The proteome of rat olfactory sensory cilia

Olfactory sensory neurons expose to the inhaled air chemosensory cilia which bind odorants and operate as transduction organelles. Odorant receptors in the ciliary membrane activate a transduction cascade which uses cAMP and Ca²⁺ for sensory signaling in the ciliary lumen. Although the canonical transduction pathway is well established, molecular components for more complex aspects of sensory transduction, like adaptation, regulation, and termination of the receptor response have not been systematically identified. Moreover, open questions in olfactory physiology include how the cilia exchange solutes with the surrounding mucus, assemble their highly polarized set of proteins, and cope with noxious substances in the ambient air. A specific ciliary proteome would promote research efforts in all of these fields. We have improved a method to detach cilia from rat olfactory sensory neurons and have isolated a preparation specifically enriched in ciliary membrane proteins. Using LC‐ESI‐MS/MS analysis, we identified 377 proteins which constitute the olfactory cilia proteome. These proteins represent a comprehensive data set for olfactory research since more than 80% can be attributed to the characteristic functions of olfactory sensory neurons and their cilia: signal processing, protein targeting, neurogenesis, solute transport, and cytoprotection. Organellar proteomics thus yielded decisive information about the diverse physiological functions of a sensory organelle.     

The archaeal sensory rhodopsin II/transducer complex: a model for transmembrane signal transfer

Archaebacterial photoreceptors mediate phototaxis by regulating cell motility through two-component signalling cascades. Homologs of this sensory pathway occur in all three kingdoms of life, most notably in enteric bacteria in which the chemotaxis has been extensively studied. Recent structural and functional studies on the sensory rhodopsin II/transducer complex mediating the photophobic response of Natronomonas pharaonis have yielded new insights into the mechanisms of signal transfer across the membrane. Electron paramagnetic resonance data and the atomic resolution structure of the receptor molecule in complex with the transmembrane segment of its cognate transducer provided a model for signal transfer from the receptor to the cytoplasmic side of the transducer. This mechanism might also be relevant for eubacterial chemoreceptor signalling.     

Sorting of soluble proteins in the secretory pathway of plants

The secretory pathway of plants is a network of organelles that communicate via vesicle transport. This process involves budding on donor membranes followed by their targeting to, recognition by and fusion with the acceptor membrane. Protein sorting through the plant secretory pathway is a process that requires the specific recognition of signals by receptor molecules. For soluble proteins, recognition takes place in the lumen of the secretory pathway. The sorting receptors must mediate signal transduction across the membrane to convey the information about the presence of cargo molecules to cytosolic factors, which regulate the formation of transport vesicles. Recently, a number of key elements in this process have been identified, providing tools to study protein sorting at the molecular level.     

Negatively calcium-modulated membrane guanylate cyclase signaling system in the rat olfactory bulb

The mechanism by which the individual odor signals are translated into the perception of smell in the brain is unknown. The signal processing occurs in the olfactory system which has three major components: olfactory neuroepithelium, olfactory bulb, and olfactory cortex. The neuroepithelial layer is composed of ciliated sensory neurons interspersed among supportive cells. The sensory neurons are the sites of odor transduction, a process that converts the odor signal into an electrical signal. The electrical signal is subsequently received by the neurons of the olfactory bulb, which process the signal and then relay it to the olfactory cortex in the brain. Apart from information about certain biochemical steps of odor transduction, there is almost no knowledge about the means by which the olfactory bulb and cortical neurons process this information. Through biochemical, functional, and immunohistochemical approaches, this study shows the presence of a Ca(2+)-modulated membrane guanylate cyclase (mGC) transduction system in the bulb portion of the olfactory system. The mGC is ROS-GC1. This is coexpressed with its specific modulator, guanylate cyclase activating protein type 1 (GCAP1), in the mitral cells. Thus, a new facet of the Ca(2+)-modulated GCAP1--ROS-GC1 signaling system, which, until now, was believed to be unique to phototransduction, has been revealed. The findings suggest a novel role for this system in the polarization and depolarization phenomena of mitral cells and also contradict the existing belief that no mGC besides GC-D exists in the olfactory neurons.     

