Regulation of Ci-SCFSlimb binding, Ci proteolysis, and hedgehog pathway activity by Ci phosphorylation

CSN-mediated deneddylation differentially modulates Ci(155) proteolysis to promote Hedgehog signalling responses

The Hedgehog (Hh) morphogen directs distinct cell responses according to its distinct signalling levels. Hh signalling stabilizes transcription factor cubitus interruptus (Ci) by prohibiting SCF^Slimb-dependent ubiquitylation and proteolysis of Ci. How graded Hh signalling confers differential SCF^Slimb-mediated Ci proteolysis in responding cells remains unclear. Here, we show that in COP9 signalosome (CSN) mutants, in which deneddylation of SCF^Slimb is inactivated, Ci is destabilized in low-to-intermediate Hh signalling cells. As a consequence, expression of the low-threshold Hh target gene dpp is disrupted, highlighting the critical role of CSN deneddylation on low-to-intermediate Hh signalling response. The status of Ci phosphorylation and the level of E1 ubiquitin-activating enzyme are tightly coupled to this CSN regulation. We propose that the affinity of substrate–E3 interaction, ligase activity and E1 activity are three major determinants for substrate ubiquitylation and thereby substrate degradation in vivo.

Hedgehog signalling gradients are required for proper wing formation in Drosophila, and Hedgehog is known to regulate the cubitus interruptus transcription factor. Here, the authors show that the COP9 signalosome has a critical role in translating a Hedgehog gradient into a cubitus interruptus gradient.

Sonic hedgehog signaling in the developing CNS where it has been and where it is going

Sonic Hedgehog (Shh) is one of three mammalian orthologs of the Hedgehog (Hh) family of secreted proteins first identified for their role in patterning the Drosophila embryo. In this review, we will highlight some of the outstanding questions regarding how Shh signaling controls embryonic development. We will mainly consider its role in the developing mammalian central nervous system (CNS) where the pathway plays a critical role in orchestrating the specification of distinct cell fates within ventral regions, a process of exquisite complexity that is necessary for the proper wiring and hence function of the mature system. Embryonic development is a process that plays out in both the spatial and the temporal dimensions, and it is becoming increasingly clear that our understanding of Shh signaling in the CNS is grounded in an appreciation for the dynamic nature of this process. In addition, any consideration of Hh signaling must by necessity include a consideration of data from many different model organisms and systems. In many cases, the extent to which insights gained from these studies are applicable to the CNS remains to be determined, yet they provide a strong framework in which to explore its role in CNS development. We will also discuss how Shh controls cell fate diversification through the regulation of patterned target gene expression in the spinal cord, a region where our understanding of the morphogenetic action of graded Shh signaling is perhaps the furthest advanced.

The hedgehog/Gli signaling paradigm in prostate cancer

Hedgehog is a ligand-activated signaling pathway that regulates Gli-mediated transcription. Although most noted for its role as an embryonic morphogen, hyperactive hedgehog also causes human skin and brain malignancies. The hedgehog-related gene anomalies found in these tumors are rarely found in prostate cancer. Yet surveys of human prostate tumors show concordance of high expression of hedgehog ligands and Gli2 that correlate with the potential for metastasis and therapy-resistant behavior. Likewise, prostate cancer cell lines express hedgehog target genes, and their growth and survival is affected by hedgehog/Gli inhibitors. To date, the preponderance of data supports the idea that prostate tumors benefit from a paracrine hedgehog microenvironment similar to the developing prostate. Uncertainty remains as to whether hedgehog's influence in prostate cancer also includes aspects of tumor cell autocrine-like signaling. The recent findings that Gli proteins interact with the androgen receptor and affect its transcriptional output have helped to identify a novel pathway through which hedgehog/Gli might affect prostate tumor behavior and raises questions as to whether hedgehog signaling in prostate cancer cells is suitably measured by the expression of Gli target genes alone.

