Scaffolding proteins organize multimolecular protein complexes for sensory signal transduction

Interactions of drosophila cryptochrome

In this study, we investigate the intricate regulatory mechanisms underlying the circadian clock in Drosophila, focusing on the light-induced conformational changes in the cryptochrome (DmCry). Upon light exposure, DmCry undergoes conformational changes that prompt its binding to Timeless and Jetlag proteins, initiating a cascade crucial for the starting of a new circadian cycle. DmCry is subsequently degraded, contributing to the desensitization of the resetting mechanism. The transient and short-lived nature of DmCry protein-protein interactions (PPIs), leading to DmCry degradation within an hour of light exposure, presents a challenge for comprehensive exploration. To address this, we employed proximity-dependent biotinylation techniques, combining engineered BioID (TurboID) and APEX (APEX2) enzymes with mass spectrometry. This approach enabled the identification of the in vitro DmCry interactome in Drosophila S2 cells, uncovering several novel PPIs associated with DmCry. Validation of these interactions through a novel co-immunoprecipitation technique enhances the reliability of our findings. Importantly, our study suggests the potential of this method to reveal additional circadian clock- or magnetic field-dependent PPIs involving DmCry. This exploration of the DmCry interactome not only advances our understanding of circadian clock regulation but also establishes a versatile framework for future investigations into light- and time-dependent protein interactions in Drosophila.

Molecular encoding of stimulus features in a single sensory neuron type enables neuronal and behavioral plasticity

Neurons modify their transcriptomes in response to an animal's experience. How specific experiences are transduced to modulate gene expression and precisely tune neuronal functions are not fully defined. Here, we describe the molecular profile of a thermosensory neuron pair in C. elegans experiencing different temperature stimuli. We find that distinct salient features of the temperature stimulus, including its duration, magnitude of change, and absolute value, are encoded in the gene expression program in this single neuron type, and we identify a novel transmembrane protein and a transcription factor whose specific transcriptional dynamics are essential to drive neuronal, behavioral, and developmental plasticity. Expression changes are driven by broadly expressed activity-dependent transcription factors and corresponding cis-regulatory elements that nevertheless direct neuron- and stimulus-specific gene expression programs. Our results indicate that coupling of defined stimulus characteristics to the gene regulatory logic in individual specialized neuron types can customize neuronal properties to drive precise behavioral adaptation.

Molecular encoding of stimulus features in a single sensory neuron type enables neuronal and behavioral plasticity

Neurons modify their transcriptomes in response to an animal’s experience. How specific experiences are transduced to modulate gene expression and precisely tune neuronal functions are not fully defined. Here, we describe the molecular profile of a thermosensory neuron pair in C. elegans experiencing different temperature stimuli. We find that distinct salient features of the temperature stimulus including its duration, magnitude of change, and absolute value are encoded in the gene expression program in this single neuron, and identify a novel transmembrane protein and a transcription factor whose specific transcriptional dynamics are essential to drive neuronal, behavioral, and developmental plasticity. Expression changes are driven by broadly expressed activity-dependent transcription factors and corresponding cis -regulatory elements that nevertheless direct neuron- and stimulus-specific gene expression programs. Our results indicate that coupling of defined stimulus characteristics to the gene regulatory logic in individual specialized neuron types can customize neuronal properties to drive precise behavioral adaptation.

An unexpected INAD PDZ tandem-mediated plcβ binding in Drosophila photo receptors

INAD assembles key enzymes of the Drosophila compound eye photo-transduction pathway into a supramolecular complex, supporting efficient and fast light signaling. However, the molecular mechanism that governs the interaction between INAD and NORPA (phospholipase Cβ, PLCβ), a key step for the fast kinetics of the light signaling, is not known. Here, we show that the NORPA C-terminal coiled-coil domain and PDZ-binding motif (CC-PBM) synergistically bind to INAD PDZ45 tandem with an unexpected mode and unprecedented high affinity. Guided by the structure of the INAD-NORPA complex, we discover that INADL is probably a mammalian counterpart of INAD. The INADL PDZ89 tandem specifically binds to PLCβ4 with a mode that is strikingly similar to that of the INAD-NORPA complex, as revealed by the structure of the INADL PDZ89-PLCβ4 CC-PBM complex. Therefore, our study suggests that the highly specific PDZ tandem - PLCβ interactions are an evolutionarily conserved mechanism in PLCβ signaling in the animal kingdom.

Calmodulin Enhances Cryptochrome Binding to INAD in Drosophila Photoreceptors

Light is the main environmental stimulus that synchronizes the endogenous timekeeping systems in most terrestrial organisms. Drosophila cryptochrome (dCRY) is a light-responsive flavoprotein that detects changes in light intensity and wavelength around dawn and dusk. We have previously shown that dCRY acts through Inactivation No Afterpotential D (INAD) in a light-dependent manner on the Signalplex, a multiprotein complex that includes visual-signaling molecules, suggesting a role for dCRY in fly vision. Here, we predict and demonstrate a novel Ca²⁺-dependent interaction between dCRY and calmodulin (CaM). Through yeast two hybrid, coimmunoprecipitation (Co-IP), nuclear magnetic resonance (NMR) and calorimetric analyses we were able to identify and characterize a CaM binding motif in the dCRY C-terminus. Similarly, we also detailed the CaM binding site of the scaffold protein INAD and demonstrated that CaM bridges dCRY and INAD to form a ternary complex in vivo. Our results suggest a process whereby a rapid dCRY light response stimulates an interaction with INAD, which can be further consolidated by a novel mechanism regulated by CaM.

Membrane protein trafficking in Drosophila photoreceptor cells

Membrane protein trafficking occurs throughout the lifetime of neurons and includes the initial protein synthesis and anterograde transport to the plasma membrane as well as internalization, degradation, and recycling of plasma membrane proteins. Defects in protein trafficking can result in neuronal degeneration and underlie blinding diseases such as retinitis pigmentosa as well as other neuronal disorders. Drosophila photoreceptor cells have emerged as a model system for identifying the components and mechanisms involved in membrane protein trafficking in neurons. Here we summarize the current knowledge about trafficking of three Drosophila phototransduction proteins, the visual pigment rhodopsin and the two light-activated ion channels TRP (transient receptor potential) and TRPL (TRP-like). Despite some common requirements shared by rhodopsin and TRP, details in the trafficking of these proteins differ considerably, suggesting the existence of several trafficking pathways for these photoreceptor proteins.

