Role of glycosylation for beta 2-adrenoceptor function in A431 cells.
A431 cells incubated with tunicamycin (0.15 micrograms/ml) for 40 h under conditions where incorporation of [3H] leucine into protein was inhibited less than 10% expressed mainly a beta-receptor species of about Mr 40,000 which was ascribed to the nonglycosylated form of the beta-receptor of about Mr 75,000 found in normal A431 cells by photoaffinity labeling. However, the tunicamycin-treated cells expressed the same number of specific beta 2-receptor-binding sites as untreated cells. Moreover, the aglycoreceptors had the same ligand binding properties as beta-adrenoceptors from control cells; but, functional tests of the receptor from tunicamycin-treated cells in reconstituted lipid vesicles showed that receptors from tunicamycin-treated cells had lost coupling efficiency. The coupling defect was at the receptor level since control experiments indicated that the other components of the signal transmission chain from beta-adrenoceptor to adenylate cyclase, the stimulatory regulatory GTP-binding protein of adenylate cyclase and adenylate cyclase, were fully functional. Homologous desensitization in tunicamycin-treated cells was characterized by export from the cell surface and sequestration of about the same number of beta-adrenoceptors as in normal desensitized cells but without further reduction of hormonally stimulated adenylate cyclase below the low level already attained in nondesensitized tunicamycin-treated cells. This was explained by assuming that the receptors removed in the course of homologous desensitization from the surface of tunicamycin-treated cells were already nonfunctional. Thus, beta-adrenergic desensitization in tunicamycin-treated cells is characterized by the functional disengagement of receptor removal and loss of adenylate cyclase activity.
Boege
F
F
Department of Physiological Chemistry, University of Würzburg Medical School, Federal Republic of Germany.
Ward
M
M
Jürss
R
R
Hekman
M
M
Helmreich
E J
EJ
D000262:Adenylyl Cyclases / Q000378:metabolism; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism; D006031:Glycosylation; D007700:Kinetics; D008562:Membrane Glycoproteins / Q000302:isolation & purification; D008970:Molecular Weight; D011943:Receptors, Adrenergic, beta / Q000096:biosynthesis / Q000378:metabolism*; D014415:Tunicamycin / Q000494:pharmacology
D008562:Membrane Glycoproteins; D011943:Receptors, Adrenergic, beta; D014415:Tunicamycin; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases