G Protein-Coupled Receptor Kinase: Phosphorylation of Muscarinic Receptors and Facilitation of Receptor Sequestration

Abstract

Agonist-bound receptors interact with G proteins composed of an αβγ -trimer and facilitate the exchange of GDP and GTP, thereby causing dissociation of α GDP βγ into α GTP - and βγ -subunits. Dissociated α GTP -subunits interact with and activate or inhibit effectors such adenylyl cyclase, phospholipase CB, or ion channels. The phosphorylation is considered to cause desensitization of the reaction induced by agonist-bound receptors. Three different processes of desensitization are known-uncoupling of receptors from G proteins, sequestration (or internalization) of receptors from the cell surface, and decrease in the numbers of receptors. Kinases that phosphorylate G protein-coupled receptors in an agonist-dependent manner are known as G protein-coupled receptor kinases (GRKs). At present, six different kinds of GRKs have been cloned. This chapter describes phosphorylation of muscarinic acetylcholine receptor m2 subtypes (m2 receptors) by GRKs belonging to the GRK2/GRK3 subgroup. The chapter focuses on synergistic activation of GRK2 by G protein βγ -subunits and agonist-bound receptors and on facilitation of m2 receptor sequestration that results from phosphorylation. The partially purified kinase, which will be termed mAChR kinase, was found to phosphorylate muscarinic receptors purified from porcine brain. The atrial m2 receptors were better substrates than brain receptors and have been used in most of the following experiments. The mAChR kinase showed essentially the same enzymatic properties as those reported for β-adrenergic receptor kinases, which include inhibition by heparin or salts and independence of second messengers or Ca 2+ ion.