Biochemical Large-Scale Interaction Analysis of Murine Olfactory Receptors and Associated Signaling Proteins with Post-Synaptic Density 95, Drosophila Discs Large, Zona-Occludens 1 (PDZ) Domains

G protein-coupled receptors (GPCRs) constitute the largest family among mammalian membrane proteins and are capable of initiating numerous essential signaling cascades. Various GPCR-mediated pathways are organized into protein microdomains that can be orchestrated and regulated through scaffolding proteins, such as PSD-95/discs-large/ZO1 (PDZ) domain proteins. However, detailed binding characteristics of PDZ-GPCR interactions remain elusive because these interactions seem to be more complex than previously thought. To address this issue, we analyzed binding modalities using our established model system. This system includes the 13 individual PDZ domains of the multiple PDZ domain protein 1 (MUPP1; the largest PDZ protein), a broad range of murine olfactory receptors (a multifaceted gene cluster within the family of GPCRs), and associated olfactory signaling proteins. These proteins were analyzed in a large-scale peptide microarray approach and continuative interaction studies. As a result, we demonstrate that canonical binding motifs were not overrepresented among the interaction partners of MUPP1. Furthermore, C-terminal phosphorylation and distinct amino acid replacements abolished PDZ binding promiscuity. In addition to the described in vitro experiments, we identified new interaction partners within the murine olfactory epithelium using pull-down-based interactomics and could verify the partners through co-immunoprecipitation. In summary, the present study provides important insight into the complexity of the binding characteristics of PDZ-GPCR interactions based on olfactory signaling proteins, which could identify novel clinical targets for GPCR-associated diseases in the future.

Novel role of cystic fibrosis transmembrane conductance regulator in maintaining adult mouse olfactory neuronal homeostasis

The olfactory epithelium (OE) of mice deficient in cystic fibrosis transmembrane conductance regulator (CFTR) exhibits ion transport deficiencies reported in human CF airways, as well as progressive neuronal loss, suggesting defects in olfactory neuron homeostasis. Microvillar cells, a specialized OE cell-subtype, have been implicated in maintaining tissue homeostasis. These cells are endowed with a PLCβ2/IP3 R3/TRPC6 signal transduction pathway modulating release of neuropeptide Y (NPY), which stimulates OE stem cell activity. It is unknown, however, whether microvillar cells also mediate the deficits observed in CFTR-null mice. Here we show that Cftr mRNA in mouse OE is exclusively localized in microvillar cells and CFTR immunofluorescence is coassociated with the scaffolding protein NHERF-1 and PLCβ2 in microvilli. In CFTR-null mice, PLCβ2 was undetectable, NHERF-1 mislocalized, and IP3 R3 more intensely stained, along with increased levels of NPY, suggesting profound alteration of the PLCβ2/IP3 R3 signaling pathway. In addition, basal olfactory neuron homeostasis was altered, shown by increased progenitor cell proliferation, differentiation, and apoptosis and by reduced regenerative capacity following methimazole-induced neurodegeneration. The importance of CFTR in microvillar cells was further underscored by decreased thickness of the OE mucus layer and increased numbers of immune cells within this tissue in CFTR-KO mice. Finally, we observed enhanced immune responses to an acute viral-like infection, as well as hyper-responsiveness to chemical and physical stimuli applied intranasally. Taken together, these data strengthen the notion that microvillar cells in the OE play a key role in maintaining tissue homeostasis and identify several mechanisms underlying this regulation through the multiple functions of CFTR.

Post-Translational Modifications of TRP Channels

Transient receptor potential (TRP) channels constitute an ancient family of cation channels that have been found in many eukaryotic organisms from yeast to human. TRP channels exert a multitude of physiological functions ranging from Ca²⁺ homeostasis in the kidney to pain reception and vision. These channels are activated by a wide range of stimuli and undergo covalent post-translational modifications that affect and modulate their subcellular targeting, their biophysical properties, or channel gating. These modifications include N-linked glycosylation, protein phosphorylation, and covalent attachment of chemicals that reversibly bind to specific cysteine residues. The latter modification represents an unusual activation mechanism of ligand-gated ion channels that is in contrast to the lock-and-key paradigm of receptor activation by its agonists. In this review, we summarize the post-translational modifications identified on TRP channels and, when available, explain their physiological role.

Photosensitive TRPs

The Drosophila "transient receptor potential" channel is the prototypical TRP channel, belonging to and defining the TRPC subfamily. Together with a second TRPC channel, trp-like (TRPL), TRP mediates the transducer current in the fly's photoreceptors. TRP and TRPL are also implicated in olfaction and Malpighian tubule function. In photoreceptors, TRP and TRPL are localised in the ~30,000 packed microvilli that form the photosensitive "rhabdomere"-a light-guiding rod, housing rhodopsin and the rest of the phototransduction machinery. TRP (but not TRPL) is assembled into multimolecular signalling complexes by a PDZ-domain scaffolding protein (INAD). TRPL (but not TRP) undergoes light-regulated translocation between cell body and rhabdomere. TRP and TRPL are also found in photoreceptor synapses where they may play a role in synaptic transmission. Like other TRPC channels, TRP and TRPL are activated by a G protein-coupled phospholipase C (PLCβ4) cascade. Although still debated, recent evidence indicates the channels can be activated by a combination of PIP2 depletion and protons released by the PLC reaction. PIP2 depletion may act mechanically as membrane area is reduced by cleavage of PIP2's bulky inositol headgroup. TRP, which dominates the light-sensitive current, is Ca(2+) selective (P Ca:P Cs >50:1), whilst TRPL has a modest Ca(2+) permeability (P Ca:P Cs ~5:1). Ca(2+) influx via the channels has profound positive and negative feedback roles, required for the rapid response kinetics, with Ca(2+) rapidly facilitating TRP (but not TRPL) and also inhibiting both channels. In trp mutants, stimulation by light results in rapid depletion of microvillar PIP2 due to lack of Ca(2+) influx required to inhibit PLC. This accounts for the "transient receptor potential" phenotype that gives the family its name and, over a period of days, leads to light-dependent retinal degeneration. Gain-of-function trp mutants with uncontrolled Ca(2+) influx also undergo retinal degeneration due to Ca(2+) cytotoxicity. In vertebrate retina, mice knockout studies suggest that TRPC6 and TRPC7 mediate a PLCβ4-activated transducer current in intrinsically photosensitive retinal ganglion cells, expressing melanopsin. TRPA1 has been implicated as a "photo-sensing" TRP channel in human melanocytes and light-sensitive neurons in the body wall of Drosophila.

