Transformation of Chick-Embryo Fibroblasts by Wild-Type and Temperature-Sensitive Rous Sarcoma Virus Alters Adenylate Cyclase Activity

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The activity of the enzyme adenylate cyclase, a component of the plasma membrane, has been determined in chick-embryo fibroblasts and in cells transformed by either Bryan high-titer strain of Rous sarcoma virus (RSV-BH) or a temperature-sensitive mutant of this virus (RSV-BH-Ta). Adenylate cyclase activity is reduced in cells transformed by the wild-type virus and also by the temperature-sensitive mutant when the cells are grown at the permissive temperature (37°). Transformation results in an altered affinity (Km) for the substrate (Mg ATP). The apparent Km ATP is 0.23 mM in normal cells and 1.1 mM in cells transformed with wild-type virus. The Km ATP of the cells infected with RSV-BH-Ta is 0.67-1.0 mM at 37° and 0.28 mM at 42°. The enzyme from normal cells appears to have two binding sites for Mg++, one at the catalytic site and a second at a regulatory site. Transformation by RSV-BH or RSV-BH-Ta (37°) apparently alters this second Mg++ site. A decrease in adenylate cyclase activity occurs within 10 min after cells infected with RSV-BH-Ta are shifted from 42° to 37°; the activity falls to one-half that of normal cells 30 min after the temperature shift. Our observations indicate that a viral function lowers cyclic AMP content by lowering the activity of adenylate cyclase, probably through some modification of the plasma membrane.

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