Photoaffinity labeling of the beta-adrenergic receptor from cultured lymphoma cells with [125I]iodoazidobenzylpindolol: loss of the label with desensitization.

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Abstract
The wild-type S49 lymphoma cell line (WT) and its coupling protein-deficient variant line (cyc-) have proven to be an invaluable model system for the study of the mechanism of hormonal regulation and desensitization of adenylate cyclase. The .beta.-adrenergic antagonist [125I]iodoazidobenzylpindolol ([125I]IABP) specifically photolabeled 2 polypeptides in membrane preparations from wild-type (WT) and coupling protein-deficient cyc- cultured S49 lymphoma cells. The MW of the 2 polypeptides determined by sodium dodecyl sulfate/polyacrylamide gel electrophoresis were 65,000 and 55,000. They were labeled in a ratio of .apprx. 1:1. Pretreatment of intact WT or cyc- cells with 1.0 .mu.M epinephrine for 15 min (desensitization) resulted in a greater loss of the 55,000 MWr polypeptide (40-60%) relative to the 65,000 MWr peptide (10-30% loss). An 18-24 h pretreatment of WT cells with terbutaline (down-regulation) led to a > 90% reduction of the photolabeling of both polypeptides, whereas a similar pretreatment of cyc- cells resulted in no further loss of labeled receptor than that observed after only a 15 min pretreatment with epinephrine. There was no indication of a change in the electrophoretic mobility of the [125I]IABP-labeled receptors after either short- or long-term agonist pretreatment. These data provide direct evidence for heterogeneity of the .beta.-adrenergic receptor in lymphoma cells. The differential loss of the [125I]IABP labeling in the 2 polypeptides suggests a functional heterogeneity as well.