Quantitative control of intracellular signaling activity through chimeric receptors incorporating multiple identical tyrosine motifs

Controlling activation levels and durations of native signaling molecules is important for efficiently controlling cellular fates. Previously we developed single-chain Fv (scFv)/cytokine receptor chimeras incorporating tyrosine motifs in the intracellular domain, which artificially control the activation of specific intracellular signaling proteins. In this study, to quantitatively control the activation levels of signaling molecules with an extended dynamic range, we constructed scFv/receptor chimeras incorporating multiple identical motifs at the different positions in the intracellular domain. We used retroviral transduction to express chimeric receptors with multiple STAT3 binding motifs connected with or without flexible linkers in a murine IL-3-dependent pro-B cell line, Ba/F3. Our results showed that the chimeric receptors can control the activation levels of STAT3 depending on ligand concentration and the number of motifs. The existence of linkers between the motifs also affected the signal intensity. Furthermore, the STAT3 activation levels significantly depended on the number of motifs rather than the distance from the JAK-binding region to the tyrosine motif.

Structural mechanism of disulphide bond-mediated redox switches

The oxidation of cysteine sulphydryl in proteins produces sulphenic acid that can form a reversible disulphide bond with another cysteine. The disulphide bond formation often triggers switches in protein structure and activity, especially when the distance between the two cysteine sulphur atoms is longer than the resulting disulphide bond distance. As an early example for the reversible disulphide bond-mediated functional switches, the reduced and oxidized forms of the bacterial transcription factor OxyR were characterized by X-ray crystallography. Recently, the Drosophila vision signalling protein, the association of inactivation-no-afterpotential D (INAD) was analysed by structural and functional methods. The two conserved cysteines of INAD were found to cycle between reduced and oxidized states during the light signal processing in Drosophila eyes, which was achieved by conformation dependent modulation of the disulphide bond redox potential. The production of the hypertension control peptide angiotensins was also shown to be controlled by the reversible disulphide bond in the precursor protein angiotensinogen. The crystal structure of the complex of angiotensiongen with its processing enzyme renin elucidated the role of the disulphide bond in stabilizing the precursor-enzyme complex facilitating the production of angiotensins. The increasing importance of the disulphide bond-mediated redox switches in normal and diseased states has implications in the development of novel antioxidant-based therapeutic approaches.

Drosophila visual transduction

Visual transduction in the Drosophila compound eye functions through a pathway that couples rhodopsin to phospholipase C (PLC) and the opening of transient receptor potential (TRP) channels. This cascade differs from phototransduction in mammalian rods and cones, but is remarkably similar to signaling in mammalian intrinsically photosensitive retinal ganglion cells (ipRGCs). In this review, I focus on recent advances in the fly visual system, including the discovery of a visual cycle and insights into the machinery and mechanisms involved in generating a light response in photoreceptor cells.

PDA (prolonged depolarizing afterpotential)-defective mutants: the story of nina's and ina's--pinta and santa maria, too

Our objective is to present a comprehensive view of the PDA (prolonged depolarizing afterpotential)-defective Drosophila mutants, nina's and ina's, from the discussion of the PDA and the PDA-based mutant screening strategy to summaries of the knowledge gained through the studies of mutants generated using the strategy. The PDA is a component of the light-evoked photoreceptor potential that is generated when a substantial fraction of rhodopsin is photoconverted to its active form, metarhodopsin. The PDA-based mutant screening strategy was adopted to enhance the efficiency and efficacy of ERG (electroretinogram)-based screening for identifying phototransduction-defective mutants. Using this strategy, two classes of PDA-defective mutants were identified and isolated, nina and ina, each comprising multiple complementation groups. The nina mutants are characterized by allele-dependent reduction in the major rhodopsin, Rh1, whereas the ina mutants display defects in some aspects of functions related to the transduction channel, TRP (transient receptor potential). The signaling proteins that have been identified and elucidated through the studies of nina mutants include the Drosophila opsin protein (NINAE), the chaperone protein for nascent opsin (NINAA), and the multifunctional protein, NINAC, required in multiple steps of the Drosophila phototransduction cascade. Also identified by the nina mutants are some of the key enzymes involved in the biogenesis of the rhodopsin chromophore. As for the ina mutants, they led to the discovery of the scaffold protein, INAD, responsible for the nucleation of the supramolecular signaling complex. Also identified by the ina mutants is one of the key members of the signaling complex, INAC (ePKC), and two other proteins that are likely to be important, though their roles in the signaling cascade have not yet been fully elucidated. In most of these cases, the protein identified is the first member of its class to be so recognized.

RNA templating the epigenome: long noncoding RNAs as molecular scaffolds

Cellular pathways must be synergized, controlled and organized to manage homeostasis. To achieve high selectivity within the crowded cellular milieu the cell utilizes scaffolding complexes whose role is to bring molecules in proximity thereby controlling and enhancing intermolecular interactions and signaling events. To date, scaffolds have been shown to be composed of proteinaceous units; however, recent evidence has supported the idea that non-coding RNAs may also play a similar role. In this point of view article we discuss recent data on ncRNA scaffolds, with particular focus on ncRNA HOTAIR. Using our current knowledge of signaling networks we discuss the role that RNA may play in writing and regulating histone modifications and the information needed for correct gene expression. Further, we speculate on additional, yet undiscovered roles that ncRNAs may be playing as molecular scaffolds.