Primary cilium-dependent and -independent Hedgehog signaling inhibits p16(INK4A)

In a genome-wide siRNA analysis of p16(INK4a) (p16) modulators, we identify the Hedgehog (Hh) pathway component SUFU and formally demonstrate that Hh signaling promotes mitogenesis by suppression of p16. A fragment of the Hh-responsive GLI2 transcription factor directly binds and inhibits the p16 promoter and senescence is associated with the loss of nuclear GLI2. Hh components partially reside in the primary cilium (PC), and the small fraction of cells in mass culture that elaborate a PC have the lowest expression of p16. Suppression of p16 is effected by both PC-dependent and -independent routes, and ablation of p16 renders cells insensitive to an Hh inhibitor and increases PC formation. These results directly link a well-established developmental mitogenic pathway with a key tumor suppressor and contribute to the molecular understanding of replicative senescence, Hh-mediated oncogenesis, and potentially the role of p16 in aging.

Inhibition of the nuclear import of cubitus interruptus by roadkill in the presence of strong hedgehog signal

Hedgehog (Hh) signalling plays an important role in various developmental processes by activating the Cubitus interruptus (Ci)/Glioblastoma (Gli) family of transcription factors. In the process of proper pattern formation, Ci activity is regulated by multiple mechanisms, including processing, trafficking, and degradation. However, it remains elusive how Ci distinctly recognizes the strong and moderate Hh signals. Roadkill (Rdx) induces Ci degradation in the anterior region of the Drosophila wing disc. Here, we report that Rdx inhibited Ci activity by two different mechanisms. In the region abutting the anterior/posterior boundary, which receives strong Hh signal, Rdx inhibited the nuclear import of Ci by releasing importin α3 from Ci. In this region, Rdx negatively regulated the expression of transcription factor Knot/Collier. In farther anterior regions receiving moderate levels of Hh signal, Rdx induced Ci degradation, as reported previously. Thus, two different mechanisms by which Rdx negatively regulates Ci may play an important role in the fine-tuning of Hh responses.

kin-19/casein kinase Iα has dual functions in regulating asymmetric division and terminal differentiation in C. elegans epidermal stem cells

Casein Kinase I (CKI) is a conserved component of the Wnt signaling pathway, which regulates cell fate determination in metazoans. We show that post-embryonic asymmetric division and fate specification of C. elegans epidermal stem cells are controlled by a non-canonical Wnt/β-catenin signaling pathway, involving the β-catenins WRM-1 and SYS-1, and that C. elegans kin-19/CKIα functions in this pathway. Furthermore, we find that kin-19 is the only member of the Wnt asymmetry pathway that functions with, or in parallel to, the heterochronic temporal patterning pathway to control withdrawal from self-renewal and subsequent terminal differentiation of epidermal stem cells. We show that, except in the case of kin-19, the Wnt asymmetry pathway and the heterochronic pathway function separately and in parallel to control different aspects of epidermal stem cell fate specification. However, given the function of kin-19/CKIα in both pathways, and that CKI, Wnt signaling pathway and heterochronic pathway genes are widely conserved in animals, our findings suggest that CKIα may function as a regulatory hub through which asymmetric division and terminal differentiation are coordinated in adult stem cells of vertebrates.

G protein-coupled receptor kinase 2 promotes high-level Hedgehog signaling by regulating the active state of Smo through kinase-dependent and kinase-independent mechanisms in Drosophila

G protein-coupled receptor kinase 2 (Gprk2/GRK2) plays a conserved role in modulating Hedgehog (Hh) pathway activity, but its mechanism of action remains unknown. Here we provide evidence that Gprk2 promotes high-level Hh signaling by regulating Smoothened (Smo) conformation through both kinase-dependent and kinase-independent mechanisms. Gprk2 promotes Smo activation by phosphorylating Smo C-terminal tail (C-tail) at Ser741/Thr742, which is facilitated by PKA and CK1 phosphorylation at adjacent Ser residues. In addition, Gprk2 forms a dimer/oligomer and binds Smo C-tail in a kinase activity-independent manner to stabilize the active Smo conformation, and promotes dimerization/oligomerization of Smo C-tail. Gprk2 expression is induced by Hh signaling, and Gprk2/Smo interaction is facilitated by PKA/CK1-mediated phosphorylation of Smo C-tail. Thus, Gprk2 forms a positive feedback loop and acts downstream from PKA and CK1 to facilitate high-level Hh signaling by promoting the active state of Smo through direct phosphorylation and molecular scaffolding.

