the transcription of target genes decapentaplegic (dpp), patched (ptc) and engrailed (en) in a dose-responsive manner. Two key components of the Hh signal transduction machinery are the kinesin-related protein Costal2 (Cos2) and the nuclear protein trafficking regulator Suppressor of Fused [Su(fu)]. Both proteins regulate the activity of the transcription factor Cubitus interruptus (Ci) in response to the Hh signal. We have analyzed the activities of mutant forms of Cos2 in vivo and found effects on differential target gene transcription. A point mutation in the motor domain of Cos2 results in a dominant-negative form of the protein that derepresses dpp but not ptc. Repression of ptc in the presence of the dominant-negative form of Cos2 requires Su(fu), which is phosphorylated in response to Hh in vivo. Overexpression of wild-type or dominant-negative cos2 represses en. Our results indicate that differential Hh target gene regulation can be accomplished by differential sensitivity of Cos2 and Su(Fu) to Hh.

An intramolecular association between two domains of the protein kinase Fused is necessary for Hedgehog signaling

The protein kinase Fused (Fu) is an integral member of the Hedgehog (Hh) signaling pathway. Although genetic studies demonstrate that Fu is required for the regulation of the Hh pathway, the mechanistic role that it plays remains largely unknown. Given our difficulty in developing an in vitro kinase assay for Fu, we reasoned that the catalytic activity of Fu might be highly regulated. Several mechanisms are known to regulate protein kinases, including self-association in either an intra- or an intermolecular fashion. Here, we provide evidence that Hh regulates Fu through intramolecular association between its kinase domain (DeltaFu) and its carboxyl-terminal domain (Fu-tail). We show that DeltaFu and Fu-tail can interact in trans, with or without the kinesin-related protein Costal 2 (Cos2). However, since the majority of Fu is found associated with Cos2 in vivo, we hypothesized that Fu-tail, which binds Cos2 directly, would be able to tether DeltaFu to Cos2. We demonstrate that DeltaFu colocalizes with Cos2 in the presence of Fu-tail and that this colocalization occurs on a subset of membrane vesicles previously characterized to be important for Hh signal transduction. Additionally, expression of Fu-tail in fu mutant flies that normally express only the kinase domain rescues the fu wing phenotype. Therefore, reestablishing the association between these two domains of Fu in trans is sufficient to restore Hh signal transduction in vivo. In such a manner we validate our hypothesis, demonstrating that Fu self-associates and is functional in an Hh-dependent manner. Our results here enhance our understanding of one of the least characterized, yet critical, components of Hh signal transduction.

Extensive phosphorylation of Smoothened in Hedgehog pathway activation

The transmembrane protein Smoothened (Smo) is activated in response to the extracellular protein signal, Hedgehog (Hh), and transmits this state of pathway activity into the cell. Previous studies in Drosophila have correlated pathway activation with Smo accumulation and increased phosphorylation. Using immunopurification and mass spectrometry, we identify here 26 serine/threonine residues within the Smo C-terminal cytoplasmic tail that are phosphorylated in Hh-stimulated cells. By systematically substituting alanine or glutamic acid to block or simulate phosphorylation, we provide evidence for a functional role of collective phosphorylation of a subset of phosphoresidues in pathway activation. This role is indicated by the ability of altered Smo proteins to produce changes in transcription of Hh-responsive genes in vivo and in cultured cells. These altered Smo proteins also affect biochemical indicators of pathway activity, such as Smo accumulation and phosphorylation of other pathway components. The prevalence and arrangement of phosphoresidues within the Smo cytoplasmic tail at recognition sites for cAMP-dependent protein kinase and casein kinase 1 suggest a role for these kinases in Smo phosphorylation, and such a role is supported by the effects of manipulating kinase activities in cultured cells. Our studies confirm and extend previous studies showing a positive effect for cAMP-dependent protein kinase and uncover a positive role for casein kinase 1alpha in Hh pathway activation.

