Calmodulin binding to peptides derived from the i3 loop of muscarinic receptors

Alzheimer's Disease: A short introduction to the calmodulin hypothesis

At the cellular level, Alzheimer's disease (AD) is characterized by the presence of intracellular plaques containing amyloid beta (Aβ) protein and neurofibrillary tangles consisting of phospho-tau (p-tau). These biomarkers are considered to contribute, at least in part, to the neurodegenerative events of the disease. But the accumulation of plaques and tangles is widely considered to be a later event with other factors likely being the cause of the disease. Calcium dysregulation-the unregulated accumulation of calcium ions-in neurons is an early event that underlies neurodegeneration. In 2002, O'Day and Myre extended this "Calcium Hypothesis" to include calmodulin (CaM) the primary target of calcium, suggesting the "Calmodulin Hypothesis" as an updated alternative. Here we overview the central role of CaM in the formation of the classic hallmarks of AD: plaques and tangles. Then some insight into CaM's binding to various risk factor proteins is given followed by a short summary of specific receptors and channels linked to the disease that are CaM binding proteins. Overall, this review emphasizes the diversity of Alzheimer's-linked CaM-binding proteins validating the hypothesis that CaM operates critically at all stages of the disease and stands out as a potential primary target for future research.

Human adenosine A2A receptor binds calmodulin with high affinity in a calcium-dependent manner

Understanding how ligands bind to G-protein-coupled receptors and how binding changes receptor structure to affect signaling is critical for developing a complete picture of the signal transduction process. The adenosine A2A receptor (A2AR) is a particularly interesting example, as it has an exceptionally long intracellular carboxyl terminus, which is predicted to be mainly disordered. Experimental data on the structure of the A2AR C-terminus is lacking, because published structures of A2AR do not include the C-terminus. Calmodulin has been reported to bind to the A2AR C-terminus, with a possible binding site on helix 8, next to the membrane. The biological meaning of the interaction as well as its calcium dependence, thermodynamic parameters, and organization of the proteins in the complex are unclear. Here, we characterized the structure of the A2AR C-terminus and the A2AR C-terminus-calmodulin complex using different biophysical methods, including native gel and analytical gel filtration, isothermal titration calorimetry, NMR spectroscopy, and small-angle X-ray scattering. We found that the C-terminus is disordered and flexible, and it binds with high affinity (Kd = 98 nM) to calmodulin without major conformational changes in the domain. Calmodulin binds to helix 8 of the A2AR in a calcium-dependent manner that can displace binding of A2AR to lipid vesicles. We also predicted and classified putative calmodulin-binding sites in a larger group of G-protein-coupled receptors.

Helix 8 and the i3 loop of the muscarinic M3 receptor are crucial sites for its regulation by the Gβ5-RGS7 complex

The muscarinic M3 receptor (M3R) is a Gq-coupled receptor and is known to interact with many intracellular regulatory proteins. One of these molecules is Gβ5-RGS7, the permanently associated heterodimer of G protein β-subunit Gβ5 and RGS7, a regulator of G protein signaling. Gβ5-RGS7 can attenuate M3R-stimulated release of Ca²⁺ from intracellular stores or enhance the influx of Ca²⁺ across the plasma membrane. Here we show that deletion of amino acids 304–345 from the central portion of the i3 loop renders M3R insensitive to regulation by Gβ5-RGS7. In addition to the i3 loop, interaction of M3R with Gβ5-RGS7 requires helix 8. According to circular dichroism spectroscopy, the peptide corresponding to amino acids 548–567 in the C-terminus of M3R assumes an α-helical conformation. Substitution of Thr553 and Leu558 with Pro residues disrupts this α-helix and abolished binding to Gβ5-RGS7. Introduction of the double Pro substitution into full-length M3R (M3R^TP/LP) prevents trafficking of the receptor to the cell surface. Using atropine or other antagonists as pharmacologic chaperones, we were able to increase the level of surface expression of the TP/LP mutant to levels comparable to that of wild-type M3R. However, M3R-stimulated calcium signaling is still severely compromised. These results show that the interaction of M3R with Gβ5-RGS7 requires helix 8 and the central portion of the i3 loop.

