Membrane compartimentalization

Controversy has arisen over why agonist competition curves are shallow in the first place. The initial explanation was based on the assumption that there is a stoichio-metric limitation to the amount of G proteins. However, direct methods of measurement have subsequently shown that G proteins may be present in large excess over the number of cognate receptor proteins. For example, there is a substantial excess of Gs relative to both P-adrenergic receptors and adenylate cyclase in S49 murine lymphoma cells (R:Gs:effector ratio ~ 1:100:3). For Ga it is believed that levels of expression are somewhat greater than those of Gas. Based on the assumed uniform accessibility of individual G proteins, agonist competition curves are expected to display a uniform population of high affinity sites in competition binding studies. To date, this has never been shown to take place. On the contrary, several experiments suggest that receptors may only have limited access to the total pool of G proteins.

• If different receptors couple to the same G protein pool and if the G proteins therein would be limiting and freely mobile, the ternary complex model predicts that binding of an agonist at one receptor should compete with other receptors for the same set of G proteins. Accordingly, an unlabelled agonist for one receptor should be able to decrease binding of a radiolabelled agonist to another receptor. Experimentally, even when G; proteins in NG108-15 neuroblastoma-glioma cells are made limiting by a partial pertussis toxin treatment, 8-opiate or muscarinic receptors still fail to inhibit radiolabelled agonist binding to the a2B-adrenergic receptors in the membranes thereof. Hence, there is no indication of cross talk with these Gi-preferring receptors (Graeser and Neubig, 1993).

• Agonist-P-adrenergic receptor-Gs complexes may be rendered permanent in the presence of the alkylating reagent N-ethylmaleimide (this effect is due to the reagent's ability to alkylate a crucial sulfhydryl group present on Gs) (Figure 158). In membrane preparations from several cell types, this produces a time-wise leftward shift of agonist competition curves and they even become biphasic at the

Figure 158 Affinity states for agonists and antagonists. These states are observed in competition binding experiments with radiolabelled antagonists.

Figure 158 Affinity states for agonists and antagonists. These states are observed in competition binding experiments with radiolabelled antagonists.

Figure 159 Only part of the p-adrenergic receptors undergo functional coupling to Gs. Turkey eythrocyte membranes were pre-incubated with isoproterenol and N-ethylmaleimide, washed and the remaining (i.e. non-coupled) pradrenergic receptors were quantified by binding of the antagonist [3H]dihydroalprenolol. Reprinted from Recent Advances in Receptor Chemistry, (C. Melchiorre and M. Giannella, Eds.), Vauquelin G., Severne Y., Convents A., Nerme V. and Abrahamsson T., Agonist-mediated activation of adrenergic receptors, pp. 43-61. Copyright (1988), with permission from Elsevier.

Figure 159 Only part of the p-adrenergic receptors undergo functional coupling to Gs. Turkey eythrocyte membranes were pre-incubated with isoproterenol and N-ethylmaleimide, washed and the remaining (i.e. non-coupled) pradrenergic receptors were quantified by binding of the antagonist [3H]dihydroalprenolol. Reprinted from Recent Advances in Receptor Chemistry, (C. Melchiorre and M. Giannella, Eds.), Vauquelin G., Severne Y., Convents A., Nerme V. and Abrahamsson T., Agonist-mediated activation of adrenergic receptors, pp. 43-61. Copyright (1988), with permission from Elsevier.

end. Yet, there is no time-wise increase in the amount of receptor sites with high agonist affinity (Figure 159). This amount appears to be fixed for each cell type (Table 18) and suggests that the P-adrenergic receptors have only limited accessibility to Gs. This may point to limitations of the mobility of the receptors and G proteins.

Table 18 Amount of coupling-prone (i.e. agonist/N-ethylmaleimide sensitive) p-adrenergic receptors in membranes from various tissues. Reprinted from Agonist-mediated activation of adrenergic receptors. In Recent Advances in Receptor Chemistry, Vauquelin, G., Severne, Y., Convents, A., Nerme, V. and Abrahamsson, T., Copyright (1988), with permission from Elsevier.

Table 18 Amount of coupling-prone (i.e. agonist/N-ethylmaleimide sensitive) p-adrenergic receptors in membranes from various tissues. Reprinted from Agonist-mediated activation of adrenergic receptors. In Recent Advances in Receptor Chemistry, Vauquelin, G., Severne, Y., Convents, A., Nerme, V. and Abrahamsson, T., Copyright (1988), with permission from Elsevier.