Costal 2 interactions with Cubitus interruptus (Ci) underlying Hedgehog-regulated Ci processing

Extracellular Hedgehog (Hh) proteins alter cellular behaviours from flies to man by regulating the activities of Gli/Ci family transcription factors. A major component of this response in Drosophila is the inhibition of proteolytic processing of the latent transcriptional activator Ci-155 to a shorter Ci-75 repressor form. Processing is thought to rely on binding of the kinesin-family protein Cos2 directly to Ci-155 domains known as CDN and CORD, allowing Cos2-associated protein kinases to phosphorylate Ci-155 efficiently and create a binding site for an E3 ubiquitin ligase complex. Here we show that the last three zinc fingers of Ci-155 also bind Cos2 in vitro and that the zinc finger region, rather than the CDN domain, functions redundantly with the CORD domain to promote Hh-regulated Ci-155 proteolysis in wing discs. We also find evidence for a unique function of Cos2 binding to CORD. Cos2 binding to CORD, but not to other regions of Ci, is potentiated by nucleotides and abrogated by the nucleotide binding variant Cos2 S182N. Removal of the CORD region alone enhances processing under a variety of conditions. Most strikingly, CORD region deletion allows Cos2 S182N to stimulate efficient Ci processing. We deduce that the CORD region has a second function distinct from Cos2 binding that inhibits Ci processing, and that Cos2 binding to CORD relieves this inhibition. We suggest that this regulatory activity of Cos2 depends on a specific nucleotide-bound conformation that may be regulated by Hh.

Drosophila Cand1 regulates Cullin3-dependent E3 ligases by affecting the neddylation of Cullin3 and by controlling the stability of Cullin3 and adaptor protein

Cullin-RING ubiquitin ligases (CRLs), which comprise the largest class of E3 ligases, regulate diverse cellular processes by targeting numerous proteins. Conjugation of the ubiquitin-like protein Nedd8 with Cullin activates CRLs. Cullin-associated and neddylation-dissociated 1 (Cand1) is known to negatively regulate CRL activity by sequestering unneddylated Cullin1 (Cul1) in biochemical studies. However, genetic studies of Arabidopsis have shown that Cand1 is required for optimal CRL activity. To elucidate the regulation of CRLs by Cand1, we analyzed a Cand1 mutant in Drosophila. Loss of Cand1 causes accumulation of neddylated Cullin3 (Cul3) and stabilizes the Cul3 adaptor protein HIB. In addition, the Cand1 mutation stimulates protein degradation of Cubitus interruptus (Ci), suggesting that Cul3-RING ligase activity is enhanced by the loss of Cand1. However, the loss of Cand1 fails to repress the accumulation of Ci in Nedd8(AN015) or CSN5(null) mutant clones. Although Cand1 is able to bind both Cul1 and Cul3, mutation of Cand1 suppresses only the accumulation of Cul3 induced by the dAPP-BP1 mutation defective in the neddylation pathway, and this effect is attenuated by inhibition of proteasome function. Furthermore, overexpression of Cand1 stabilizes the Cul3 protein when the neddylation pathway is partially suppressed. These data indicate that Cand1 stabilizes unneddylated Cul3 by preventing proteasomal degradation. Here, we propose that binding of Cand1 to unneddylated Cul3 causes a shift in the equilibrium away from the neddylation of Cul3 that is required for the degradation of substrate by CRLs, and protects unneddylated Cul3 from proteasomal degradation. Cand1 regulates Cul3-mediated E3 ligase activity not only by acting on the neddylation of Cul3, but also by controlling the stability of the adaptor protein and unneddylated Cul3.

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.

Not lost in space: trafficking in the hedgehog signaling pathway

Compartmentalization within cells provides spatial organization of signaling pathways and ensures the specificity of signaling. In vertebrates, the primary cilium, a tiny microtubule-based protrusion present on most cells, is essential for organizing events during Hedgehog signal transduction. When cells are stimulated with Hedgehog ligands, proteins in the pathway move in and out of the cilia. Protein kinase A (PKA), which is implicated in diverse cellular processes including protein trafficking, is a component of the Hedgehog signaling pathway. PKA has been localized near primary cilia, at a location suitable for regulating the localization of other proteins in the pathway.