Calmodulin Binding Proteins and Alzheimer's Disease

The small, calcium-sensor protein, calmodulin, is ubiquitously expressed and central to cell function in all cell types. Here the literature linking calmodulin to Alzheimer's disease is reviewed. Several experimentally-verified calmodulin-binding proteins are involved in the formation of amyloid-β plaques including amyloid-β protein precursor, β-secretase, presenilin-1, and ADAM10. Many others possess potential calmodulin-binding domains that remain to be verified. Three calmodulin binding proteins are associated with the formation of neurofibrillary tangles: two kinases (CaMKII, CDK5) and one protein phosphatase (PP2B or calcineurin). Many of the genes recently identified by genome wide association studies and other studies encode proteins that contain putative calmodulin-binding domains but only a couple (e.g., APOE, BIN1) have been experimentally confirmed as calmodulin binding proteins. At least two receptors involved in calcium metabolism and linked to Alzheimer's disease (mAchR; NMDAR) have also been identified as calmodulin-binding proteins. In addition to this, many proteins that are involved in other cellular events intimately associated with Alzheimer's disease including calcium channel function, cholesterol metabolism, neuroinflammation, endocytosis, cell cycle events, and apoptosis have been tentatively or experimentally verified as calmodulin binding proteins. The use of calmodulin as a potential biomarker and as a therapeutic target is discussed.

Serotonin 5-HT3 receptor-mediated vomiting occurs via the activation of Ca2+/CaMKII-dependent ERK1/2 signaling in the least shrew (Cryptotis parva)

Stimulation of 5-HT₃ receptors (5-HT₃Rs) by 2-methylserotonin (2-Me-5-HT), a selective 5-HT₃ receptor agonist, can induce vomiting. However, downstream signaling pathways for the induced emesis remain unknown. The 5-HT₃R channel has high permeability to extracellular calcium (Ca²⁺) and upon stimulation allows increased Ca²⁺ influx. We examined the contribution of Ca²⁺/calmodulin-dependent protein kinase IIα (Ca²⁺/CaMKIIα), interaction of 5-HT₃R with calmodulin, and extracellular signal-regulated kinase 1/2 (ERK1/2) signaling to 2-Me-5-HT-induced emesis in the least shrew. Using fluo-4 AM dye, we found that 2-Me-5-HT augments intracellular Ca²⁺ levels in brainstem slices and that the selective 5-HT₃R antagonist palonosetron, can abolish the induced Ca²⁺ signaling. Pre-treatment of shrews with either: i) amlodipine, an antagonist of L-type Ca²⁺ channels present on the cell membrane; ii) dantrolene, an inhibitor of ryanodine receptors (RyRs) Ca²⁺-release channels located on the endoplasmic reticulum (ER); iii) a combination of their less-effective doses; or iv) inhibitors of CaMKII (KN93) and ERK1/2 (PD98059); dose-dependently suppressed emesis caused by 2-Me-5-HT. Administration of 2-Me-5-HT also significantly: i) enhanced the interaction of 5-HT₃R with calmodulin in the brainstem as revealed by immunoprecipitation, as well as their colocalization in the area postrema (brainstem) and small intestine by immunohistochemistry; and ii) activated CaMKIIα in brainstem and in isolated enterochromaffin cells of the small intestine as shown by Western blot and immunocytochemistry. These effects were suppressed by palonosetron. 2-Me-5-HT also activated ERK1/2 in brainstem, which was abrogated by palonosetron, KN93, PD98059, amlodipine, dantrolene, or a combination of amlodipine plus dantrolene. However, blockade of ER inositol-1, 4, 5-triphosphate receptors by 2-APB, had no significant effect on the discussed behavioral and biochemical parameters. This study demonstrates that Ca²⁺ mobilization via extracellular Ca²⁺ influx through 5-HT₃Rs/L-type Ca²⁺ channels, and intracellular Ca²⁺ release via RyRs on ER, initiate Ca²⁺-dependent sequential activation of CaMKIIα and ERK1/2, which contribute to the 5-HT₃R-mediated, 2-Me-5-HT-evoked emesis.