Ex vivo identification of protein-protein interactions involving the dopamine transporter

The dopamine (DA) transporter (DAT) is a key regulator of dopaminergic signaling as it mediates the reuptake of extrasynaptic DA and thereby terminates dopaminergic signaling. Emerging evidence indicates that DAT function is influenced through interactions with other proteins. The current report describes a method to identify such interactions following DAT immunoprecipitation from a rat striatal synaptosomal preparation. This subcellular fraction was selected since DAT function is often determined ex vivo by measuring DA uptake in this preparation and few reports investigating DAT-protein interactions have utilized this preparation. Following SDS-PAGE and colloidal Coomassie staining, selected protein bands from a DAT-immunoprecipitate were excised, digested with trypsin, extracted, and analyzed by liquid chromatography tandem mass spectrometry (LC/MS/MS). From the analysis of the tryptic peptides, several proteins were identified including DAT, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) β, CaMKII δ, protein kinase C (PKC) β, and PKC γ. Co-immunoprecipitation of PKC, CaMKII, and protein interacting with C kinase-1 with DAT was confirmed by Western blotting. Thus, the present study highlights a method to immunoprecipitate DAT and to identify co-immunoprecipitating proteins using LC/MS/MS and Western blotting. This method can be utilized to evaluate DAT protein-protein interactions but also to assess interactions involving other synaptic proteins. Ex vivo identification of protein-protein interactions will provide new insight into the function and regulation of a variety of synaptic, membrane-associated proteins, including DAT.

Why Drosophila to study phototransduction?

This review recounts the early history of Drosophila phototransduction genetics, covering the period between approximately 1966 to 1979. Early in this period, the author felt that there was an urgent need for a new approach in phototransduction research. Through inputs from a number of colleagues, he was led to consider isolating Drosophila mutants that are defective in the electroretinogram. Thanks to the efforts of dedicated associates and technical staff, by the end of this period, he was able to accumulate a large number of such mutants. Particularly important in this effort was the use of the mutant assay protocol based on the "prolonged depolarizing afterpotential." This collection of mutants formed the basis of the subsequent intensive investigations of the Drosophila phototransduction cascade by many investigators.

Scaffolds are 'active' regulators of signaling modules

Signaling cascades, in addition to proteins with obvious signaling-relevant activities (e.g. protein kinases or receptors), also employ dedicated 'inactive' proteins whose functions appear to be the organization of the former components into higher order complexes through protein-protein interactions. The core function of signaling adaptors, anchors and scaffolds is the recruitment of proteins into one macromolecular complex. Several recent studies have demonstrated that the recruiter and the recruited molecules mutually influence each other in a scaffolded complex. This yields fundamentally novel properties for the signaling complex as a whole. Because these are not merely additive to the properties of the individual components, scaffolded signaling complexes may behave as functionally distinct modules.

The TRPC2 channel forms protein-protein interactions with Homer and RTP in the rat vomeronasal organ

BACKGROUND

The signal transduction cascade operational in the vomeronasal organ (VNO) of the olfactory system detects odorants important for prey localization, mating, and social recognition. While the protein machinery transducing these external cues has been individually well characterized, little attention has been paid to the role of protein-protein interactions among these molecules. Development of an in vitro expression system for the transient receptor potential 2 channel (TRPC2), which establishes the first electrical signal in the pheromone transduction pathway, led to the discovery of two protein partners that couple with the channel in the native VNO.

RESULTS

Homer family proteins were expressed in both male and female adult VNO, particularly Homer 1b/c and Homer 3. In addition to this family of scaffolding proteins, the chaperones receptor transporting protein 1 (RTP1) and receptor expression enhancing protein 1 (REEP1) were also expressed. RTP1 was localized broadly across the VNO sensory epithelium, goblet cells, and the soft palate. Both Homer and RTP1 formed protein-protein interactions with TRPC2 in native reciprocal pull-down assays and RTP1 increased surface expression of TRPC2 in in vitro assays. The RTP1-dependent TRPC2 surface expression was paralleled with an increase in ATP-stimulated whole-cell current in an in vitro patch-clamp electrophysiological assay.

CONCLUSIONS

TRPC2 expression and channel activity is regulated by chaperone- and scaffolding-associated proteins, which could modulate the transduction of chemosignals. The developed in vitro expression system, as described here, will be advantageous for detailed investigations into TRPC2 channel activity and cell signalling, for a channel protein that was traditionally difficult to physiologically assess.

Phototransduction and the evolution of photoreceptors

Photoreceptors in metazoans can be grouped into two classes, with their photoreceptive membrane derived either from cilia or microvilli. Both classes use some form of the visual pigment protein opsin, which together with 11-cis retinaldehyde absorbs light and activates a G-protein cascade, resulting in the opening or closing of ion channels. Considerable attention has recently been given to the molecular evolution of the opsins and other photoreceptor proteins; much is also known about transduction in the various photoreceptor types. Here we combine this knowledge in an attempt to understand why certain photoreceptors might have conferred particular selective advantages during evolution. We suggest that microvillar photoreceptors became predominant in most invertebrate species because of their single-photon sensitivity, high temporal resolution, and large dynamic range, and that rods and a duplex retina provided primitive chordates and vertebrates with similar sensitivity and dynamic range, but with a smaller expenditure of ATP.

A forward genetic screen in Drosophila melanogaster to identify mutations affecting INAD localization in photoreceptor cells

In Drosophila photoreceptors, the multivalent PDZ protein INAD interacts with multiple signaling components and localizes complexes to the rhabdomere, a subcellular compartment specialized for phototransduction. Since this localization is critical for signaling, we conducted a genetic screen of the third chromosome for mutations that result in mislocalization of an INAD-GFP fusion protein. We identified seven mutant lines that fall into two complementation groups, idl (INAD localization)-A and idl-B. We show that idl-A mutants fail to complement with chaoptic (chp) mutants. Since chaoptin is a structural component of the rhabdomere, mislocalization of INAD may be a secondary effect of the retinal degeneration in chp and idl-A mutants. Genetic complementation and DNA sequencing reveal that the two idl-B mutants represent new alleles of trp, a gene encoding the major light-activated channel. The molecular change in each allele affects a highly conserved residue in either an ankyrin domain on the N-terminus or in the S6 transmembrane domain of TRP. These changes lead to the loss of TRP protein. TRP has previously been shown to anchor INAD in the rhabdomeres, therefore the independent identification of two trp alleles validates our screen for INAD-GFP localization. One possibility is that a limited number of proteins are required for localizing INAD-signaling complexes. A similar screen of the X and second chromosomes may be required to find the remaining players involved.

