Signal transduction. A new thread in an intricate web

Generation and characterization of novel human IRAS monoclonal antibodies

Imidazoline receptors were first proposed by Bousquet et al., when they studied antihypertensive effect of clonidine. A strong candidate for I1R, known as imidazoline receptor antisera-selected protein (IRAS), has been cloned from human hippocampus. We reported that IRAS mediated agmatine-induced inhibition of opioid dependence in morphine-dependent cells. To elucidate the functional and structure properties of I1R, we developed the newly monoclonal antibody against the N-terminal hIRAS region including the PX domain (10–120aa) through immunization of BALB/c mice with the NusA-IRAS fusion protein containing an IRAS N-terminal (10–120aa). Stable hybridoma cell lines were established and monoclonal antibodies specifically recognized full-length IRAS proteins in their native state by immunoblotting and immunoprecipitation. Monoclonal antibodies stained in a predominantly punctate cytoplasmic pattern when applied to IRAS-transfected HEK293 cells by indirect immunofluorescence assays and demonstrated excellent reactivity in flow immunocytometry. These monoclonal antibodies will provide powerful reagents for the further investigation of hIRAS protein functions.

Sorting Nexins - Unifying Trends and New Perspectives

The sorting nexins (SNXs) are a family of PX domain-containing proteins found in yeast and mammalian cells that have been proposed to regulate intracellular trafficking. Mammalian SNXs have been suggested to function variously in pro-degradative sorting, internalization, endosomal recycling, or simply in endosomal sorting. In yeast, the defining function for these proteins is a regulation of cargo retrieval. Here we examine recent data on the SNX family of proteins and attempt to draw out unifying themes between the work performed in yeast and mammalian systems.

Regulation of neuromodulator receptor efficacy—implications for whole-neuron and synaptic plasticity

Membrane receptors for neuromodulators (NM) are highly regulated in their distribution and efficacy-a phenomenon which influences the individual cell's response to central signals of NM release. Even though NM receptor regulation is implicated in the pharmacological action of many drugs, and is also known to be influenced by various environmental factors, its functional consequences and modes of action are not well understood. In this paper we summarize relevant experimental evidence on NM receptor regulation (specifically dopamine D1 and D2 receptors) in order to explore its significance for neural and synaptic plasticity. We identify the relevant components of NM receptor regulation (receptor phosphorylation, receptor trafficking and sensitization of second-messenger pathways) gained from studies on cultured cells. Key principles in the regulation and control of short-term plasticity (sensitization) are identified, and a model is presented which employs direct and indirect feedback regulation of receptor efficacy. We also discuss long-term plasticity which involves shifts in receptor sensitivity and loss of responsivity to NM signals. Finally, we discuss the implications of NM receptor regulation for models of brain plasticity and memorization. We emphasize that a realistic model of brain plasticity will have to go beyond Hebbian models of long-term potentiation and depression. Plasticity in the distribution and efficacy of NM receptors may provide another important source of functional plasticity with implications for learning and memory.

Physiological actions of regulators of G-protein signaling (RGS) proteins

Regulators of G-protein signaling (RGS) proteins are a family of proteins, which accelerate GTPase-activity intrinsic to the alpha subunits of heterotrimeric G-proteins and play crucial roles in the physiological control of G-protein signaling. If RGS proteins were active unrestrictedly, they would completely suppress various G-protein-mediated cell signaling as has been shown in the over-expression experiments of various RGS proteins. Thus, physiologically the modes of RGS-action should be under some regulation. The regulation can be achieved through the control of either the protein function and/or the subcellular localization. Examples for the former are as follows: (i) Phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) inhibits RGS-action, which can be recovered by Ca(2+)/calmodulin. This underlies a voltage-dependent "relaxation" behavior of G-protein-gated K(+) channels. (ii) A modulatory protein, 14-3-3, binds to the RGS proteins phosphorylated by PKA and inhibits their actions. For the latter mechanism, additional regulatory modules, such as PDZ, PX, and G-protein gamma subunit-like (GGL) domains, identified in several RGS proteins may be responsible: (i) PDZ domain of RGS12 interacts with a G-protein-coupled chemokine receptor, CXCR2, and thus facilitates its GAP action on CXCR2-mediated G-protein signals. (ii) RGS9 forms a complex with a type of G-protein beta-subunit (Gbeta5) via its GGL domain, which facilitates the GAP function of RGS9. Both types of regulations synergistically control the mode of action of RGS proteins in the physiological conditions, which contributes to fine tunings of G-protein signalings.