Membrane source

Receptor

Coupling

Turkey erythrocyte

ft

50%

Human adipose

ft

46%

Friend erythroleukemia

ft

69%

S49 Lymphoma (WT)

ft

65%

(unc, cyc")

ft

0%

Human uterus

ft

50%

Calf trapezius muscle

ft

45%

Rat brain

ft > ft

36%

Rat lung

ft > ft

50%

Calf lung

ft

25%

Bound (fmols/cell) t(min)

Figure 160 A reduction in the ^-opioid receptor concentration in C6 rat glioma cells produces a similar reduction in ^-agonist-mediated [35S]GTPyS binding. Left: Scatchard plot for cells with different radioligand Bmax values. Right: [35S]GPPyS binding versus the incubation time, same symbols). Reproduced from Alt, A., Mcfadyen, I. J., Fan, C. D., Woods, J. H. and Traynor, J. R. (2001) Journal of Pharmacology & Experimental Therapeutics, 298, 116-121, with permission from the American Society for Pharmacology and Experimental Theraputics.

Bound (fmols/cell) t(min)

Figure 160 A reduction in the ^-opioid receptor concentration in C6 rat glioma cells produces a similar reduction in ^-agonist-mediated [35S]GTPyS binding. Left: Scatchard plot for cells with different radioligand Bmax values. Right: [35S]GPPyS binding versus the incubation time, same symbols). Reproduced from Alt, A., Mcfadyen, I. J., Fan, C. D., Woods, J. H. and Traynor, J. R. (2001) Journal of Pharmacology & Experimental Therapeutics, 298, 116-121, with permission from the American Society for Pharmacology and Experimental Theraputics.

• The ternary complex model predicts a reduction in receptor concentration would decrease the likelihood of a random encounter. This should decrease the rate of G protein activation, but should not affect the maximum number of G proteins activated. Yet, a reduction of the ^-opioid receptor concentration in C6 rat glioma cell membranes produced a similar decrease in the maximum ^-agonist-mediated G protein activation (maximal [35S]GTPyS binding) but did not affect the rate of [35S]GTPyS binding (Figure 160). Similar data were found for digitonin-permeabilized cells (digitonin is a cholesterol-binding detergent that makes pores in the membrane so that [35S]GTP yS can get within the cell). These findings suggest that the receptors do not have access to the whole pool of G proteins, but that each receptor is only surrounded by a fixed, limited number of G proteins.

• Cells typically have multiple types of receptors, G proteins and effectors, and it is difficult to understand how specific receptor-effector communication would result from a multitude of promiscuous protein interactions. Still, receptor-effector communication does appear to be quite specific in living cells. Moreover, it is difficult to reconcile the low absolute concentration of GPCR, G protein and effector (ranging from femtomole to low picomole per milligram of protein) with the observed rapid, highly selective interaction of components required for signal transduction in cell membranes.

To account for these considerations, the current dogma is that GPCR signalling components are held in close association with one another as 'prearranged signalling complexes': i.e. they are not freely floating or dependent on random collision to interact. Several mechanisms imply some form of organization:

• One explanation is that only some of the receptors in cell membrane preparations are actually present at the cell surface. Indeed, only 5% of the membranes of eukaryotic cells are present at the cell surface. The other 95% are intracellular and make part of, for example, the endoplasmic reticulum, the golgi apparatus and small vesicles. Receptors may be present in the membranes of such intracellular compartments on their way to their translocation to the membrane or after their endocytosis and display only limited access to G proteins (e.g. G proteins not present, post-transcriptional processing of the receptor not yet complete, desensitization) when compared to the receptors at the cell surface.

• Another explanation is that receptors and G proteins are compartmentalized by cytoskeletal divisions of the cellular membrane. In this respect, cell plasma membranes are indeed compartmentalized in large specialized domains such as luminal and basolateral surfaces in epithelial cells. These have a distinct segregation of proteins, including receptors and other signalling molecules. The postsy-naptic regions of neuronal target cells also, typically, have high concentrations of certain receptors, transporters and enzymes.

• Another explanation is that G proteins shuttle between receptors and effectors within restricted microdomains in the membrane. In this respect, evidence has been gathered for many signalling molecules to be enriched in lipid rafts/caveolae in the membrane (Figure 161).