The Hedgehog response network: sensors, switches, and routers

The Hedgehog (Hh) signaling pathway is intimately linked to cell growth and differentiation, with normal roles in embryonic pattern formation and adult tissue homeostasis and pathological roles in tumor initiation and growth. Recent advances in our understanding of Hh response have resulted from the identification of new pathway components and new mechanisms of action for old pathway components. The most striking new finding is that signal transmission from membrane to cytoplasm proceeds through recruitment, by the seven-transmembrane protein Smoothened, of an atypical kinesin, which routes pathway activation by interaction with other components of a complex that includes the latent zinc finger transcription factor, Ci.

Multiple Cos2/Ci interactions regulate Ci subcellular localization through microtubule dependent and independent mechanisms

The Hedgehog (Hh) family of secreted proteins governs many developmental processes in both vertebrates and invertebrates. In Drosophila, Hh acts by blocking the formation of a truncated repressor form of Cubitus interruptus (Ci) and by stimulating the nuclear translocation and activity of full-length Ci (Ci155). In the absence of Hh, Ci155 is sequestered in the cytoplasm by forming protein complexes with Costal2 (Cos2), Fused (Fu) and Suppressor of Fused [Su(fu)]. How complex formation regulates Ci155 subcellular localization is not clear. We find that Cos2 interacts with two distinct domains of Ci155, an amino (N)-terminal domain (CDN) and a carboxyl (C)-terminal domain (CORD), and Cos2 competes with Su(fu) for binding to the N-terminal region of Ci155. We provide evidence that both N- and C-terminal Cos2 binding domains are involved in the cytoplasmic retention of Ci155 in imaginal discs. Treating imaginal discs with microtubule-destabilizing reagent nocodazole promotes nuclear translocation of Ci155, suggesting that the microtubule network plays an important role in the cytoplasmic retention of Ci155. In addition, we find that adding a nuclear localization signal (NLS) to exposed regions of Ci155 greatly facilitates its nuclear translocation, suggesting that the cytoplasmic retention of Ci155 may also depend on NLS masking.

Regulation of Hedgehog signaling: a complex story

The Hedgehog (Hh) signal transduction pathway plays critical instructional roles during development. Activating mutations in human Hh signaling components predispose to a variety of tumor types, and have been observed in sporadic tumors occurring in a wide range of organs. Multiple insights into the regulation of Hh signaling have been achieved through studies using Drosophila melanogaster as a model organism. In Drosophila, regulation of the transcription factor Cubitus interruptus (Ci) is the ultimate target of the Hh pathway. Ci is regulated through communication of the membrane proteins Patched (Ptc) and Smoothened (Smo) to the intracellular Hedgehog Signaling Complex (HSC) in response to a graded concentration of Hh ligand. The HSC consists of the Kinesin Related Protein, Costal2 (Cos2), the serine-threonine protein kinase. Fused (Fu) and Ci. In the absence of Hh stimulation, the HSC is involved in processing of Ci to a truncated repressor protein. In response to Hh binding to Ptc, processing of Ci is blocked to allow for accumulation of full-length Ci activator protein(s). Differential concentrations of Hh ligand stimulate production of Ci transcriptional activators of varying strength, which facilitate activation of distinct subsets of target genes. The mechanism(s) by which Ptc and Smo communicate with the HSC in response to differential ligand concentrations to regulate Ci function are not yet fully elucidated. Here, we review what is known about regulation of individual Hh signaling components, concentrating on the mechanisms by which the Hh signal is propagated through Smo to the HSC.

Hedgehog signal transduction via Smoothened association with a cytoplasmic complex scaffolded by the atypical kinesin, Costal-2

The seven-transmembrane protein Smoothened (Smo) transduces extracellular activation of the Hedgehog (Hh) pathway by an unknown mechanism to increase transcriptional activity of the latent cytoplasmic transcription factor Ci (Cubitus interruptus). Here, we present evidence that Smo associates directly with a Ci-containing complex that is scaffolded and stabilized by the atypical kinesin, Costal-2 (Cos2). This complex constitutively suppresses pathway activity, but Hh signaling reverses its regulatory effect to promote Ci-mediated transcription. In response to Hh activation of Smo, Cos2 mediates accumulation and phosphorylation of Smo at the membrane as well as phosphorylation of the cytoplasmic components Fu and Su(fu). Positive response of Cos2 to Hh stimulation requires a portion of the Smo cytoplasmic tail and the Cos2 cargo domain, which interacts directly with Smo.