Variations on a molecular switch: transport and sensory signalling by archaeal rhodopsins

The archaeal rhodopsins are a family of seven-transmembrane-helix, visual pigment-like proteins found in Halobacterium salinarum and related halophilic Archaea. Two, bacteriorhodopsin (BR) and halorhodopsin (HR), are transport rhodopsins that carry out light-driven electrogenic translocation of protons and chloride, respectively, across the cell membrane. The other two, sensory rhodopsins I and II (SRI and SRII), are phototaxis receptors that send signals to tightly bound transducer proteins that in turn control a phosphorylation cascade modulating the cell's flagellar motors. Recent progress has cast light on how nature has modified the common design of these proteins to carry out their distinctly different functions: electrogenic ion transport and non-electrogenic signal transduction. A key shared mechanism between BR and SRII appears to be an interhelical salt bridge locked conformational switch that is released by photoisomerization of retinal. In BR disruption of the lock opens a cytoplasmic half-channel that ensures uptake of the transported proton from the cytoplasmic side of the membrane at a critical time in the pumping cycle. Transducer-free SRI uses the same mechanism to carry out light-driven proton transport, but interaction with its transducer blocks the cytoplasmic half-channel thereby interrupting the transport cycle. In SRI, transducer interaction also disrupts the salt bridge in the dark, poising the receptor in an intermediate conformation able to produce opposite signals depending on the colour of the stimulus light. A model for signalling is proposed in which the salt bridge-controlled half-channel is used to modulate interaction with the Htr proteins when the receptor signalling states are formed.     

TonB and the Gram-negative dilemma

TonB protein serves as an energy transducer to couple cytoplasmic membrane energy to high-affinity active transport of iron siderophores and vitamin B₁₂ across the outer membranes of Gram-negative bacteria. The biochemical mechanism of the energy transduction remains to be determined, but important details are already known. TonB is targeted to and anchored in the cytoplasmic membrane by a single membrane-spanning domain and spans the periplasm to physically interact with outer-membrane receptors of the transport ligands. TonB-dependent energy transduction is modulated by ExbB protein, which stabilizes TonB, and possibly by several other proteins including ExbC, ExbD, and TolQ. TonB has a relatively short functional half-life that is accelerated when rates of active transport across the outer membrane are increased. A model that incorporates this information, as well as some tempered speculation, is presented.     

S100B-modulated Ca2+-dependent ROS-GC1 transduction machinery in the gustatory epithelium: a new mechanism in gustatory transduction

Gustatory transduction is a biochemical process by which the gustatory signal generates the electric signal. The microvilli of the taste cells in the gustatory epithelium are the sites of gustatory transduction. This study documents the biochemical, molecular, and functional identity of the Ca2+-modulated membrane guanylate cyclase transduction machinery in the bovine gustatory epithelium. The machinery is a two-component system: the Ca2+-sensor protein, S100B; and the transducer, ROS-GC1. S100B senses increments in free Ca2+, undergoes conformational change, binds to the domain amino acids (aa) Gly962-Asn981 and via the transduction domain aa Ile1030-Gln1041 activates ROS-GC1, generating the second messenger, cyclic GMP. In a recent study, operational presence of this machinery has been demonstrated in the photoreceptor bipolar synapse [Duda et al., EMBO J. 21 (2002) 2547]. Thus, the machinery has a broader role in sensory perceptions, vision in the retinal neurons and gustation in the tongue. The entry of the ROS-GC transduction machinery defines the beginning of a new paradigm of Ca2+ signaling in the tongue.     

Membrane Bound GSK-3 Activates Wnt Signaling through Disheveled and Arrow

Wnt ligands and their downstream pathway components coordinate many developmental and cellular processes. In adults, they regulate tissue homeostasis through regulation of stem cells. Mechanistically, signal transduction through this pathway is complicated by pathway components having both positive and negative roles in signal propagation. Here we examine the positive role of GSK-3/Zw3 in promoting signal transduction at the plasma membrane. We find that targeting GSK-3 to the plasma membrane activates signaling in Drosophila embryos. This activation requires the presence of the co-receptor Arrow-LRP5/6 and the pathway activating protein Disheveled. Our results provide genetic evidence for evolutionarily conserved, separable roles for GSK-3 at the membrane and in the cytosol, and are consistent with a model where the complex cycles from cytosol to membrane in order to promote signaling at the membrane and to prevent it in the cytosol.     

