Review
G protein antagonists

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Abstract

Heterotrimeric G proteins couple membrane-bound heptahelical receptors to their cellular effector systems (ion channels or enzymes generating a second messenger). In current pharmacotherapy, the input to G protein-regulated signalling is typically manipulated by targeting the receptor with appropriate agonists or antagonists and, to a lesser extent, by altering second messenger levels, most notably by inhibiting phosphodiesterases that hydrolyse cyclic nucleotides. When stimulated, G proteins undergo a cycle of activation and deactivation in which the α-subunits and the βγ-dimers sequentially expose binding sites for their reaction partners (receptors, guanine nucleotides and effectors, as well as regulatory proteins). These domains can be blocked by inhibitors and this produces effects that cannot be achieved by receptor antagonists. Here, the structural and mechanistic information on G protein antagonists is summarized and an outline of the arguments supporting the hypothesis that G proteins per se are also potential drug targets is provided.

Section snippets

Signalling mechanism and potential target sites for drug action

The cycle of G protein activation can be broken down into a four-step reaction1 (Fig. 1):

(1) The basal state (Fig. 1a), in which the G protein is an αβγ-heterotrimer with GDP bound to the α-subunit. In the absence of activation by a receptor, the rate of GDP release (koff ⩽;0.1 min−1) is much lower than the rate of GTP hydrolysis (kcat ⩾3 min−1); this kinetic feature clamps the system in the ‘off' position.

(2) Receptor-mediated GDP-release (Fig. 1b): The agonist-liganded, activated receptor

Targeting the guanine-nucleotide-binding pocket with modified guanine nucleotides

Guanine nucleotides bind with very high affinity to the G-protein α-subunit in a cleft between the RAS-like and the helical domain of Gα. In theory, this tight binding lends itself to being blocked by modified guanine nucleotides, although certain substitutions on the guanine ring are tolerated without loss in the ability of these nucleotides to combine with the binding site12. In addition, oGTP, the 2′,3′-dialdehyde analogue of GTP, is an effective G protein antagonist (1a in Fig. 1); the

Insect venoms

The wasp venom mastoparan, a peptide of 14 amino acids, was the first rigorously characterized direct G protein activator20, 21. Mastoparan and mastoparan-X, a related and more potent peptide22, activate preferentially Gi and Go. If Ala10 in mastoparan is replaced by α-aminoisobutyric acid, the resulting peptide (mastoparan-S) selectively activates Gsα (Ref. 23); mellitin efficiently stimulates Gα11 and Giα−1 but inhibits the spontaneous guanine nucleotide exchange of Gsα (Ref. 24). It has long

Suramin as a lead in the development of nonpeptide G protein antagonists

Suramin, a symmetric polysulphonated naphtylamine-benzamide-derivative, has been used for more than 70 years in the treatment of African sleeping disease and river blindness. Apart from being effective against various Trypanosoma species and the adult filariae of Onchocerca volvulus, suramin exerts mostly inhibitory effects on an array of mammalian targets such as P2 receptors, DNA-polymerases and growth-factor receptors38. Among the targets are G protein α-subunits; initially suramin was

Prenylated compounds target the interface between receptors and G protein βγ-dimers

As already mentioned, receptors require Gβγ-dimers to promote GDP release from the α-subunits and it is likely that the receptor contacts the βγ-dimer directly. Accordingly, a peptide from the third intracellular loop of the α2-adrenoceptor can be cross-linked to the C-terminus of the β-subunit45. In addition, the lipid-modified γ-carboxy terminus contributes to the receptor–G protein interface; this conclusion is based on the observation that an isoprenylated peptide comprising the last 12

Direct G protein antagonism – a useful concept?

From the perspective of experimental pharmacology, it is beneficial to have compounds that can be used as appropriate research tools; G protein antagonists are likely to be of use in this respect. The very fact that a potential target site exists does not necessarily justify a search for inhibitors; clearly, it is more gratifying if a pharmacodynamic principle is ultimately relevant to pharmacotherapy. It is not obvious why one would like to target a G protein with an inhibitor rather than

Acknowledgements

E.B-C. is supported by the EU-sponsored ENBST-research network. Work from the authors' laboratory is supported by grants from the Austrian Science Foundation (12750 to MF and 12125 to CN).

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