The C. elegans VIG-1 and FRM-1 modulate carbachol-stimulated ERK1/2 activation in chinese hamster ovary cells expressing the muscarinic acetylcholine receptor GAR-3

Many neurotransmitter receptors are known to interact with a variety of intracellular proteins that modulate signaling processes. In an effort to understand the molecular mechanism by which acetylcholine (ACh) signaling is modulated, we searched for proteins that interact with GAR-3, the Caenorhabditis elegans homolog of muscarinic ACh receptors. We isolated two proteins, VIG-1 and FRM-1, in a yeast two-hybrid screen of a C. elegans cDNA library using the third intracellular (i3) loop of GAR-3 as bait. To test whether these proteins regulate ACh signaling, we utilized Chinese hamster ovary (CHO) cells stably expressing GAR-3 (GAR-3/CHO cells). Previously we have shown that the cholinergic agonist carbachol stimulates extracellular signal-regulated kinases 1 and 2 (ERK1/2) activation in an atropine-sensitive manner in this cell line. When VIG-1 was transiently expressed in GAR-3/CHO cells, carbachol-stimulated ERK1/2 activation was substantially reduced. In contrast, transient expression of FRM-1 significantly enhanced carbachol-stimulated ERK1/2 activation. Neither VIG-1 nor FRM-1 expression appeared to alter the affinity between GAR-3 and carbachol. In support of this notion, expression of these proteins did not affect GAR-3-mediated phospholipase C activation. To verify the modulation of ERK1/2 activity by VIG-1 and FRM-1, we used an i3 loop deletion mutant of GAR-3 (termed GAR-3Δi3). Carbachol treatment evoked robust ERK1/2 activation in CHO cells stably expressing the deletion mutant (GAR-3Δi3/CHO cells). However, transient expression of either VIG-1 or FRM-1 had little effect on carbachol-stimulated ERK1/2 activation in GAR-3Δi3/CHO cells. Taken together, these results indicate that VIG-1 and FRM-1 regulate GAR-3-mediated ERK1/2 activation by interacting with the i3 loop of GAR-3.

Between promiscuity and specificity: novel roles of EF-hand calcium sensors in neuronal Ca2+ signalling

In recent years, substantial progress has been made towards an understanding of the physiological function of EF-hand calcium sensor proteins of the Calmodulin (CaM) superfamily in neurons. This deeper appreciation is based on the identification of novel target interactions, structural studies and the discovery of novel signalling mechanisms in protein trafficking and synaptic plasticity, in which CaM-like sensor proteins appear to play a role. However, not all interactions are of plausible physiological relevance and in many cases it is not yet clear how the CaM signaling network relates to the proposed function of other EF-hand sensors. In this review, we will summarize these findings and address some of the open questions on the functional role of EF-hand calcium binding proteins in neurons.

Regulation of M₃ muscarinic receptor expression and function by transmembrane protein 147

The M₃ muscarinic acetylcholine receptor (M3R) regulates many fundamental physiological functions. To identify novel M3R-interacting proteins, we used a recently developed yeast two-hybrid screen (split ubiquitin method) to detect interactions among membrane proteins. This screen led to the identification of many novel M3R-associated proteins, including the putative membrane protein transmembrane protein 147 (Tmem147). The amino acid sequence of Tmem147 is highly conserved among mammals, but its physiological roles are unknown at present. We initially demonstrated that Tmem147 could be coimmunoprecipitated with M3Rs in cotransfected mammalian cells (COS-7 cells). Confocal imaging studies showed that Tmem147 was localized to endoplasmic reticulum (ER) membranes and that the Tmem147/M3R interaction occurred in the ER of cotransfected COS-7 cells, resulting in impaired trafficking of the M3R to the cell surface. To study the role of Tmem147 in modulating M3R function in a more physiologically relevant setting, we carried out studies with H508 human colon cancer cells that endogenously express M3Rs and Tmem147. Treatment of H508 cells with carbachol, a hydrolytically stable acetylcholine analog, promoted H508 cell proliferation and activation of the mitogenic kinase, p90RSK. Small interfering RNA-mediated knockdown of Tmem147 expression significantly augmented the stimulatory effects of carbachol on H508 cell proliferation and p90RSK activation. These effects were associated with an increase in the density of cell surface M3Rs. Our data clearly indicate that Tmem147 represents a potent negative regulator of M3R function, most likely by interacting with M3Rs in an intracellular compartment (ER). These findings may lead to new strategies aimed at modulating M3R activity for therapeutic purposes.