Olfactory receptor signaling is regulated by the post-synaptic density 95, Drosophila discs large, zona-occludens 1 (PDZ) scaffold multi-PDZ domain protein 1

The unique ability of mammals to detect and discriminate between thousands of different odorant molecules is governed by the diverse array of olfactory receptors expressed by olfactory sensory neurons in the nasal epithelium. Olfactory receptors consist of seven transmembrane domain G protein-coupled receptors and comprise the largest gene superfamily in the mammalian genome. We found that approximately 30% of olfactory receptors possess a classical post-synaptic density 95, Drosophila discs large, zona-occludens 1 (PDZ) domain binding motif in their C-termini. PDZ domains have been established as sites for protein-protein interaction and play a central role in organizing diverse cell signaling assemblies. In the present study, we show that multi-PDZ domain protein 1 (MUPP1) is expressed in the apical compartment of olfactory sensory neurons. Furthermore, on heterologous co-expression with olfactory sensory neurons, MUPP1 was shown to translocate to the plasma membrane. We found direct interaction of PDZ domains 1 + 2 of MUPP1 with the C-terminus of olfactory receptors in vitro. Moreover, the odorant-elicited calcium response of OR2AG1 showed a prolonged decay in MUPP1 small interfering RNA-treated cells. We have therefore elucidated the first building blocks of the putative 'olfactosome', brought together by the scaffolding protein MUPP1, a possible central nucleator of the olfactory response.

Phototransduction motifs and variations

Seeing begins in the photoreceptors, where light is absorbed and signaled to the nervous system. Throughout the animal kingdom, photoreceptors are diverse in design and purpose. Nonetheless, phototransduction-the mechanism by which absorbed photons are converted into an electrical response-is highly conserved and based almost exclusively on a single class of photoproteins, the opsins. In this Review, we survey the G protein-coupled signaling cascades downstream from opsins in photoreceptors across vertebrate and invertebrate species, noting their similarities as well as differences.

Lipids in Ca2+ signalling--an introduction

Lipids and lipid-derived metabolites are increasingly recognised as bonafide signalling molecules that regulate many cellular processes. These include the well-established InsP(3), diacylglycerol (DAG), PIP(2), PIP(3) and arachidonic acid (AA), as well as other poly-unsaturated fatty acids (PUFAs), lysophospholipids, sphingolipids, endocannabinoids and endovanilloids. They regulate a plethora of molecules that are involved in Ca(2+) signalling, including various ion channels, pumps and transporters, thereby triggering, modulating and fine-tuning Ca(2+) signals. Although appreciated individually, it seems timely to highlight the overall impact of lipids as signalling molecules and their role in Ca(2+) signalling, and this is the aim of this special issue of Cell Calcium.

Primary processes in sensory cells: current advances

In the course of evolution, the strong and unremitting selective pressure on sensory performance has driven the acuity of sensory organs to its physical limits. As a consequence, the study of primary sensory processes illustrates impressively how far a physiological function can be improved if the survival of a species depends on it. Sensory cells that detect single-photons, single molecules, mechanical motions on a nanometer scale, or incredibly small fluctuations of electromagnetic fields have fascinated physiologists for a long time. It is a great challenge to understand the primary sensory processes on a molecular level. This review points out some important recent developments in the search for primary processes in sensory cells that mediate touch perception, hearing, vision, taste, olfaction, as well as the analysis of light polarization and the orientation in the Earth's magnetic field. The data are screened for common transduction strategies and common transduction molecules, an aspect that may be helpful for researchers in the field.

Dynamic scaffolding in a G protein-coupled signaling system

The INAD scaffold organizes a multiprotein complex that is essential for proper visual signaling in Drosophila photoreceptor cells. Here we show that one of the INAD PDZ domains (PDZ5) exists in a redox-dependent equilibrium between two conformations--a reduced form that is similar to the structure of other PDZ domains, and an oxidized form in which the ligand-binding site is distorted through formation of a strong intramolecular disulfide bond. We demonstrate transient light-dependent formation of this disulfide bond in vivo and find that transgenic flies expressing a mutant INAD in which PDZ5 is locked in the reduced state display severe defects in termination of visual responses and visually mediated reflex behavior. These studies demonstrate a conformational switch mechanism for PDZ domain function and suggest that INAD behaves more like a dynamic machine rather than a passive scaffold, regulating signal transduction at the millisecond timescale through cycles of conformational change.

Light-induced recruitment of INAD-signaling complexes to detergent-resistant lipid rafts in Drosophila photoreceptors

Here, we reveal a novel feature of the dynamic organization of signaling components in Drosophila photoreceptors. We show that the multi-PDZ protein INAD and its target proteins undergo light-induced recruitment to detergent-resistant membrane (DRM) rafts. Reduction of ergosterol, considered to be a key component of lipid rafts in Drosophila, resulted in a loss of INAD-signaling complexes associated with DRM fractions. Genetic analysis demonstrated that translocation of INAD-signaling complexes to DRM rafts requires activation of the entire phototransduction cascade, while constitutive activation of the light-activated channels resulted in recruitment of complexes to DRM rafts in the dark. Mutations affecting INAD and TRP showed that PDZ4 and PDZ5 domains of INAD, as well as the INAD-TRP interaction, are required for translocation of components to DRM rafts. Finally, selective recruitment of phosphorylated, and therefore activatable, eye-PKC to DRM rafts suggests that DRM domains are likely to function in signaling, rather than trafficking.