The output of Hedgehog signaling is controlled by the dynamic association between Suppressor of Fused and the Gli proteins

The transcriptional program orchestrated by Hedgehog signaling depends on the Gli family of transcription factors. Gli proteins can be converted to either transcriptional activators or truncated transcriptional repressors. We show that the interaction between Gli3 and Suppressor of Fused (Sufu) regulates the formation of either repressor or activator forms of Gli3. In the absence of signaling, Sufu restrains Gli3 in the cytoplasm, promoting its processing into a repressor. Initiation of signaling triggers the dissociation of Sufu from Gli3. This event prevents formation of the repressor and instead allows Gli3 to enter the nucleus, where it is converted into a labile, differentially phosphorylated transcriptional activator. This key dissociation event depends on Kif3a, a kinesin motor required for the function of primary cilia. We propose that the Sufu-Gli3 interaction is a major control point in the Hedgehog pathway, a pathway that plays important roles in both development and cancer.

Mechanisms of protein kinase A anchoring

The second messenger cyclic adenosine monophosphate (cAMP), which is produced by adenylyl cyclases following stimulation of G-protein-coupled receptors, exerts its effect mainly through the cAMP-dependent serine/threonine protein kinase A (PKA). Due to the ubiquitous nature of the cAMP/PKA system, PKA signaling pathways underlie strict spatial and temporal control to achieve specificity. A-kinase anchoring proteins (AKAPs) bind to the regulatory subunit dimer of the tetrameric PKA holoenzyme and thereby target PKA to defined cellular compartments in the vicinity of its substrates. AKAPs promote the termination of cAMP signals by recruiting phosphodiesterases and protein phosphatases, and the integration of signaling pathways by binding additional signaling proteins. AKAPs are a heterogeneous family of proteins that only display similarity within their PKA-binding domains, amphipathic helixes docking into a hydrophobic groove formed by the PKA regulatory subunit dimer. This review summarizes the current state of information on compartmentalized cAMP/PKA signaling with a major focus on structural aspects, evolution, diversity, and (patho)physiological functions of AKAPs and intends to outline newly emerging directions of the field, such as the elucidation of AKAP mutations and alterations of AKAP expression in human diseases, and the validation of AKAP-dependent protein-protein interactions as new drug targets. In addition, alternative PKA anchoring mechanisms employed by noncanonical AKAPs and PKA catalytic subunit-interacting proteins are illustrated.

Regulation of protein stability by GSK3 mediated phosphorylation

Glycogen synthase kinase-3 (GSK3) plays important roles in numerous signaling pathways that regulate a variety of cellular processes including cell proliferation, differentiation, apoptosis and embryonic development. In the canonical Wnt signaling pathway, GSK3 phosphorylation mediates proteasomal targeting and degradation of beta-catenin via the destruction complex. We recently reported a biochemical screen that discovered multiple additional protein substrates whose stability is regulated by Wnt signaling and/or GSK3 and these have important implications for Wnt/GSK3 regulation of different cellular processes.(1) In this article, we also present a bio-informatics based screen for proteins whose stability may be controlled by GSK3 and beta-Trcp, the SCF E3 ubiquitin ligase that is responsible for beta-catenin degradation in the Wnt signaling pathway. Furthermore, we review various GSK3 regulated proteolysis substrates described in the literature. We propose that GSK3 phosphorylation dependent proteolysis is a widespread mechanism that the cell employs to regulate a variety of cell processes in response to signals.

Multiple Ser/Thr-rich degrons mediate the degradation of Ci/Gli by the Cul3-HIB/SPOP E3 ubiquitin ligase

The Cul3-based E3 ubiquitin ligases regulate many cellular processes using a large family of BTB domain-containing proteins as their target recognition components, but how they recognize targets remains unknown. Here we identify and characterize degrons that mediate the degradation of the Hedgehog pathway transcription factor cubitus interruptus (Ci)/Gli by Cul3-Hedghog-induced MATH and BTB domain-containing protein (HIB)/SPOP. Ci uses multiple Ser/Thr (S/T)-rich motifs that bind HIB cooperatively to mediate its degradation. We provide evidence that both HIB and Ci form dimers/oligomers and engage in multivalent interactions, which underlies the in vivo cooperativity among individual HIB-binding sites. We find that similar S/T-rich motifs are present in Gli proteins as well as in numerous HIB-interacting proteins and mediate Gli degradation by SPOP. Our results provide a mechanistic insight into how HIB/SPOP recognizes its substrates and have important implications for the genome-wide prediction of substrates for Cul3-based E3 ligases.

