Synthesis, insertion into the plasma membrane, and turnover of alpha-bungarotoxin receptors in chick sympathetic neurons.

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Abstract
Alpha-Bungarotoxin was used to identify an integral membrane protein in the plasma membrane of chick sympathetic neurons. The synthesis, insertion into the plasma membrane, and turnover of the alpha-bungarotoxin receptor were studied using isotopically labeled amino acids (2H, 13C, 15N) to directly label receptor molecules. Neurons incubated in medium containing dense amino acids continued to insert unlabeled receptors from a pool of previously synthesized molecules for 2 h. Density-labeled receptors began to appear in the plasma membrane after this 2-h period. Synthesis of receptors, but not insertion into the surface, was blocked by cycloheximide (100 microgram/ml). Neither colchicine (0.05 microgram/ml) of actinomycin D (5 microgram/ml) has any effect on alpha-bungarotoxin receptor synthesis or insertion. Autoradiographic studied revealed that receptors occur on growth cones, axons, and cell bodies of single neurons and explanted ganglia. The rate of insertion of newly synthesized receptors into the plasma membrane of axons extending from explanted sympathetic ganglia was approximately the same as that into the cell body portion of the ganglion. Cytochalasin B (2 microgram/ml) rapidly distrupted growth cones but had no effect on receptor insertion. These experiments suggested that the growth cone is not the sole or even the primary site for insertion of this membrane protein. The kinetics of turnover of the alpha-bungarotoxin receptor were a first-order exponential with t 1/2 = 11 h. Neurons that had their surface receptors labeled with 125I-alpha-bungarotoxin produced [125I]iodotyrosine. This process was inhibited by low temperature (23 degrees C) and also by a metabolic inhibitor. This is interpreted as evidence that receptors turn over by a mechanism in which they are internalized and then proteolytically degraded.