Variability of spike trains and the processing of temporal patterns of acoustic signals—problems,constraints, and solutions

Object recognition and classification by sensory pathways is rooted in spike trains provided by sensory neurons. Nervous systems had to evolve mechanisms to extract information about relevant object properties, and to separate these from spurious features. In this review, problems caused by spike train variability and counterstrategies are exemplified for the processing of acoustic signals in orthopteran insects. Due to size limitations of their nervous system we expect to find solutions that are stripped to the computational basics. A key feature of auditory systems is temporal resolution, which is likely limited by spike train variability. Basic strategies to reduce such variability are to integrate over time, or to average across several neurons. The first strategy is constrained by its possible interference with temporal resolution. Grasshoppers do not seem to explore temporal integration much, in spite of the repetitive structure of their songs, which invites for multiple looks at the signal. The benefits of averaging across neurons depend on uncorrelated responses, a factor that may be crucial for the performance and evolution of small nervous systems. In spite of spike train variability the temporal information necessary for the recognition of conspecifics is preserved to a remarkable degree in the auditory pathway.Abbreviations dB decibel - CV coefficient of variation - SNR signal to noise ratio     

Role of the JAK/STAT signal transduction pathway in the regulation of gene expression in CNS

Over the last 20 years the JAK/STAT signal transduction pathway has been extensively studied. An enormous amount of data on different cell signal transduction pathways is now available. The JAK/STAT signal transduction pathway is one of the intracellular signaling pathways activated by cytokines and growth factors that was first studied in the hematopoietic system, but recent data demonstrate that this signal transduction is also greatly utilized by other systems. The JAK/STAT pathway is a signaling cascade that links the activation of specific cell membrane receptors to nuclear gene expression. This review is focused on the role of JAK/STAT signal transduction pathway activation in the central nervous system (CNS).     

A new concept to reveal protein dynamics based on energy dissipation

Protein dynamics is essential for its function, especially for intramolecular signal transduction. In this work we propose a new concept, energy dissipation model, to systematically reveal protein dynamics upon effector binding and energy perturbation. The concept is applied to better understand the intramolecular signal transduction during allostery of enzymes. The E. coli allosteric enzyme, aspartokinase III, is used as a model system and special molecular dynamics simulations are designed and carried out. Computational results indicate that the number of residues affected by external energy perturbation (i.e. caused by a ligand binding) during the energy dissipation process shows a sigmoid pattern. Using the two-state Boltzmann equation, we define two parameters, the half response time and the dissipation rate constant, which can be used to well characterize the energy dissipation process. For the allostery of aspartokinase III, the residue response time indicates that besides the ACT2 signal transduction pathway, there is another pathway between the regulatory site and the catalytic site, which is suggested to be the β15-αK loop of ACT1. We further introduce the term "protein dynamical modules" based on the residue response time. Different from the protein structural modules which merely provide information about the structural stability of proteins, protein dynamical modules could reveal protein characteristics from the perspective of dynamics. Finally, the energy dissipation model is applied to investigate E. coli aspartokinase III mutations to better understand the desensitization of product feedback inhibition via allostery. In conclusion, the new concept proposed in this paper gives a novel holistic view of protein dynamics, a key question in biology with high impacts for both biotechnology and biomedicine.     

The B-cell antigen receptor signals through a preformed transducer module of SLP65 and CIN85

Spleen tyrosine kinase Syk and its substrate SLP65 (also called BLNK) are proximal signal transducer elements of the B-cell antigen receptor (BCR). Yet, our understanding of signal initiation and processing is limited owing to the incomplete list of SLP65 interaction partners and our ignorance of their association kinetics. We have now determined and quantified the in vivo interactomes of SLP65 in resting and stimulated B cells by mass spectrometry. SLP65 orchestrated a complex signal network of about 30 proteins that was predominantly based on dynamic interactions. However, a stimulation-independent and constant association of SLP65 with the Cbl-interacting protein of 85 kDa (CIN85) was requisite for SLP65 phosphorylation and its inducible plasma membrane translocation. In the absence of a steady SLP65/CIN85 complex, BCR-induced Ca(2+) and NF-κB responses were abrogated. Finally, live cell imaging and co-immunoprecipitation experiments further confirmed that both SLP65 and CIN85 are key components of the BCR-associated primary transducer module required for the onset and progression phases of BCR signal transduction.     