• An even more provoking explanation is that receptors and G proteins may be held in constant physical proximity of each other.

Of particular note is that a variety of G proteins and a large number of G proteincoupled receptors have been shown to be enriched in lipid rafts or caveolae. These membrane structures may therefore serve as scaffolding centres for components involved in GPCR signalling:

• Lipid rafts are specialized membrane domains enriched in certain lipids like cholesterol and proteins. Due to the presence of cholesterol, a lipid raft forms a domain that exhibits less fluidity than the surrounding plasma membrane

Figure 161 Lipid rafts: Glycosphingolipids and other lipids with long, straight acyl chains are depicted in orange.

Lipid raft

Figure 161 Lipid rafts: Glycosphingolipids and other lipids with long, straight acyl chains are depicted in orange.

Figure 162 Caveolae and coated pit structures in the osteoblast plasma membrane. Shown: transmission electron micrograph of a murine MC3T3-E1 osteoblast. Reproduced from Journal of Bone and Mineral Research 2000, 15, 2391-2401, with permission of the American Society for Bone and Mineral Research.

(Figure 161). Glycosphingolipids and other lipids with long, straight acyl chains are preferentially incorporated into the rafts. The fatty-acid chains of lipids within the rafts tend to be extended and so more tightly packed, creating domains with higher order. It is therefore thought that rafts exist in a separate ordered phase that 'floats' in a sea of poorly ordered lipids. Lipid rafts have an average diameter in the range of 100 to 200 nm and produce a rather extensive coverage of the plasma membrane surface.

• Caveolae are lipid rafts, which contain the cholesterol-binding protein caveolin-1. Caveolae are identified as 50-100 nm 'flask-shaped' invaginations of the plasma membrane (Figure 162). They are found in a variety of cell types, especially endothelial cells, but none exist as classical invaginated caveolae in neuronal tissues. Caveolin-1 is palmitoylated and forms an oligomeric coat structure around the bulb of caveolae. It binds cholesterol. This appears to be required for its role in maintaining caveolar structure.

Several techniques have been employed to investigate the presence of proteins and protein complexes in lipid rafts/caveolae:

• Fluorescence microscopy: the small size and apparently even distribution of lipid rafts/caveolae might result in an apparently even distribution of the constituent proteins, as visualized by this technique. Nevertheless, this approach has been used to reveal the co-localization of certain proteins with caveolin-1 (Figure 163).

Figure 163 Immunofluorescence microscopy of G protein and caveolin-1 in isolated luminal endothelial cell plasma membranes from rat lung. All G proteins are present in discrete microdomains (i.e. punctate staining). Gq shows the greatest degree of co-localization with caveolin (Oh and Schnitzer, 2001, reproduced by permission of the American Society for Cell Biology).

Figure 163 Immunofluorescence microscopy of G protein and caveolin-1 in isolated luminal endothelial cell plasma membranes from rat lung. All G proteins are present in discrete microdomains (i.e. punctate staining). Gq shows the greatest degree of co-localization with caveolin (Oh and Schnitzer, 2001, reproduced by permission of the American Society for Cell Biology).

• The traditional method of preparation of detergent-resistant lipid rafts and caveo-lae involves scraping cells into cold buffer containing 1% of the detergent Triton X-100, and homogenizing the lysate. Rafts are isolated by flotation in a 5 to 30% linear sucrose density gradient where they distribute in the top few fractions of the gradient. The caveolin-containing lipid rafts can be further separated from non-invaginated rafts by anti-caveolin immunoaffinity purification.

• An indirect approach for studying the function of lipid rafts involves depleting cells of cholesterol with agents (such as filipin or methyl-P-cyclodextrin) that sequester or remove cholesterol. Lipid rafts and caveolae are disassembled and the constituent molecules are dispersed to a more random distribution over the cell surface.

Many proteins and lipids are known to be enriched in caveolae (Table 19). This may, at least in part, be related to the ability of caveolin to recruit proteins bearing caveolin-binding motifs in these structures (see also Figure 153). Caveolin-1 and caveolin-2 are most prevalent in endothelial cells, smooth muscle cells, skeletal myoblasts, fibroblasts and adipocytes. Caveolin-3 is exclusively present in muscle cells, including cardiac myocytes and cells of the arterial vasculature. Caveolin-binding proteins comprise

Table 19 Signalling molecules expressed in caveola. Reproduced from Ostrom, R. S., Post, R. and Insel, P. (2000) Journal of Pharmacology and Experimental Therapeutics, 294, 407-412, with permission from the American Society for Pharmacology and Experimental Therapeutics.