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.

A quantification of pathway components supports a novel model of Hedgehog signal transduction

The secreted protein Hedgehog (Hh) plays a critical instructional role during metazoan development. In Drosophila, Hh signaling is interpreted by a set of conserved, downstream effectors that differentially localize and interact to regulate the stability and activity of the transcription factor Cubitus interruptus. Two essential models that integrate genetic, cell biological, and biochemical information have been proposed to explain how these signaling components relate to one another within the cellular context. As the molar ratios of the signaling effectors required in each of these models are quite different, quantitating the cellular ratio of pathway components could distinguish these two models. Here, we address this important question using a set of purified protein standards to perform a quantitative analysis of Drosophila cell lysates for each downstream pathway component. We determine each component's steady-state concentration within a given cell, demonstrate the molar ratio of Hh signaling effectors differs more than two orders of magnitude and that this ratio is conserved in vivo. We find that the G-protein-coupled transmembrane protein Smoothened, an activating component, is present in limiting amounts, while a negative pathway regulator, Suppressor of Fused, is present in vast molar excess. Interestingly, despite large differences in the steady-state ratio, all downstream signaling components exist in an equimolar membrane-associated complex. We use these quantitative results to re-evaluate the current models of Hh signaling and now propose a novel model of signaling that accounts for the stoichiometric differences observed between various Hh pathway components.

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.

Variations in Hedgehog signaling: divergence and perpetuation in Sufu regulation of Gli

The Hedgehog (Hh) proteins play a universal role in metazoan development. Nevertheless, fundamental differences exist between Drosophila and vertebrates in the transduction of the Hh signal, notably regarding the role of primary cilia in mammalian cells. In this issue of Genes & Development, Chen and colleagues (pp. 1910-1928) demonstrate that mouse Suppressor of fused (Sufu) regulates the stability of the transcription factors Gli2 and Gli3 by antagonizing the conserved Gli degradation device mediated by Hib/Spop in a cilia-independent manner.

The role of Parafibromin/Hyrax as a nuclear Gli/Ci-interacting protein in Hedgehog target gene control

The Hedgehog (Hh) pathway, an evolutionarily conserved key regulator of embryonic patterning and tissue homeostasis, controls its target genes by managing the processing and activities of the Gli/Ci transcription factors. Little is known about the nuclear co-factors the Gli/Ci proteins recruit, and how they mechanistically control Hh target genes. Here, we provide evidence for the involvement of Parafibromin/Hyx as a positive component in Hh signaling. We found that hyx RNAi impaired Hh pathway activity in Drosophila cell culture. Consistent with an evolutionarily conserved function in Hh signaling, RNAi-mediated knockdown of Parafibromin in mammalian cell culture experiments diminished the transcriptional activity of Gli1 and Gli2. In vivo, in Drosophila, genetic impairment of hyx decreased the expression of the high-threshold Hh target gene knot/collier. Conversely, hyx overexpression ameliorated the inhibitory activity of Ptc and Ci(75) misexpression during Drosophila wing development. We subsequently found that Parafibromin can form a complex with all three Glis, and provide evidence that Parafibromin/Hyx directly binds Region 1, the Su(fu) interaction domain, in the N-terminus of all Glis and Ci. Taken together, our results suggest a target gene-selective involvement of the PAF1 complex in Hh signaling via the Parafibromin/Hyx-mediated recruitment to Gli/Ci.

Hedgehog signaling in development and cancer

The Hedgehog (Hh) family of secreted proteins governs a wide variety of processes during embryonic development and adult tissue homeostasis. Here we review the current understanding of the molecular and cellular basis of Hh morphogen gradient formation and signal transduction, and the multifaceted roles of Hh signaling in development and tumorigenesis. We discuss how the Hh pathway has diverged during evolution and how it integrates with other signaling pathways to control cell growth and patterning.