Proteomic analysis of a membrane preparation from rat olfactory sensory cilia

The cilia of mammalian olfactory receptor neurons (ORNs) represent the sensory interface that is exposed to the air within the nasal cavity. The cilia are the site where odorants bind to specific receptors and initiate olfactory transduction that leads to excitation of the neuron. This process involves a multitude of ciliary proteins that mediate chemoelectrical transduction, amplification, and adaptation of the primary sensory signal. Many of these proteins were initially identified by their enzymatic activities using a membrane protein preparation from olfactory cilia. This so-called "calcium-shock" preparation is a versatile tool for the exploration of protein expression, enzyme kinetics, regulatory mechanisms, and ciliary development. To support such studies, we present a first proteomic analysis of this membrane preparation. We subjected the cilia preparation to liquid chromatography-electrospray ionisation (LC-ESI-MS/MS) tandem mass spectrometry and identified 268 proteins, of which 49% are membrane proteins. A detailed analysis of their cellular and subcellular localization showed that the cilia preparation obtained by calcium shock not only is highly enriched in ORN proteins but also contains a significant amount of nonciliary material. Although our proteomic study does not identify the entire set of ciliary and nonciliary proteins, it provides the first estimate of the purity of the calcium-shock preparation and provides valuable biochemical information for further research.     

Expression, purification and activities of the entire family of intact membrane sensor kinases from Enterococcus faecalis

Two-component signal transduction systems are the main mechanism by which bacteria sense and respond to their environment, and their membrane-located histidine protein kinases generally constitute the sensory components of these systems. Relatively little is known about their fundamental mechanisms and precise nature of the molecular signals sensed, because of the technical challenges of producing sufficient quantities of these hydrophobic membrane proteins. This study evaluated the heterologous production, purification and activities of the 16 intact membrane sensor kinases of Enterococcus faecalis. Following the cloning of the genes into expression plasmid pTTQ18His, all but one kinase was expressed successfully in Escherichia coli inner membranes. Purification of the hexa-histidine 'tagged' recombinant proteins was achieved for 13, and all but one were verified as intact. Thirteen intact kinases possessed autophosphorylation activity with no added signal when assayed in membrane vesicles or as purified proteins. Signal testing of two functionally-characterized kinases, FsrC and VicK, was successful examplifying the potential use of in vitro activity assays of intact proteins for systematic signal identification. Intact FsrC exhibited an approximately 10-fold increase in activity in response to a two-fold molar excess of synthetic GBAP pheromone, whilst glutathione, and possibly redox potential, were identified for the first time as direct modulators of VicK activity in vitro. The impact of DTT on VicK phosphorylation resulted in increased levels of phosphorylated VicR, the downstream response regulator, thereby confirming the potential of this in vitro approach for investigations of modulator effects on the entire signal transduction process of two-component systems.     

An archaeal photosignal-transducing module mediates phototaxis in Escherichia coli

