Involvement of tyrosine residues located in the carboxyl tail of the human beta 2-adrenergic receptor in agonist-induced down-regulation of the receptor.

Abstract
Chronic exposure of various cell types to adrenergic agonists leads to a decrease in cell surface .beta.2-adrenergic receptor (.beta.2AR) number. Sequestration of the receptor away from the cell surface as well as a down-regulation of the total number of cellular receptors are believed to contribute to this agonist-mediated regulation of receptor number. However, the molecular mechanisms underlying these phenomena are not well characterized. Recently, tyrosine residues located in the cytoplasmic tails of several membrane receptors, such s the low density lipoprotein and mannose-6-phosphate receptors, have been suggested as playing an important role in the agonist-induced internalization of these receptors. Accordingly, we assessed the potential role of two tyrosine residues in the carboxyl tail of the human .beta.2AR in agonist-induced sequestration and down-regulation of the receptor. Tyr-350 and Tyr-354 of the human .beta.2AR were replaced with alanine residues by site-directed mutagenesis and both wild-type and mutant .beta.2AR were stably expressed in transformed Chinese hamster fibroblasts. The mutation dramatically decreased the ability of the .beta.2AR to undergo isoproterenol-induced down-regulation. However, the substitution of Tyr-350 and Tyr-354 did not affect agonist-induced sequestration of the receptor. These results suggest that tyrosine residues in the cytoplasmic tail of human .beta.2AR are crucial determinants involved in its down-regulation.