Molecular organization of the complex between the muscarinic M3 receptor and the regulator of G protein signaling, Gbeta(5)-RGS7

The complex of the regulator of G protein signaling (RGS), Gbeta(5)-RGS7, can inhibit signal transduction via the M3 muscarinic acetylcholine receptor (M3R). RGS7 consists of three distinct structural entities: the DEP domain and its extension DHEX, the Ggamma-like (GGL) domain, which is permanently bound to Gbeta subunit Gbeta(5), and the RGS domain responsible for the interaction with Galpha subunits. Inhibition of the M3R by Gbeta(5)-RGS7 is independent of the RGS domain but requires binding of the DEP domain to the third intracellular loop of the receptor. Recent studies identified the dynamic intramolecular interaction between the Gbeta(5) and DEP domains, which suggested that the Gbeta(5)-RGS7 dimer could alternate between the "open" and "closed" conformations. Here, we identified point mutations that weaken DEP-Gbeta(5) binding, presumably stabilizing the open state, and tested their effects on the interaction of Gbeta(5)-RGS7 with the M3R. We found that these mutations facilitated binding of Gbeta(5)-RGS7 to the recombinant third intracellular loop of the M3R but did not enhance its ability to inhibit M3R-mediated Ca(2+) mobilization. This led us to the idea that the M3R can effectively induce the Gbeta(5)-RGS7 dimer to open; such a mechanism would require a region of the receptor distinct from the third loop. Indeed, we found that the C-terminus of M3R interacts with Gbeta(5)-RGS7. Truncation of the C-terminus rendered the M3R insensitive to inhibition by wild-type Gbeta(5)-RGS7; however, the open mutant of Gbeta(5)-RGS7 was able to inhibit signaling by the truncated M3R. The GST fusion of the M3R C-tail could not bind to wild-type Gbeta(5)-RGS7 but could associate with its open mutant as well as with the separated recombinant DEP domain or Gbeta(5). Taken together, our data are consistent with the following model: interaction of the M3R with Gbeta(5)-RGS7 causes the DEP domain and Gbeta(5) to dissociate from each other and bind to the C-tail, and the DEP domain also binds to the third loop, thereby inhibiting M3R-mediated signaling.

Structure and mechanism of calmodulin binding to a signaling sphingolipid reveal new aspects of lipid-protein interactions

Lipid-protein interactions are rarely characterized at a structural molecular level due to technical difficulties; however, the biological significance of understanding the mechanism of these interactions is outstanding. In this report, we provide mechanistic insight into the inhibitory complex formation of the lipid mediator sphingosylphosphorylcholine with calmodulin, the most central and ubiquitous regulator protein in calcium signaling. We applied crystallographic, thermodynamic, kinetic, and spectroscopic approaches using purified bovine calmodulin and bovine cerebral microsomal fraction to arrive at our conclusions. Here we present 1) a 1.6-Å resolution crystal structure of their complex, in which the sphingolipid occupies the conventional hydrophobic binding site on calmodulin; 2) a peculiar stoichiometry-dependent binding process: at low or high protein-to-lipid ratio calmodulin binds lipid micelles or a few lipid molecules in a compact globular conformation, respectively, and 3) evidence that the sphingolipid displaces calmodulin from its targets on cerebral microsomes. We have ascertained the specificity of the interaction using structurally related lipids as controls. Our observations reveal the structural basis of selective calmodulin inhibition by the sphingolipid. On the basis of the crystallographic and biophysical characterization of the calmodulin-sphingosylphosphorylcholine interaction, we propose a novel lipid-protein binding model, which might be applicable to other interactions as well.

CaMKIIalpha interacts with M4 muscarinic receptors to control receptor and psychomotor function

Muscarinic acetylcholine receptors (mAChRs) are widely expressed in the mammalian brain and are essential for neuronal functions. These receptors are believed to be actively regulated by intracellular signals, although the underlying mechanisms are largely unknown. In this study, we show that Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) binds directly and selectively to one of five mAChR subtypes, M4 receptors (M4Rs), at their C-terminal regions of second intracellular loops. This binding relies on Ca(2+) activation of the kinase and leads to the phosphorylation of M4Rs at a specific threonine site (Thr145). Complementary in vivo studies in rat striatal neurons enriched with M4Rs confirm that rising Ca(2+) recruits CaMKIIalpha to M4Rs to potentiate receptor signalling, which controls behavioural sensitivity to dopamine stimulation in an activity-dependent manner. Our data identify a new model of protein-protein interactions. In a Ca(2+)-sensitive manner, CaMKIIalpha regulates M4R efficacy and controls the acetylcholine-dopamine balance in the basal ganglia and also the dynamics of movement.