A positive role for Patched in Hedgehog signaling revealed by the intracellular trafficking of Sex-lethal, the Drosophila sex determination master switch

The sex determination master switch, Sex-lethal(Sxl), controls sexual development as a splicing and translational regulator. Hedgehog (Hh) is a secreted protein that specifies cell fate during development. We show that Sxl is in a complex that contains all of the known Hh cytoplasmic components, including Cubitus interruptus (Ci)the only known target of Hh signaling. Hh promotes the entry of Sxl into the nucleus in the wing disc. In the anterior compartment, the Hh receptor Patched(Ptc) is required for this effect, revealing Ptc as a positive effector of Hh. Some of the downstream components of the Hh signaling pathway also alter the rate of Sxl nuclear entry. Mutations in Suppressor of Fused or Fused with altered ability to anchor Ci are also impaired in anchoring Sxl in the cytoplasm. The levels, and consequently, the ability of Sxl to translationally repress downstream targets in the sex determination pathway, can also be adversely affected by mutations in Hh signaling genes. Conversely,overexpression of Sxl in the domain that Hh patterns negatively affects wing patterning. These data suggest that the Hh pathway impacts on the sex determination process and vice versa and that the pathway may serve more functions than the regulation of Ci.

The Kinesin-related protein Costal2 associates with membranes in a Hedgehog-sensitive, Smoothened-independent manner

In Drosophila, Hedgehog (Hh) signal transduction has been shown to require a multiprotein complex (Hedgehog signaling complex (HSC)), which includes the Kinesin-related protein Costal2 (Cos2), the serine/threonine protein kinase Fused (Fu), and the transcription factor Cubitus interruptus (Ci). We present evidence that a biologically relevant fraction of the HSC is found in association with cellular membranes. We demonstrate that Cos2 is capable of tethering an exogenous protein to vesicular membranes and that Cos2 association with membranes is Hh-sensitive. In addition, we demonstrate that Cos2 associates with membranes in cells that lack the transmembrane protein Smoothened (Smo) through a domain of Cos2 distinct from its recently characterized Smo binding domain. We suggest that an Hh-regulated membrane binding activity of Cos2 is part of the mechanism by which Cos2 contributes to Hh signaling. We propose a model in which there are two distinct HSCs with discrete subcellular localizations and activities: one is endosome-associated and facilitates production of a repressor form of Ci (HSC-R), and one is Smo-associated and promotes Ci activation (HSC-A). In response to Hh and through interaction with Cos2, Smo mediates both inhibition of the endosome-associated HSC-R and activation of HSC-A at the plasma membrane.