The Multi-PDZ domain protein MUPP1 as a lipid raft-associated scaffolding protein controlling the acrosome reaction in mammalian spermatozoa

The success of acrosomal exocytosis, a complex process with a variety of interrelated steps, relies on the coordinated interaction of participating signaling molecules. Since scaffolding proteins are known to spatially organize sequential signaling pathways, we examined whether the Multi-PDZ domain protein MUPP1, recently identified in mammalian spermatozoa, is functionally active in controlling acrosomal secretion in mammalian sperm cells. To address this question, permeabilized mouse sperm were loaded with inhibitory antibodies against MUPP1 as well as with a photosensitive Ca(2+) chelator which allows a controlled release of acrosomal Ca(2+). The results revealed that MUPP1 controls initial tethering and docking of the acrosomal vesicle, whereas syntaxin 2, a t-SNARE protein also expressed in the acrosomal cap of mammalian spermatozoa, appears to take part in the final process of acrosomal fusion. Interestingly, using immunogold electron microscopy, it was found that MUPP1 is detectable in the region of the periacrosomal membrane. Furthermore, in isolated detergent-insoluble glycolipid-enriched membrane domains from epididymal spermatozoa, MUPP1 was found to show a striking association with the Triton X-100 insoluble membrane fraction, which did not change significantly upon sperm capacitation or partial chemical extraction of cholesterol. This evidence points to a role of MUPP1 as a membrane raft-associated molecular organizer, and suggests that mammalian spermatozoa may use a scaffolding protein and distinct membrane subdomains to spatially organize components involved in the process of acrosomal exocytosis.

G-Protein-coupled receptor-protein interactions: basis for new concepts on receptor structure and function

1. G-Protein-coupled receptors (GPCRs) constitute a large family of cell surface proteins. Their primary function is to transmit extracellular stimuli to intracellular signals. It is estimated that the human genome contains more than 1000 genes that code for proteins of the GPCR structure. These receptors also comprise the most important class of therapeutic drug targets. 2. The mechanism of GPCR signalling was initially envisioned as involving coupling to the heterotrimeric G-proteins only. However, recent developments in the field suggest that such a simplistic model cannot be sustained any longer. The emerging view is that a wide range of accessory proteins are involved in the regulation of every aspect of GPCR activity. 3. G-Protein-coupled receptor-interacting proteins are implicated in the regulation of several aspects of GPCR biology, including receptor targeting to the respective sites of action, receptor anchoring, signalling and receptor desensitization. In some cases (e.g. receptor activity modifying proteins), they may contribute to the receptor structure and form a part of the ligand-binding domain. 4. These findings have contributed to new concepts of cellular organization in which modular protein-protein interactions provide a network through which signalling pathways are assembled and controlled.

Ggamma13 interacts with PDZ domain-containing proteins

The G protein gamma13 subunit (Ggamma13) is expressed in taste and retinal and neuronal tissues and plays a key role in taste transduction. We identified PSD95, Veli-2, and other PDZ domain-containing proteins as binding partners for Ggamma13 by yeast two-hybrid and pull-down assays. In two-hybrid assays, Ggamma13 interacted specifically with the third PDZ domain of PSD95, the sole PDZ domain of Veli-2, and the third PDZ domain of SAP97, a PSD95-related protein. Ggamma13 did not interact with the other PDZ domains of PSD95. Coexpression of Ggamma13 with its Gbeta1 partner did not interfere with these two-hybrid interactions. The physical interaction of Ggamma13 with PSD95 in the cellular milieu was confirmed in pull-down assays following heterologous expression in HEK293 cells. The interaction of Ggamma13 with the PDZ domain of PSD95 was via the C-terminal CAAX tail of Ggamma13 (where AA indicates the aliphatic amino acid); alanine substitution of the CTAL sequence at the C terminus of Ggamma13 abolished its interactions with PSD95 in two-hybrid and pull-down assays. Veli-2 and SAP97 were identified in taste tissue and in Ggamma13-expressing taste cells. Coimmunoprecipitation of Ggamma13 and PSD95 from brain and of Ggamma13 and SAP97 from taste tissue indicates that Ggamma13 interacts with these proteins endogenously. This is the first demonstration that PDZ domain proteins interact with heterotrimeric G proteins via the CAAX tail of Ggamma subunits. The interaction of Ggamma13 with PDZ domain-containing proteins may provide a means to target particular Gbetagamma subunits to specific subcellular locations and/or macromolecular complexes involved in signaling pathways.

Homo- and heteromeric assembly of TRP channel subunits

Mammalian homologues of the Drosophila melanogaster transient receptor potential (TRP) channels are the second largest cation channel family within the superfamily of hexahelical cation channels. Most mammalian TRP channels function as homooligomers and mediate mono- or divalent cation entry upon activation by a variety of stimuli. Because native TRP channels may be multimeric proteins of possibly complex composition, it is difficult to compare cation conductances in native tissues to those of clearly defined homomeric TRP channel complexes in living cells. Therefore, the possibility of heteromeric TRP channel assembly has been investigated in recent years by several groups. As a major conclusion of these studies, most heteromeric TRP channel complexes appear to consist of subunit combinations only within relatively narrow confines of phylogenetic subfamilies. Although the general capability of heteromer formation between closely related TRP channel subunits is now clearly established, we are only beginning to understand whether these heteromeric complexes are of physiological significance. This review summarizes the current knowledge on the promiscuity and specificity of the assembly of channel complexes composed of TRPC-, TRPV- and TRPM-subunits of mammalian TRP channels.

Rhodopsin patterning in central photoreceptor cells of the blowfly Calliphora vicina: cloning and characterization of Calliphora rhodopsins Rh3, Rh5 and Rh6

The ommatidia that constitute the compound eyes of flies contain eight photoreceptor cells, which are divided into two classes: the peripheral photoreceptors, R1–6, and the central photoreceptors, R7 and R8. In the fruit fly, Drosophila, R1–6 express the same rhodopsin (Rh1),whilst the R7 and R8 of a given ommatidium express either Rh3 and Rh5, or Rh4 and Rh6, respectively. We have studied whether this expression pattern of rhodopsins is conserved in the blowfly Calliphora vicina. We have cloned three novel Calliphora rhodopsins, which are homologues of Drosophila Rh3, Rh5 and Rh6, with an amino acid sequence identity of 80.7%, 60.9% and 86.1%, respectively. Immunocytochemical studies with antibodies specific for Rh3, Rh5 and Rh6 revealed that Rh3 is expressed in a subset of R7 cells, while Rh5 and Rh6 are expressed in a non-overlapping subset of R8 cells. Rh3 and Rh5 are present in most cases in the same ommatidia, which account for approximately 27% of all ommatidia, and Rh6 is found in the complementary 73%. The similarity of the rhodopsin expression pattern of Calliphora with that of Drosophila suggests that the developmental mechanism regulating the terminal differentiation of R7 and R8 cells are highly conserved between these fly species.