Phosphorylation of Gli2 by protein kinase A is required for Gli2 processing and degradation and the Sonic Hedgehog-regulated mouse development

In mice, Gli2 and Gli3 are the transcription factors that mediate the initial Hedgehog (Hh) signaling. In the absence of Hh signaling, the majority of the full-length Gli3 protein undergoes proteolytic processing into a repressor, while only a small fraction of the full-length Gli2 protein is processed. Gli3 processing is dependent on phosphorylation of the first four of the six protein kinase A (PKA) sites at its C-terminus. However, whether the same phosphorylation of Gli2 by PKA is required for Gli2 processing and, if so, whether such phosphorylation regulates additional Gli2 function are unknown. To address these questions, we mutated these PKA sites in the mouse Gli2 locus to create the Gli2(P1-4) allele. Gli2(P1-4) homozygous embryos die in utero and exhibit exencephaly, defects in neural tube closure, enlarged craniofacial structures, and an extra anterior digit. Analysis of spinal cord patterning shows that domains of motoneurons and V2, V1, and V0 interneurons are expanded to different degrees in both Gli2(P1-4) single and Gli2(P1-4);Shh double mutants. Furthermore, Gli2(P1-4) expression prevents massive cell death and promotes cell proliferation in Shh mutant. Analysis of Gli2(P1-4) protein in vivo reveals that the mutant protein is not processed and is twice as stable as wild type Gli2 protein. We also show that the Gli2 repressor can effectively antagonize Gli2P1-4 activity. Together, these results indicate that phosphorylation of Gli2 by PKA induces Gli2 processing and destabilization in vivo and plays an important role in the Hh-regulated mouse embryonic patterning.

The phospho-occupancy of an atypical SLIMB-binding site on PERIOD that is phosphorylated by DOUBLETIME controls the pace of the clock

A common feature of animal circadian clocks is the progressive phosphorylation of PERIOD (PER) proteins, which is highly dependent on casein kinase Idelta/epsilon (CKIdelta/epsilon; termed DOUBLETIME [DBT] in Drosophila) and ultimately leads to the rapid degradation of hyperphosphorylated isoforms via a mechanism involving the F-box protein, beta-TrCP (SLIMB in Drosophila). Here we use the Drosophila melanogaster model system, and show that a key step in controlling the speed of the clock is phosphorylation of an N-terminal Ser (S47) by DBT, which collaborates with other nearby phosphorylated residues to generate a high-affinity atypical SLIMB-binding site on PER. DBT-dependent increases in the phospho-occupancy of S47 are temporally gated, dependent on the centrally located DBT docking site on PER and partially counterbalanced by protein phosphatase activity. We propose that the gradual DBT-mediated phosphorylation of a nonconsensus SLIMB-binding site establishes a temporal threshold for when in a daily cycle the majority of PER proteins are tagged for rapid degradation. Surprisingly, most of the hyperphosphorylation is unrelated to direct effects on PER stability. We also use mass spectrometry to map phosphorylation sites on PER, leading to the identification of a number of "phospho-clusters" that explain several of the classic per mutants.

A unique protection signal in Cubitus interruptus prevents its complete proteasomal degradation

The limited proteolysis of Cubitus interruptus (Ci), the transcription factor for the developmentally and medically important Hedgehog (Hh) signaling pathway, triggers a critical switch between transcriptional repressor and activator forms. Ci repressor is formed when the C terminus of full-length Ci is degraded by the ubiquitin-proteasome pathway, an unusual reaction since the proteasome typically completely degrades its substrates. We show that several regions of Ci are required for generation of the repressor form: the zinc finger DNA binding domain, a single lysine residue (K750) near the degradation end point, and a 163-amino-acid region at the C terminus. Unlike other proteins that are partially degraded by the proteasome, dimerization is not a key feature of Ci processing. Using a pulse-chase assay in cultured Drosophila cells, we distinguish between regions required for initiation of degradation and those required for the protection of the Ci N terminus from degradation. We present a model whereby the zinc finger region and K750 together form a unique protection signal that prevents the complete degradation of Ci by the proteasome.

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.

Hedgehog signaling: a smoothened conformational switch

An intracellular conformational switch in the serpentine transmembrane protein Smoothened appears to underlie Hedgehog pathway activation. The switch is gated by electrostatic interactions that are regulated by multiple phosphorylations, potentially endowing a dose-dependent response.