Magnesium regulation of the beta-receptor-adenylate cyclase complex. I. Effects of manganese on receptor binding and cyclase activation
Cech, S.Y.; Maguire, M.E.
Molecular Pharmacology 22(2): 267-273
1982
ISSN/ISBN: 0026-895X PMID: 6292688 Document Number: 196857
The ability of Mn2+ to interact with specific sites for free Mg2+ on the .beta.-adrenergic receptor-adenylate cyclase complex was investigated in murine S49 lymphoma cell variants possessing specific defects in receptor-cyclase coupling. Free Mg2+ activates adenylate cyclase in the S49 unc variant in a manner identical with the activation seen in wild-type S49 cells. Unc is defective in coupling of .beta.-adrenergic receptors to the adenylate cyclase catalytic complex, and Mg2+ cannot alter agonist affinity for the .beta.-receptor. Mg2+ is able to alter agonist affinity for .beta.-receptors of the S49 variant H21a hepatoma line. These cell lines have functional receptor and coupling protein, but the cyclase catalytic unit is either uncoupled or inactive, respectively. In wild-type S49 cell membranes, free Mn2+ increases the Vmax of the fluoride-stimulated adenylate cyclase activity but decreases the Vmax of hormone/GTP-stimulated activity. Free Mn2+ also increases the Vmax in the cyc- S49 variant which lacks functional coupling protein but has a functional cyclase catalytic unit. Free Mn2+ has no effect on the Km for the substrate MnATP2- in either wild-type or cyc- S49 cell membranes. Free Mn2+ blocks the effect of Mg2+ to increase agonist affinity for .beta.-adrenergic receptors but has no effect by itself. The presence of 2 independent metal-binding sites on the receptor-cyclase complex is indicated, 1 site on the coupling protein(s) mediating metal ion effects on agonist affinity for the receptor and a 2nd site on the cyclase catalytic subunit mediating metal ion activation of cyclase activity.