title: G-protein-mediated interconversions of cell-surface cAMP receptors and their involvement in excitation and desensitization of guanylate cyclase in Dictyostelium discoideum.
abstract: In Dictyostelium discoideum cells, extracellular cAMP induces the rapid (within 2 s) activation of guanylate cyclase, which is followed by complete desensitization after about 10 s. cAMP binding to these cells is heterogeneous, showing a subclass of fast dissociating sites coupled to adenylate cyclase (A-sites) and a subclass of slowly dissociating sites coupled to guanylate cyclase (B-sites). The kinetics of the B-sites were further investigated on a seconds time scale. Statistical analysis of the association of [3H]cAMP to the B-sites and dissociation of the complex revealed that the receptor can exist in three states which interconvert according to the following scheme. (formula; see text). cAMP binds to the BF-state (off-rate 2.5 s) which rapidly (t1/2 = 3 s) converts to the BS-state (off-rate 15 s) and subsequently (without a detectable delay) into the BSS-state (off-rate 150 s). In membranes, both the BS- and BSS-states are converted to the BF-state by GTP and GDP, suggesting the involvement of a G-protein. Densensitized cells show a 80% reduction of the formation of the BSS-state, but no reduction of the BF- or BS-state. These data are combined into a model in which the transitions of the B-sites are mediated by a G-protein; activation of the G-protein and guanylate cyclase is associated with the transition of the BS- to the BSS-state of the receptor, whereas desensitization is associated with the inhibition of this transition.
authors.forename: P J
authors.initials: PJ
authors.lastname: van Haastert
authors.forename: R J
authors.initials: RJ
authors.lastname: de Wit
authors.forename: P M
authors.initials: PM
authors.lastname: Janssens
authors.forename: F
authors.initials: F
authors.lastname: Kesbeke
authors.forename: J
authors.initials: J
authors.lastname: DeGoede
mesh_terms: D002462:Cell Membrane / Q000378:metabolism; D000242:Cyclic AMP / Q000378:metabolism; D004023:Dictyostelium / Q000201:enzymology / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006153:Guanosine Diphosphate / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000494:pharmacology; D006162:Guanylate Cyclase / Q000378:metabolism*; D007700:Kinetics; D008433:Mathematics; D011953:Receptors, Cyclic AMP / Q000378:metabolism*; D014316:Tritium
chemical_list: D011953:Receptors, Cyclic AMP; D014316:Tritium; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D006162:Guanylate Cyclase