The Gbeta5-RGS7 complex selectively inhibits muscarinic M3 receptor signaling via the interaction between the third intracellular loop of the receptor and the DEP domain of RGS7

Regulators of G protein signaling (RGS) make up a diverse family primarily known as GTPase-activating proteins (GAPs) for heterotrimeric G proteins. In addition to the RGS domain, which is responsible for GAP activity, most RGS proteins contain other distinct structural motifs. For example, members of the R7 family of RGS proteins contain a DEP, GGL, and novel DHEX domain and are obligatory dimers with G protein beta subunit Gbeta5. Here we show that the Gbeta5-RGS7 complex can inhibit Ca2+ mobilization elicited by muscarinic acetylcholine receptor type 3 (M3R), but not by other Gq-coupled receptors such as M1, M5, histamine H1, and GNRH receptors. The isolated DEP domain of RGS7 is sufficient for the inhibition of M3R signaling, whereas the deletion of the DEP domain renders the Gbeta5-RGS7 complex ineffective. Deletion of a portion of the third intracellular loop allowed the receptor (M3R-short) to signal but rendered it insensitive to the effect of the Gbeta5-RGS7 complex. Accordingly, the recombinant DEP domain bound in vitro to the GST-fused i3 loop of the M3R. These results identify a novel molecular mechanism that can impart receptor subtype selectivity on signal transduction via Gq-coupled muscarinic receptors.

Dissociation of calmodulin-target peptide complexes by the lipid mediator sphingosylphosphorylcholine: implications in calcium signaling

Previously we have identified the lipid mediator sphingosylphosphorylcholine (SPC) as the first potentially endogenous inhibitor of the ubiquitous Ca2+ sensor calmodulin (CaM) (Kovacs, E., and Liliom, K. (2008) Biochem. J. 410, 427-437). Here we give mechanistic insight into CaM inhibition by SPC, based on fluorescence stopped-flow studies with the model CaM-binding domain melittin. We demonstrate that both the peptide and SPC micelles bind to CaM in a rapid and reversible manner with comparable affinities. Furthermore, we present kinetic evidence that both species compete for the same target site on CaM, and thus SPC can be considered as a competitive inhibitor of CaM-target peptide interactions. We also show that SPC disrupts the complex of CaM and the CaM-binding domain of ryanodine receptor type 1, inositol 1,4,5-trisphosphate receptor type 1, and the plasma membrane Ca2+ pump. By interfering with these interactions, thus inhibiting the negative feedback that CaM has on Ca2+ signaling, we hypothesize that SPC could lead to Ca2+ mobilization in vivo. Hence, we suggest that the action of the sphingolipid on CaM might explain the previously recognized phenomenon that SPC liberates Ca2+ from intracellular stores. Moreover, we demonstrate that unlike traditional synthetic CaM inhibitors, SPC disrupts the complex between not only the Ca2+-saturated but also the apo form of the protein and the target peptide, suggesting a completely novel regulation for target proteins that constitutively bind CaM, such as ryanodine receptors.

Interactions between calmodulin, adenosine A2A, and dopamine D2 receptors

The Ca(2+)-binding protein calmodulin (CaM) has been shown to bind directly to cytoplasmic domains of some G protein-coupled receptors, including the dopamine D(2) receptor. CaM binds to the N-terminal portion of the long third intracellular loop of the D(2) receptor, within an Arg-rich epitope that is also involved in the binding to G(i/o) proteins and to the adenosine A(2A) receptor, with the formation of A(2A)-D(2) receptor heteromers. In the present work, by using proteomics and bioluminescence resonance energy transfer (BRET) techniques, we provide evidence for the binding of CaM to the A(2A) receptor. By using BRET and sequential resonance energy transfer techniques, evidence was obtained for CaM-A(2A)-D(2) receptor oligomerization. BRET competition experiments indicated that, in the A(2A)-D(2) receptor heteromer, CaM binds preferentially to a proximal C terminus epitope of the A(2A) receptor. Furthermore, Ca(2+) was found to induce conformational changes in the CaM-A(2A)-D(2) receptor oligomer and to selectively modulate A(2A) and D(2) receptor-mediated MAPK signaling in the A(2A)-D(2) receptor heteromer. These results may have implications for basal ganglia disorders, since A(2A)-D(2) receptor heteromers are being considered as a target for anti-parkinsonian agents.