Receptors

Postreceptor components

ß-Adrenergic

G proteins (a and ß-y)

Raf-1

Bradykinin

Endothelial nitric-oxide synthase

Rac-l

Endothelin

Mitogen-activated protein kinase

RhoA

M2 muscarinic acetylcholine

Adenylyl cyclase

Src kinases

Adenosine Al

PKA (catalytic subunit)

Shc

Cholecystokinin

PKC (a)

Calmodulin

Platelet-derived growth factor

Diacylglycerol

IP3 receptor

Epidermal growth factor

GRKs

Insulin

Ras

GPCRs and other receptors, including many growth factor receptors (EGF receptor, PDGF receptor, insulin receptor, etc.), as well as signal molecules like heterotrimeric G proteins, protein kinase C, Shc, SOS, Raf1 and Src family tyrosine kinases. Thus, the enrichment of receptors and signal molecules in lipid rafts/caveolae enables them to be in close contact with each other and, hence, to facilitate their interaction.

The enrichment of certain GPCRs and G proteins in lipid rafts/caveolae tend to limit the utility of analyzing the total cellular expression (and stoichiometry) of such proteins.

Instead, our conceptual models should take account of:

• The potential compartmentation of molecules in vivo. This may be necessary to provide rapid, efficient and specific propagation of extracellular stimuli to intracellular targets. As an example (Figure 164): Pradrenergic receptors are significantly enriched in caveolae in cardiac myocytes while PGE2 receptors are excluded. This explains why overexpression of the type 6 adenyl cyclase enzyme (AC6), which is almost exclusively expressed in caveolae, enhances the maximal cAMP production to P1-receptor activation, but not to PGE2 receptor activation (not shown).

• The potential movement (or translocation) of receptors between cellular compartments (Table 20). In this respect, certain GPCRs reportedly translocate out of (e.g. cardiac P2-adrenergic receptors) or into caveolae (e.g. bradykinin Bj receptors) upon activation by an agonist.

In the first example, P2-adrenergic receptors are enriched in caveolae of myo-cytes but, upon stimulation, they translocate out of these structures (Figure 165). This may be attributed to receptor desensitization by GRK2. Indeed, when this desensitization is blunted (with PARKct, the C-terminal peptide of GRK2, which blocks activation of endogenous GRK2 by sequestering P-y), they no longer translocate out of caveolae upon agonist exposure.

log [isoproterenol], M

Figure 164 Top: Immunoblot after SDS-PAGE of caveolin-enriched and depleted cardiac myocyte membranes. Bottom: Effect of isoproterenol on cAMP production in membranes of control and in AC6-overexpressed cells (Ostrom et al., 2001, reproduced by permission of the American Society for Biochemistry and Molecular Biology).

log [isoproterenol], M

Figure 164 Top: Immunoblot after SDS-PAGE of caveolin-enriched and depleted cardiac myocyte membranes. Bottom: Effect of isoproterenol on cAMP production in membranes of control and in AC6-overexpressed cells (Ostrom et al., 2001, reproduced by permission of the American Society for Biochemistry and Molecular Biology).

Table 20 Movement of receptors between compartments (Pike, 2003, reproduce by permission of the American Society for Biochemistry).

Moves into

Moves out of

Unaffected by

Receptor

Rafts

Rafts

Agonist

Tyrosine kinases

EGF

X

ErbB2

X

Insulin

X

X

NGF

X

PDGF

X

G protein-coupled

Adenosine Al

X

Angiotensin II type 1

X

ß2-Adrenergic

X

ß1-Adrenergic

X

M2 Muscarinic cholinergic

X

Bradykinin 1,2

X

Endothelin

X

Rhodopsin

X

Figure 165 Immunoblot after SDS-PAGE of caveolin-enriched cardiac myocyte membranes. Effect of agonist (isoprotenorol, ISO) exposure on the presence of Pj- and p2-adrenergic receptors in caveolae with (control cells) or without (cells transfected with pARKct) endogenous GRK2 activity (Ostrom et al., 2001, reproduced by permission of the American Society for Biochemistry and Molecular Biology).