GPCR interacting proteins (GIP)

G protein-coupled receptors (GPCR) interact not only with heterotrimeric G proteins but also with accessory proteins called GPCR interacting proteins (GIP). These proteins have important functions. They are implicated in GPCR targeting to specific cellular compartments, in their assembling into large functional complexes called "receptosomes," in their trafficking to and from the plasma membrane, and in the fine-tuning of their signaling properties. There are several types of GIPs. Some are transmembrane proteins such as another GPCR (homodimerization and heterodimerization), ionic channels, ionotropic receptors, and single transmembrane proteins. The latter is implicated in the fine-tuning of receptor pharmacology or signaling. Other GIPs are soluble proteins interacting mainly with the "magic" C-terminal tail. Among them, PDZ domain-containing proteins are the most abundant. They generally, but not always, interact with the extreme C-terminal domain of GPCRs. Some GIPs interact with specific sequences of the C-terminal such as the Homer binding sequence (-PPxxFR-), the dopamine receptor interacting protein (DRIP) binding sequence (-FxxxFxxxF-), etc. Finally, only few GIPs have been found thus far to interact with the third intracellular loop of GPCRs. The future will tell us if this situation is only due to technical reasons.

Specificity of membrane specializations in mechanoreceptors of birds--A freeze-etching study

The detailed knowledge of the molecular process of mechanotransduction is still an unsolved question. The investigation of the intramembranous structure of the cutaneous mechanoreceptors may play an important role in elucidating this problem. In this relation, Herbst sensory corpuscles in ducks were studied for the first time using the freeze-etching and thin sectioning techniques. Herbst corpuscles have the basic structural components valid for most of the encapsulated mechanoreceptors in mammals: a capsule made of perineural cells, a lamellar complex of modified Schwann cells, surrounding the non-myelinated part of the receptor nerve fiber and its ending. Freeze-etching replicas reveal that the plasmalemmae of the capsule cells, modified Schwann cells and axolemmae of parts of the nerve fiber differ in both density and pattern of distribution of intramembranous particles (IMPs) as well as IMP size. On all the plasmalemmae the IMP density is higher on the P-face (2000-3300 microm(-2)) than the respective E-face (800-1500 microm(-2)). The axolemma of the ending of the receptor nerve fiber expresses higher density of IMPs than its shaft. The mean IMP size for all the plasmalemmae varies between 5.5 and 7.5 nm. Many tight junctions occur between the capsule cells. These results indicate that the non-myelinated axolemma as well as the plasmalemmae of other components of Herbst corpuscles are specialized in terms of content and distribution of IMPs. The IMPs may represent various kinds of mechanosensitive channel proteins or related membrane-bound proteins participating in the process of mechanotransduction.

Calcium sensitivity of a sodium-activated nonselective cation channel in lobster olfactory receptor neurons

We report that a Na+-activated nonselective cation channel described previously in lobster olfactory neurons, in which phosphoinositide signaling mediates olfactory transduction, can also be activated by Ca2+. Ca2+ activates the channel in the presence of Na+, increasing the open probability of the channel with a K1/2 of 490 nM and a Hill coefficient of 1.3. Ca2+ also increases the sensitivity of the channel to Na+. In some cells, the same channel is Ca2+ insensitive in a cell-specific manner. The nonspecific activator of protein phosphatases, protamine, applied to the intracellular face of patches containing the channel irreversibly eliminates the sensitivity to Ca2+. This effect can be blocked by okadaic acid, a nonspecific blocker of protein phosphatases, and restored by the catalytic subunit of protein kinase A in the presence of MgATP. The Ca2+-sensitive form of the channel is predominantly expressed in the transduction zone of the cells in situ. These findings imply that the Ca2+ sensitivity of the channel, and possibly its regulation by phosphorylation, play a role in olfactory transduction and help tie activation of the channel to the canonical phosphoinositide turnover pathway.

Regulation of TRP channels via lipid second messengers

In Drosophila photoreceptors, the light-sensitive current is mediated downstream of phospholipase C by TRP (transient receptor potential) channels. Recent evidence suggests that Drosophila TRP channels are activated by diacylglycerol (DAG) or its metabolites (polyunsaturated fatty acids), possibly in combination with the reduction in phosphatidyl inositol 4,5 bisphosphate (PIP2). Consistent with this view, diacylglycerol kinase is identified as a key enzyme required for response termination. Signaling is critically dependent upon efficient PIP2 synthesis; mutants of this pathway in combination with genetically targeted PIP2 reporters provide unique insights into the kinetics and regulation of PIP2 turnover. Recent evidence indicates that a growing number of mammalian TRP homologues are also regulated by lipid messengers, including DAG, arachidonic acid, and PIP2.