Albumin: a Galpha(s)-specific guanine nucleotide dissociation inhibitor and GTPase activating protein

Heterotrimeric GTP binding protein (G protein)-mediated signal transduction events are regulated by their effectors and regulators of G protein signaling (RGS) protein family. The latter proteins function as GTPase activating proteins (GAPs) for G protein alpha subunits and terminate signaling events. In a search for proteins that modulate the activity of the stimulatory G protein of adenylyl cyclase (Galpha(s)), we found that bovine serum albumin (BSA) inhibits the steady-state GTPase activity of Galpha(s), but not the inhibitory G protein (Galpha(i1)). This effect of BSA is mediated by decreasing the rate of GDP dissociation from Galpha(s) and decreasing the rate of GTP binding. Thus, BSA functions as a guanine nucleotide dissociation inhibitor for Galpha(s). Moreover, BSA also increased the intrinsic GTPase activity of Galpha(s), but not Galpha(i1), demonstrating that BSA functions as a Galpha(s)-specific GAP. Using mutants of Galpha(s) (Q227L, Q227N, R201C, and R201K), we demonstrate that BSA mediates its GAP function by modulating the ability of R201 to increase GTPase activity. Moreover, using wild-type and Q227N forms of Galpha(s), our studies demonstrate that the GDI function of BSA decreases the ability of Galpha(s) to stimulate adenylyl cyclase. These findings assign a novel function to BSA as a regulator of G protein signaling.

Biochemical and genetic evidence for a family of heterotrimeric G-proteins in Trichomonas vaginalis

We have cloned a single copy gene from the human parasite Trichomonas vaginalis that encodes a putative protein of 402 amino acids with approximately 35% sequence identity to known alpha subunits of heterotrimeric G-proteins. It contains the characteristic GTP binding domains G-1 to G-5 with the key residues conserved. The new sequence has an unusual N-terminal extension of approximately 70 residues that cannot be aligned to reference G-proteins and which is characterised by proline-rich repeats. To investigate the expression and cellular localisation of the protein we produced specific antisera against a recombinant fusion protein. The antisera recognised a protein of an apparent molecular mass of 51 kDa in protein extracts from T. vaginalis and immunofluorescent microscopy established that the protein is localised to discrete endomembranes. Using a protocol designed to purify mammalian heterotrimeric G-proteins incorporating a GTPgammaS binding assay, we isolated two proteins from Trichomonas that are recognised by an heterologous GA/1 antisera raised to a peptide of the conserved G-1 domain of G-protein alpha subunits. These two proteins have an apparent molecular mass of 61 and 48 kDa, respectively, larger and smaller than the translation product of the cloned gene. Consistent with these results, the GA/1 antisera did not cross-react with the fusion protein produced from the gene we have cloned. These data suggest T. vaginalis possesses more than one heterotrimeric G-protein alpha subunit. Based on the sequence features of the cloned gene and the biochemical properties of the purified proteins, we suggest that these alpha subunits are likely to be part of classic heterotrimeric G-protein complexes.

Regulators of G-protein signalling: multifunctional proteins with impact on signalling in the cardiovascular system

Regulator of G-protein signalling (RGS) proteins form a superfamily of at least 25 proteins, which are highly diverse in structure, expression patterns, and function. They share a 120 amino acid homology domain (RGS domain), which exhibits GTPase accelerating activity for alpha-subunits of heterotrimeric G-proteins, and thus, are negative regulators of G-protein-mediated signalling. Based on the organisation of the Rgs genes, structural similarities, and differences in functions, they can be divided into at least six subfamilies of RGS proteins and three more families of RGS-like proteins. Many of these proteins regulate signalling processes within cells, not only via interaction with G-protein alpha-subunits, but are G-protein-regulated effectors, Gbetagamma scavenger, or scaffolding proteins in signal transduction complexes as well. The expression of at least 16 different RGS proteins in the mammalian or human myocardium have been described. A subgroup of at least eight was detected in a single atrial myocyte. The exact functions of these proteins remain mostly elusive, but RGS proteins such as RGS4 are involved in the regulation of G(i)-protein betagamma-subunit-gated K(+) channels. An up-regulation of RGS4 expression has been consistently found in human heart failure and some animal models. Evidence is increasing that the enhanced RGS4 expression counter-regulates the G(q/11)-induced signalling caused by hypertrophic stimuli. In the vascular system, RGS5 seems to be an important signalling regulator. It is expressed in vascular endothelial cells, but not in cultured smooth muscle cells. Its down-regulation, both in a model of capillary morphogenesis and in an animal model of stroke, render it a candidate gene, which may be involved in the regulation of capillary growth, angiogenesis, and in the pathophysiology of stroke.

Sorting out the cellular functions of sorting nexins

Sorting nexins are a diverse group of cellular trafficking proteins that are unified by the presence of a phospholipid-binding motif - the PX domain. The ability of these proteins to bind specific phospholipids, as well as their propensity to form protein-protein complexes, points to a role for these proteins in membrane trafficking and protein sorting. It will be interesting to determine whether the various sorting nexins have specialized or generalized roles in protein trafficking.