Figure 165 Immunoblot after SDS-PAGE of caveolin-enriched cardiac myocyte membranes. Effect of agonist (isoprotenorol, ISO) exposure on the presence of Pj- and p2-adrenergic receptors in caveolae with (control cells) or without (cells transfected with pARKct) endogenous GRK2 activity (Ostrom et al., 2001, reproduced by permission of the American Society for Biochemistry and Molecular Biology).

In the second example (Ostrom, 2002) (Figure 166), bradykinin receptors are predominantly localized in non-caveolar domains in unstimulated cells (A). Upon agonist exposure they generate a response (increase in [Ca2+]j and stimulation of intracellular phospholipase A2) via Gaq and PLC-P and translocate to caveolin-rich domains (B). Since PLC-P and its substrate (PIP2)

Bradykinin Receptor Cyclase

Figure 166 Schematic diagram illustrating the localization and translocation of bradykinin B1 receptors and the potential effect on their signalling. Reproduced from Ostrom, R. S. (2002) Molecular Pharmacology, 61, 473-476, with permission from the American Society for Pharmacology and Experimental Therapeutics.

Figure 166 Schematic diagram illustrating the localization and translocation of bradykinin B1 receptors and the potential effect on their signalling. Reproduced from Ostrom, R. S. (2002) Molecular Pharmacology, 61, 473-476, with permission from the American Society for Pharmacology and Experimental Therapeutics.

are enriched in caveolae, the response is facilitated. The increase in [Ca2+]j is only transient but, as the receptors do not internalize, they become able to activate ERK/MAP kinase signalling (again resulting in the stimulation of intracellular phospholipase A2 enzymes) (C). This signalling is again facilitated by the fact that caveolae are enriched in upstream components of the ERK/ MAP kinase pathway.

The overall picture is that lipid rafts/caveolae may exert both positive and negative control on signal transduction:

• In their positive role, receptors, coupling factors, effector enzymes and substrates would be co-localized in (or recruited to) single lipid rafts/caveolae. Signal transduction would occur rapidly and efficiently because of the spatial proximity of the interacting components. In this respect, it is of interest that cholesterol depletion generally impairs G protein-mediated signalling, suggesting that these signalling events require intact lipid rafts/caveolae and proceed within these membrane domains. Hence, it is believed that the spatial proximity of signalling components might be the rule rather than the exception for GPCR-mediated signalling as well as for the cross talk between GPCRs and other signalling systems, such as the ERK/MAP kinase system.

• In their negative role, rafts may spatially segregate interacting components to block non-specific pathways of activation. They may also directly suppress the activity of signalling proteins present in rafts (and explain why cholesterol depletion may increase signalling in a limited number of cases), favour the exit of activated receptors or even favour their desensitization and internalization. In this respect, it has been anticipated that caveolae are sites of endocytosis. This is due to their similarity in appearance to clathrin coated pits as they pinch off the plasma membrane. Various stimuli can lead to internalization of caveolae but, under normal conditions, they represent a largely immobile plasma membrane compartment not involved in constitutive endocytosis. It should be emphasized that caveola-mediated endocytosis is distinct from that of coated pits. Coated pit inhibitors do not affect caveolae internalization whereas the cholesterol-binding agent fillipin inhibits caveolae internalization without affecting coated pits. In the example (shown in Figure 167), fillipin was unable to inhibit the agonist-mediated internalization of AT1A receptors. This suggests that the AT1A receptor internalization process predominantly occurs via clathrin-coated vesicles. Additional evidence for such mechanism is produced by:

■ The requirement of P-arrestins as well as dynamin.

■ The inhibition by sucrose (which inhibits clathrin-coated vesicle-mediated endocytosis).

Figure 167 Treatment of AT1A receptor-expressing CHO cells with filipin does not affect Ang II-mediated endocytosis of the receptor (i.e. acid resistant [125I]-Ang II binding). Reproduced from Gaborik, Z., Szaszak, M., Szidonya, L., Balla, B., Paku, S., Catt, K. J., Clark, A. J. L. and Hunyady, L. (2001) Molecular Pharmacology, 59, 239-247, with permission from the American Society for Pharmacology and Experimental Therapeutics.

Figure 167 Treatment of AT1A receptor-expressing CHO cells with filipin does not affect Ang II-mediated endocytosis of the receptor (i.e. acid resistant [125I]-Ang II binding). Reproduced from Gaborik, Z., Szaszak, M., Szidonya, L., Balla, B., Paku, S., Catt, K. J., Clark, A. J. L. and Hunyady, L. (2001) Molecular Pharmacology, 59, 239-247, with permission from the American Society for Pharmacology and Experimental Therapeutics.