Agonist-induced phosphorylation of the serotonin 5-HT2C receptor regulates its interaction with multiple PDZ protein 1

Multiple PDZ domain protein 1 (MUPP1), a putative scaffolding protein containing 13 PSD-95, Dlg, ZO-1 (PDZ) domains, was identified by a yeast two-hybrid screen as a serotonin2C receptor (5-HT2C R)-interacting protein (Ullmer, C., Schmuck, K., Figge, A., and Lubbert, H. (1998) FEBS Lett. 424, 63-68). MUPP1 PDZ domain 10 (PDZ 10) associates with Ser458-Ser-Val at the carboxyl-terminal tail of the 5-HT2C R. Both Ser458 and Ser459 are phosphorylated upon serotonin stimulation of the receptor (Backstrom, J. R., Price, R. D., Reasoner, D. T., and Sanders-Bush, E. (2000) J. Biol. Chem. 275, 23620-23626). To investigate whether phosphorylation of these serines in the receptor regulates MUPP1 interaction, we used several approaches. First, we substituted the serines in the receptor carboxyl tail with aspartates to mimic phosphorylation (S458D, S459D, or S458D/S459D). Pull-down assays demonstrated that Asp mutations at Ser458 significantly decreased receptor tail interaction with PDZ 10. Next, serotonin treatment of 5-HT2C R/3T3 cells resulted in a dose-dependent reduction of receptor interaction with PDZ 10. Effects of serotonin on receptor-PDZ 10 binding could be blocked by pretreatment with a receptor antagonist. Alkaline phosphatase treatment reverses the effect of serotonin, indicating that agonist-induced phosphorylation at Ser458 resulted in a loss of MUPP1 association and also revealed a significant amount of basal phosphorylation of the receptor. We conclude that 5-HT2C R interaction with MUPP1 is dynamically regulated by phosphorylation at Ser458.

Proteins modulating TRP channel function

TRP channels are involved in different signaling cascades; TRP channels can be activated via hormones and neurotransmitter in a receptor/G-protein-mediated manner or by osmotic, thermic or mechanic stimuli. The overall functional role of TRP channels within these processes of hormonal cellular control, nociception or cellular calcium homeostasis is still unclear, as these complex processes often involve macromolecular structures. Whereas the integration of Drosophila TRP in the phototransduction process is becoming clear, the understanding of the participation of mammalian TRP channels in signal transduction complexes is only beginning. TRP channels have been demonstrated to interact with PDZ domain proteins, and both scaffold and regulatory function have been shown for INAD, the PDZ domain protein of the Drosophila phototransduction complex. In mammalian cells, the interaction of NHERF and TRPC4 has been shown and it is anticipated that NHERF may abolish the apparent store-dependent regulation of TRPC4 and TRPC5. Whereas TRP channels and PDZ domain proteins form permanent heterodimeric proteins, the interaction of calcium-binding proteins is dependent on the calcium concentration and is, therefore, dynamic. The prototype of calcium-binding protein used for experiments is calmodulin; whether or not calmodulin is also the natural interaction partner of TRP channels is an open question.

TRP channels in Drosophila photoreceptors: the lipid connection

The light-sensitive current in Drosophila photoreceptors is mediated by transient receptor potential (TRP) channels, at least two members of which (TRP and TRPL) are activated downstream of phospholipase C (PLC) in response to light. Recent evidence is reviewed suggesting that Drosophila TRP channels are activated by one or more lipid products of PLC activity: namely diacylglycerol (DAG), its metabolites (polyunsaturated fatty acids) or the reduction in phosphatidylinositol 4,5-bisphosphate (PIP(2)). The most compelling evidence for this view comes from analysis of rdgA mutants which are unable to effectively metabolise DAG due to a defect in DAG kinase. The rdgA mutation leads to constitutive activation of both TRP and TRPL channels and dramatically increases sensitivity to light in hypomorphic mutations of PLC and G protein.

Regulation of G protein-coupled receptor signaling by scaffold proteins

The actions of many hormones and neurotransmitters are mediated through stimulation of G protein-coupled receptors. A primary mechanism by which these receptors exert effects inside the cell is by association with heterotrimeric G proteins, which can activate a wide variety of cellular enzymes and ion channels. G protein-coupled receptors can also interact with a number of cytoplasmic scaffold proteins, which can link the receptors to various signaling intermediates and intracellular effectors. The multicomponent nature of G protein-coupled receptor signaling pathways makes them ideally suited for regulation by scaffold proteins. This review focuses on several specific examples of G protein-coupled receptor-associated scaffolds and the roles they may play in organizing receptor-initiated signaling pathways in the cardiovascular system and other tissues.

The PDZ scaffold protein INAD abolishes apparent store-dependent regulation of the light-activated cation channel TRP

In fly photoreceptor cells, light initiates a G protein-coupled phospholipase Cb-dependent signaling cascade that results in the depolarization of the cell membrane, which is mediated by the cation channels TRP and TRPL. Together with phospholipase Cb and an eye-specific protein kinase C, TRP is tethered to the scaffolding protein INAD, which forms a multimolecular signaling complex. Divergent data from expressed TRP and studies from photoreceptor cells have brought up a controversy whether or not a capacitative calcium entry (CCE) mechanism is involved in the Drosophila phototransduction pathway. Our initial characterization of TRP from photoreceptors of Calliphora vicina supported the hypothesis of a CCE mechanism, as heterologously expressed TRP was stimulated after application of thapsigargin. The situation changed when the PDZ domain protein INAD was coexpressed with TRP. In cells coexpressing TRP and INAD, no calcium entry was detectable on application of store depletion protocols. Suppression of CCE by INAD was not observed when the described interaction was disrupted by mutations in TRP and INAD. Our data show that apparent activation of TRP by CCE is abolished by INAD. Within the complex, the proteins necessary for phototransduction mutually influence their activities. The results support the hypothesis of a store-independent activation of TRP.

From flies' eyes to our ears: mutations in a human class III myosin cause progressive nonsyndromic hearing loss DFNB30

Normal vision in Drosophila requires NINAC, a class III myosin. Class III myosins are hybrid motor-signaling molecules, with an N-terminal kinase domain, highly conserved head and neck domains, and a class III-specific tail domain. In Drosophila rhabdomeres, NINAC interacts with actin filaments and with a PDZ scaffolding protein to organize the phototransduction machinery into a signaling complex. Recessive null mutations in Drosophila NINAC delay termination of the photoreceptor response and lead to progressive retinal degeneration. Here, we show that normal hearing in humans requires myosin IIIA, the human homolog of NINAC. In an extended Israeli family, nonsyndromic progressive hearing loss is caused by three different recessive, loss-of-function mutations in myosin IIIA. Of 18 affected relatives in Family N, 7 are homozygous and 11 are compound heterozygous for pairs of mutant alleles. Expression of mammalian myosin IIIA is highly restricted, with the strongest expression in retina and cochlea. The involvement of homologous class III myosins in both Drosophila vision and human hearing is an evolutionary link between these sensory systems.