Differential role of the carboxyl-terminal tyrosine in down-regulation and sequestration of the m2 muscarinic acetylcholine receptor.

Muscarinic acetylcholine receptor (mAChR) number can be altered in response to sustained agonist exposure. Short term agonist exposure (seconds to minutes) causes a rapid removal of mAChR from the cell surface (sequestration) while agonist exposure for longer periods of time (hours) causes a decrease in total receptor number (down-regulation). Tyrosine residues located in the cytoplasmic tails of a number of membrane receptors have been demonstrated to be important in the regulation by either sequestration, as is the case with the mannose 6-phosphate receptor and other receptors endocytosed via clathrin coated vesicles, or down-regulation, as is the case with the beta 2-adrenergic receptor. Mutation of the lone cytoplasmic tail tyrosine residue (Tyr-459) of the mammalian m2 mAChR to Phe, Trp, or Ala did not affect agonist-induced sequestration, although it significantly attenuated agonist-induced down-regulation. Conversion of m2 Tyr-459 to Ile did not affect the rate or extent of agonist-induced sequestration or down-regulation, but the sensitivity of this mutant receptor to agonist-induced down-regulation was slightly decreased. Agonist and antagonist binding as well as functional coupling to the inhibition of cAMP accumulation was unaffected by any of the mutations to Tyr-459. These results are the first to identify a site in a mAChR involved in the down-regulation of receptor in response to agonist.