title: Heterologous desensitization mediated by G protein-specific binding to caveolin.
abstract: We examined the notion that sequestration of G protein subunits by binding to caveolin impedes G protein reassociation and leads to transient, G protein-specific desensitization of response in dispersed smooth muscle cells. Cholecystokinin octapeptide (CCK-8) and substance P (SP) were used to activate G(q/11), cyclopentyl adenosine (CPA) was used to activate G(i3), and acetylcholine (ACh) was used to activate both G(q/11) and G(i3) via m3 and m2 receptors, respectively. CCK-8 and SP increased only Galpha(q/11), and CPA increased only Galpha(i3) in caveolin immunoprecipitates; caveolin and other G proteins were not increased. ACh increased both Galpha(q/11) and Galpha(i3) in a time- and concentration-dependent fashion: only Galpha(q/11) was increased in the presence of an m2 antagonist, and only Galpha(i3) was increased in the presence of an m3 antagonist. To determine whether transient G protein binding to caveolin affected subsequent responses mediated by the same G protein, PLC-beta activity was measured in cells stimulated sequentially with two different agonists that activate either the same or a different G protein. After treatment of the cells with ACh and an m2 antagonist, the phospholipase C-beta (PLC-beta) response to CCK-8 and SP, but not CPA, was decreased; conversely, after treatment of the cells with ACh and an m3 antagonist, the PLC-beta response to CPA, but not CCK-8 or SP, was decreased. Similarly, after treatment with CCK-8 or SP, the PLC-beta response mediated by G(q/11) only was decreased, whereas after treatment with CPA, the PLC-beta response mediated by G(i3) only was decreased. A caveolin-binding Galpha(q/11) fragment blocked the binding of activated Galpha(q/11) but not Galpha(i3) to caveolin-3 and prevented desensitization of the PLC-beta response mediated only by other G(q/11)-coupled receptors. A caveolin-binding Galpha(i3) fragment had the reverse effect. Thus, transient binding of receptor-activated G protein subunits to caveolin impedes reassociation of the heterotrimeric species and leads to desensitization of response mediated by other receptors coupled to the same G protein.
authors.affiliation: Departments of Physiology and Medicine, Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia 23298-0711, USA. skarnam@hsc.vcu.edu
authors.forename: K S
authors.initials: KS
authors.lastname: Murthy
authors.forename: G M
authors.initials: GM
authors.lastname: Makhlouf
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology; D000241:Adenosine / Q000031:analogs & derivatives; D000818:Animals; D051242:Caveolin 1; D022461:Caveolins / Q000302:isolation & purification / Q000378:metabolism*; D002451:Cell Compartmentation; D020962:Heterotrimeric GTP-Binding Proteins / Q000378:metabolism*; D007422:Intestines / Q000166:cytology; D009130:Muscle, Smooth / Q000166:cytology / Q000502:physiology*; D011485:Protein Binding; D011817:Rabbits; D011956:Receptors, Cell Surface / Q000378:metabolism; D011976:Receptors, Muscarinic / Q000378:metabolism; D015398:Signal Transduction; D012844:Sincalide / Q000494:pharmacology; D013347:Subcellular Fractions / Q000737:chemistry; D013373:Substance P / Q000494:pharmacology
chemical_list: D051242:Caveolin 1; D022461:Caveolins; D011956:Receptors, Cell Surface; D011976:Receptors, Muscarinic; D013373:Substance P; D020962:Heterotrimeric GTP-Binding Proteins; D000241:Adenosine; D012844:Sincalide; D000109:Acetylcholine