Goldman P S, Nathanson N M
Department of Pharmacology, University of Washington, Seattle 98195.
J Biol Chem. 1994 Jun 3;269(22):15640-5.
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.
毒蕈碱型乙酰胆碱受体（mAChR）的数量可因持续暴露于激动剂而发生改变。短期激动剂暴露（数秒至数分钟）会导致mAChR从细胞表面快速移除（隔离），而长时间（数小时）的激动剂暴露则会导致总受体数量减少（下调）。已证明，许多膜受体胞质尾中的酪氨酸残基在通过隔离进行的调节中很重要，如甘露糖6 - 磷酸受体和其他通过网格蛋白包被小泡内吞的受体；在通过下调进行的调节中也很重要，如β2 - 肾上腺素能受体。将哺乳动物m2 mAChR唯一的胞质尾酪氨酸残基（Tyr - 459）突变为苯丙氨酸、色氨酸或丙氨酸，虽然显著减弱了激动剂诱导的下调，但不影响激动剂诱导的隔离。将m2 Tyr - 459转换为异亮氨酸不影响激动剂诱导的隔离或下调的速率或程度，但该突变受体对激动剂诱导的下调的敏感性略有降低。激动剂和拮抗剂结合以及与抑制cAMP积累的功能偶联不受Tyr - 459任何突变的影响。这些结果首次确定了mAChR中一个与激动剂诱导的受体下调有关的位点。