Smoothened translates Hedgehog levels into distinct responses

In the Drosophila wing, Hedgehog is made by cells of the posterior compartment and acts as a morphogen to pattern cells of the anterior compartment. High Hedgehog levels instruct L3/4 intervein fate, whereas lower levels instruct L3 vein fate. Transcriptional responses to Hedgehog are mediated by the balance between repressor and activator forms of Cubitus interruptus, CiR and CiA. Hedgehog regulates this balance through its receptor, Patched, which acts through Smoothened and thence a regulatory complex that includes Fused, Costal, Suppressor of Fused and Cubitus interruptus. It is not known how the Hedgehog signal is relayed from Smoothened to the regulatory complex nor how responses to different levels of Hedgehog are implemented. We have used chimeric and deleted forms of Smoothened to explore the signaling functions of Smoothened. A Frizzled/Smoothened chimera containing the Smo cytoplasmic tail (FFS) can induce the full spectrum of Hedgehog responses but is regulated by Wingless rather than Hedgehog. Smoothened whose cytoplasmic tail is replaced with that of Frizzled (SSF) mimics fused mutants, interfering with high Hedgehog responses but with no effect on low Hedgehog responses. The cytoplasmic tail of Smoothened with no transmembrane or extracellular domains (SmoC) interferes with high Hedgehog responses and allows endogenous Smoothened to constitutively initiate low responses. SmoC mimics costal mutants. Genetic interactions suggest that SSF interferes with high signaling by titrating out Smoothened, whereas SmoC drives constitutive low signaling by titrating out Costal. These data suggest that low and high signaling (1) are qualitatively different, (2) are mediated by distinct configurations of the regulatory complex and (3) are initiated by distinct activities of Smoothened. We present a model where low signaling is initiated when a Costal inhibitory site on the Smoothened cytoplasmic tail shifts the regulatory complex to its low state. High signaling is initiated when cooperating Smoothened cytoplasmic tails activate Costal and Fused, driving the regulatory complex to its high state. Thus, two activities of Smoothened translate different levels of Hedgehog into distinct intracellular responses.

The topology of the regulatory interactions predicts the expression pattern of the segment polarity genes in Drosophila melanogaster

Expression of the Drosophila segment polarity genes is initiated by a pre-pattern of pair-rule gene products and maintained by a network of regulatory interactions throughout several stages of embryonic development. Analysis of a model of gene interactions based on differential equations showed that wild-type expression patterns of these genes can be obtained for a wide range of kinetic parameters, which suggests that the steady states are determined by the topology of the network and the type of regulatory interactions between components, not the detailed form of the rate laws. To investigate this, we propose and analyse a Boolean model of this network which is based on a binary ON/OFF representation of mRNA and protein levels, and in which the interactions are formulated as logical functions. In this model the spatial and temporal patterns of gene expression are determined by the topology of the network and whether components are present or absent, rather than the absolute levels of the mRNAs and proteins and the functional details of their interactions. The model is able to reproduce the wild-type gene expression patterns, as well as the ectopic expression patterns observed in overexpression experiments and various mutants. Furthermore, we compute explicitly all steady states of the network and identify the basin of attraction of each steady state. The model gives important insights into the functioning of the segment polarity gene network, such as the crucial role of the wingless and sloppy paired genes, and the network's ability to correct errors in the pre-pattern.

Similarities between the Hedgehog and Wnt signaling pathways

Hedgehog and Wnt proteins are signaling molecules that direct many aspects of metazoan development through signal transduction pathways that are just beginning to be understood. Recently, the common use of glycogen synthase kinase 3 and casein kinase 1 has been added to a growing list of straightforward similarities between Hedgehog and Wnt signaling pathways. These kinases silence both pathways by labeling a key transcription factor (Cubitus interruptus) or co-activator (beta-catenin) for proteolysis, and it is possible that reversal of these phosphorylation events is, in each case, central to pathway activation. This review compares the two pathways to explore whether our more extensive knowledge of Wnt pathways can be of predictive value for investigating Hedgehog signaling.

Distinct protein degradation mechanisms mediated by Cul1 and Cul3 controlling Ci stability in Drosophila eye development

The ubiquitin-like protein, Nedd8, covalently modifies members of the Cullin family. Cullins are the major components of a series of ubiquitin ligases that control the degradation of a broad range of proteins. We found that Nedd8 modifies Cul1 in Drosophila. In Drosophila Nedd8 and Cul1 mutants, protein levels of the signal transduction effectors, Cubitus interruptus (Ci) and Armadillo (Arm), and the cell cycle regulator, Cyclin E (CycE), are highly accumulated, suggesting that the Cul1-based SCF complex requires Nedd8 modification for the degradation processes of Ci, Arm, and CycE in vivo. We further show that two distinct degradation mechanisms modulating Ci stability in the developing eye disc are separated by the morphogenetic furrow (MF) in which retinal differentiation is initiated. In cells anterior to the MF, Ci proteolytic processing promoted by PKA requires the activity of the Nedd8-modified Cul1-based SCF(Slimb) complex. In posterior cells, Ci degradation is controlled by a mechanism that requires the activity of Cul3, another member of the Cullin family. This posterior Ci degradation mechanism, which partially requires Nedd8 modification, is activated by Hedgehog (Hh) signaling and is PKA-independent.