Restricted GPCR-G protein coupling: effector activity

The ternary complex model by De Lean et al. (1980) is a shuttling model. The ligand-activated receptor activates G proteins, which freely diffuse to the effector enzymes allowing more G proteins to be activated. This should result in 'unlimited' G protein activation. When the numbers of receptors and effector proteins are similar, the shuttling model predicts that, upon increasing the amount of G proteins present, the maximal response (e.g. cAMP production) should increase first (Figure 168). When the amount of G proteins is increased further, it will produce a leftward shift of the agonist concentration response curves.

The two next models both comprise restricted mobility of GPCRs and G proteins:

• In the complexing model, the activated G protein remains bound to the receptor during its interaction with the effector. This blocks the interaction of the receptor with more G proteins (Figure 169). When the numbers of receptors and effector proteins are similar, the G protein activation (and therefore even the activity of the effector) is limited by the amount of receptors available, and no marked leftward shift of the response curve is observed. However, shifts may be observed if receptor levels are slightly increased.

• In the pre-coupled mode, the G protein is bound to the receptor even in the absence of the ligand and remains bound to the receptor during its interaction with the effector enzyme. This also blocks the interaction of the receptor with more

Adrenergic Receptors

Figure 168 Top: Schematic representation of the classical 'shuttling model'. Bottom: Simulated agonist concentration versus (left) G protein activation and (right) cAMP production curves for different (indicated) G protein:receptor ratios. Reprinted from Trends in Pharmacological Sciences, 22, Kukkonen, J. P., Nasman, J. and Akerman, K. E., Modelling of promiscuous receptor-Gi/Gs-protein coupling and effector response, 616-622, © (2001), with permission from Elsevier.

Figure 168 Top: Schematic representation of the classical 'shuttling model'. Bottom: Simulated agonist concentration versus (left) G protein activation and (right) cAMP production curves for different (indicated) G protein:receptor ratios. Reprinted from Trends in Pharmacological Sciences, 22, Kukkonen, J. P., Nasman, J. and Akerman, K. E., Modelling of promiscuous receptor-Gi/Gs-protein coupling and effector response, 616-622, © (2001), with permission from Elsevier.

Figure 169 Top: Schematic representation of the 'complexing model'. Bottom: Simulated agonist concentration versus (left) G protein activation and (right) cAMP production curves for different (indicated) G protein/receptor ratios. Reprinted from Trends in Pharmacological Sciences, 22, Kukkonen, J. P., Nasman, J. and Akerman, K. E., Modelling of promiscuous receptor-Gi/Gs-protein coupling and effector response, 616-622, © (2001), with permission from Elsevier.

Figure 169 Top: Schematic representation of the 'complexing model'. Bottom: Simulated agonist concentration versus (left) G protein activation and (right) cAMP production curves for different (indicated) G protein/receptor ratios. Reprinted from Trends in Pharmacological Sciences, 22, Kukkonen, J. P., Nasman, J. and Akerman, K. E., Modelling of promiscuous receptor-Gi/Gs-protein coupling and effector response, 616-622, © (2001), with permission from Elsevier.

(c) Precoupled mode!

(c) Precoupled mode!

Figure 170 Schematic representation of the 'pre-coupled model'. Simulated agonist concentration versus G protein activation and cAMP production curves are similar to those for the 'com-plexing model'. Reprinted from Trends in Pharmacological Sciences, 22, Kukkonen, J. P., Nasman, J. and Akerman, K. E., Modelling of promiscuous receptor-Gi/Gs-protein coupling and effector response, 616-622, © (2001), with permission from Elsevier.

Figure 170 Schematic representation of the 'pre-coupled model'. Simulated agonist concentration versus G protein activation and cAMP production curves are similar to those for the 'com-plexing model'. Reprinted from Trends in Pharmacological Sciences, 22, Kukkonen, J. P., Nasman, J. and Akerman, K. E., Modelling of promiscuous receptor-Gi/Gs-protein coupling and effector response, 616-622, © (2001), with permission from Elsevier.

G proteins (Figure 170). The simulated data according to this model are very similar to those obtained for the complexing model.

Was this article helpful?

0 0

Post a comment