How calmodulin interacts with the adenosine A(2A) and the dopamine D(2) receptors

Receptor heteromerization is a mechanism used by G protein-coupled receptors to diversify their properties and function. We previously demonstrated that these interactions occur through salt bridge formation between epitopes of the involved receptors. Recent studies claim that calmodulin (CaM) binds to an Arg-rich epitope located in the amino-terminus of the dopamine D(2) receptor third intracellular loop. This is the same epitope involved in adenosine A(2A)-D(2) receptor heteromerization, through Coulombic interaction between the Arg residues and a phosphorylated serine (pS) located in the medial segment of the C-terminus of the A(2A) receptor. Mass spectrometric analysis indicates that an electrostatic interaction involving the D(2) receptor Arg-rich epitope and several CaM acidic epitopes are mainly responsible for the D(2) receptor-CaM binding. CaM could also form multiple noncovalent complexes by means of electrostatic interactions with an epitope localized in the proximal segment of the C-terminus of the A(2A) receptor. Ca(2+) disrupted the binding of CaM to the D(2) but not to the A(2A) receptor epitope, and CaM disrupted the electrostatic interactions between the D(2) receptor epitope and the more distal A(2A) receptor epitope. A model is introduced with the possible functional implications of A(2A)-D(2)-CaM interactions. These in vitro findings imply a possible regulatory role for CaM in receptor heteromers formation.

Sphingosylphosphorylcholine as a novel calmodulin inhibitor

S1P (sphingosine 1-phosphate) and SPC (sphingosylphosphorylcholine) have been recently recognized as important mediators of cell signalling, regulating basic cellular processes such as growth,differentiation, apoptosis, motility and Ca2+ homoeostasis.Interestingly, they can also act as first and second messengers. Although their activation of cell-surface G-protein-coupled receptors has been studied extensively, not much is known about heir intracellular mechanism of action, and their target proteins are yet to be identified. We hypothesized that these sphingolipids might bind to CaM (calmodulin), the ubiquitous intracellular Ca2+sensor. Binding assays utilizing intrinsic tyrosine fluorescence of the protein, dansyl-labelled CaM and surface plasmon resonance revealed that SPC binds to both apo- and Ca2+-saturated CaM selectively, when compared with the related lysophospholipid mediators S1P, LPA (lysophosphatidic acid) and LPC (lysophosphatidylcholine). Experiments carried out with the model CaM-binding domain melittin showed that SPC dissociates the CaM-target peptide complex, suggesting an inhibitory role. The functional effect of the interaction was examined on two target enzymes, phosphodiesterase and calcineurin, and SPC inhibited the Ca2+/CaM-dependent activity of both. Thus we propose that CaM might be an intracellular receptor for SPC, and raise the possibility of a novel endogenous regulation of CaM.

The proto-oncogene SET interacts with muscarinic receptors and attenuates receptor signaling

G protein-coupled receptors mediate cell responses to extracellular stimuli and likely function in the context of a larger signal transduction complex. Utilizing the third intracellular loop of a G protein-coupled receptor in glutathione S-transferase pulldown assays from rat brain lysates coupled with high sensitivity detection methods and subsequent functional studies, we report the identification of SET as a regulator of muscarinic receptor signaling. SET is a putative oncogene reported to inhibit protein phosphatase 2A and regulate gene transcription. SET binds the carboxyl region of the M3-muscarinic receptor i3 loop, and endogenous SET co-immunoprecipitates with intact M3 muscarinic receptor expressed in cells. Small interfering RNA knockdown of endogenous SET in Chinese hamster ovary cells stably expressing the M3 muscarinic receptor augmented receptor-mediated mobilization of intracellular calcium by approximately 35% with no change in agonist EC(50), indicating that interaction of SET with the M3 muscarinic receptor reduces its signaling capacity. SET knockdown had no effect on the mobilization of intracellular calcium by the P2-purinergic receptor, ionomycin, or a direct activator of phospholipase C, indicating a specific regulation of M3 muscarinic receptor signaling. These data provide expanded functionality for SET and a previously unrecognized mechanism for regulation of GPCR signaling capacity.