Acetylcholine receptor: effects of proteolysis on receptor metabolism.

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Abstract
Sarcoma virus [Oncornavirus] transformation and tumor promoters reduce the cell surface concentration of acetylcholine receptors (AChR) in differentiating chick embryo myogenic cultures. Both of these agents also induce high rates of plasminogen activator (PA) synthesis in myogenic cultures. Proteolysis in chick embryo myogenic cultures was modulated by 1 or more of the following: stimulation of PA synthesis, direct addition of plasmin, removal of plasminogen, or addition of plasmin inhibitors. When the rates of proteolysis were raised either by addition of plasmin or by stimulating PA synthesis in the presence of plasminogen, both the steady-state concentration and the half-life [t1/2] of surface AChR decreased, but the rate of receptor synthesis was unaffected. The magnitude of these effects, and their dependence on added plasminogen, indicated that proteolysis initiated by plasminogen activation could account almost entirely for the reduction in receptor t1/2 produced by sarcoma virus transformation and phorbol ester. The rate of receptor synthesis, which is also reduced by viral transformation and tumor promoters, was not modified by proteolysis; hence plasmin action may be responsible for a large part, but not all of the change in surface receptor under these conditions. The plasmin-catalyzed changes in receptor parameters appear to occur in response to modified membrane metabolism resulting from proteolysis of surface components other than AChR itself.