Drosophila Roc1a encodes a RING-H2 protein with a unique function in processing the Hh signal transducer Ci by the SCF E3 ubiquitin ligase

Substrate specificity of SCF E3 ubiquitin ligases is thought to be determined by the F box protein subunit. Another component of SCF complexes is provided by members of the Roc1/Rbx1/Hrt1 gene family, which encode RING-H2 proteins. Drosophila contains three members of this gene family. We show that Roc1a mutant cells fail to proliferate. Further, while the F box protein Slimb is required for Cubitus interruptus (Ci) and Armadillo/beta-catenin (Arm) proteolysis, Roc1a mutant cells hyperaccumulate Ci but not Arm. This suggests that Slimb and Roc1a function in the same SCF complex to target Ci but that a different RING-H2 protein acts with Slimb to target Arm. Consequently, the identity of the Roc subunit may contribute to the selection of substrates by metazoan SCF complexes.

Hedgehog signal transduction proteins: contacts of the Fused kinase and Ci transcription factor with the kinesin-related protein Costal2

BACKGROUND

Hedgehog signaling proteins play important roles in development by controlling growth and patterning in various animals including Drosophila and mammals. Hedgehog signaling triggers changes in responsive cells through a novel transduction mechanism that ultimately controls the transcription of specific target genes via the activity of zinc finger transcription factors of the Cubitus interruptus/GLI family. In flies, key Hedgehog signal transduction components have been identified including the kinesin-related protein Costal2, the serinethreonine kinase Fused, and the PEST-containing protein Suppressor of Fused. These proteins control Cubitus interruptus cleavage, nucleo-cytoplasmic localization and activation. In fly embryos, Costal2, Fused, Suppressor of Fused and Cubitus interruptus are associated in at least one cytoplasmic complex, which interacts with the microtubules in a Hedgehog-dependent manner.

RESULTS

Here we identified and mapped direct interactions between Cos2, Fu, and Ci using an in vitro affinity assay and the yeast two-hybrid system.

CONCLUSIONS

Our results provide new insights into the possible mechanism of the cytosolic steps of Hedgehog transduction.

The carboxyl-terminal domain of the protein kinase fused can function as a dominant inhibitor of hedgehog signaling

The secreted protein hedgehog (Hh) plays a critical role in the developmental patterning of multiple tissues. In Drosophila melanogaster, a cytosolic multiprotein signaling complex appears necessary for Hh signaling. Genes that encode components of this Hh signaling complex (HSC) were originally identified and characterized based on their genetic interactions with hh, as well as with each other. It is only in recent years that the mechanistic functions of these components have begun to be unraveled. Here, we have investigated the relationship between two components of the HSC, the serine/threonine protein kinase Fused (Fu) and the kinesin-related protein Costal2 (Cos2). We have reconstituted a Fu/Cos2 complex in vitro and shown that Fu is able to directly associate with Cos2, forming a complex whose molecular size is similar to a previously described complex found in Drosophila cell extracts. We have also determined that the carboxyl-terminal domain of Fu is necessary and sufficient for the direct binding of Fu to Cos2. To validate the physiological relevance of this interaction, we overexpressed the carboxyl-terminal domain of Fu in wild-type flies. These flies exhibit a phenotype similar to that seen in fu mutants and consistent with an hh loss-of-function phenotype. We conclude that the carboxyl-terminal domain of Fu can function in a dominant negative manner, by preventing endogenous Fu from binding to Cos2. Thus, we provide the first evidence that Hh signaling can be compromised by targeting the HSC for disruption.