Halophilic archaea, such as Halobacterium salinarum and Natronobacterium pharaonis, alter their swimming behavior by phototaxis responses to changes in light intensity and color using visual pigment-like sensory rhodopsins (SRs). In N. pharaonis, SRII (NpSRII) mediates photorepellent responses through its transducer protein, NpHtrII. Here we report the expression of fusions of NpSRII and NpHtrII and fusion hybrids with eubacterial cytoplasmic domains and analyze their function in vivo in haloarchaea and in eubacteria. A fusion in which the C terminus of NpSRII is connected by a short flexible linker to NpHtrII is active in phototaxis signaling for H. salinarum, showing that the fusion does not inhibit functional receptor-transducer interactions. We replaced the cytoplasmic portions of this fusion protein with the cytoplasmic domains of Tar and Tsr, chemotaxis transducers from enteric eubacteria. Purification of the fusion protein from H. salinarum and Tar fusion chimera from Escherichia coli membranes shows that the proteins are not cleaved and exhibit absorption spectra characteristic of wild-type membranes. Their photochemical reaction cycles in H. salinarum and E. coli membranes, respectively, are similar to those of native NpSRII in N. pharaonis. These fusion chimeras mediate retinal-dependent phototaxis responses by Escherichia coli, establishing that the nine-helix membrane portion of the receptor-transducer complex is a modular functional unit able to signal in heterologous membranes. This result confirms a current model for SR-Htr signal transduction in which the Htr transducers are proposed to interact physically and functionally with their cognate sensory rhodopsins via helix-helix contacts between their transmembrane segments.     

The twin-arginine translocation pathway is necessary for correct membrane insertion of the Rieske Fe/S protein in Legionella pneumophila

The twin-arginine translocation (Tat) pathway translocates folded proteins across the cytoplasmic membrane. Proteins transported through this secretion system typically carry two arginine residues in their signal peptide that is cleaved off during translocation. Recently, we demonstrated the presence of the Tat pathway in Legionella pneumophila Philadelphia-1 and the Rieske Fe/S protein PetA was one of the predicted Tat substrates. Because we observed that the signal peptide of PetA is not processed and that this protein is still membrane associated in the tat mutants, correct membrane insertion was assayed using a trypsin sensitivity assay. We conclude that the Tat pathway is necessary for correct membrane insertion of L. pneumophila PetA.     

Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

The organization of multicellular organisms depends on cell-cell communication. The signal molecules are often soluble components in the extracellular fluid, but also include odors and light. A large array of surface receptors is involved in the detection of these signals. Signals are then transduced across the plasma membrane so that enzymes at the inner face of the membrane are activated, producing second messengers, which by a complex network of interactions activate target proteins or genes. Vertebrate cells have been used to study hormone and neurotransmitter action, vision, the regulation of cell growth and differentiation. Sensory transduction in lower eukaryotes is predominantly used for other functions, notably cell attraction for mating and food seeking. By comparing sensory transduction in lower and higher eukaryotes general principles may be recognized that are found in all organisms and deviations that are present in specialised systems. This may also help to understand the differences between cell types within one organism and the importance of a particular pathway that may or may not be general. In a practical sense, microorganisms have the advantage of their easy genetic manipulation, which is especially advantageous for the identification of the function of large families of signal transducing components.     

Protein footprinting in a complex milieu: identifying the interaction surfaces of the chemotaxis adaptor protein CheW

Characterizing protein-protein interactions in a biologically relevant context is important for understanding the mechanisms of signal transduction. Most signal transduction systems are membrane associated and consist of large multiprotein complexes that undergo rapid reorganization—circumstances that present challenges to traditional structure determination methods. To study protein-protein interactions in a biologically relevant complex milieu, we employed a protein footprinting strategy based on isotope-coded affinity tag (ICAT) reagents. ICAT reagents are valuable tools for proteomics. Here, we show their utility in an alternative application—they are ideal for protein footprinting in complex backgrounds because the affinity tag moiety allows for enrichment of alkylated species prior to analysis. We employed a water-soluble ICAT reagent to monitor cysteine accessibility and thereby to identify residues involved in two different protein-protein interactions in the Escherichia coli chemotaxis signaling system. The chemotaxis system is an archetypal transmembrane signaling pathway in which a complex protein superstructure underlies sophisticated sensory performance. The formation of this superstructure depends on the adaptor protein CheW, which mediates a functionally important bridging interaction between transmembrane receptors and histidine kinase. ICAT footprinting was used to map the surfaces of CheW that interact with the large multidomain histidine kinase CheA, as well as with the transmembrane chemoreceptor Tsr in native E. coli membranes. By leveraging the affinity tag, we successfully identified CheW surfaces responsible for CheA-Tsr interaction. The proximity of the CheA and Tsr binding sites on CheW suggests the formation of a composite CheW-Tsr surface for the recruitment of the signaling kinase to the chemoreceptor complex.