LUTZ BIRNBAUMER*, JUAN CODINA*, RAFAEL MATTERA*, RICHARD A. CERIONE†, JOHN D. HILDEBRANDT*1, TERESA SUNYER*, FRANCISCO J. ROJAS*2, MARC G. CARON†, ROBERT J. LEFKOWITZ† and RAVI IYENGAR*,     *Department of Cell Biology, Baylor College of Medicine, Houston, Texas; †Howard Hughes Medical Institute, Duke University Medical Center, Durham, North Carolina

Publisher Summary

Receptors that affect cyclic adenosine monophosphate (cAMP) are sub-classified into two subtypes: Rs receptors, which increase cAMP levels by stimulating the enzyme adenylyl cyclase, and Ri receptors, which decrease cAMP levels by inhibiting the cAMP-forming enzyme. This chapter discusses the transduction mechanism to which Rs- and Ri-type receptors couple to modulate adenylyl cyclase activity. At the center of this transduction mechanism are two oligomeric coupling proteins called N or G proteins. These proteins have properties to bind and hydrolyze guanosine triphosphate and regulate hormone affinity for receptors and the catalytic activity of the cAMP-forming enzyme. This complex receptor-coupling protein-adenylyl cyclase system is approached by first reviewing structural and functional aspects that regulate cAMP formation. The chapter also discusses the basic structure and regulation of adenylyl cyclase by nucleotides and magnesium. It also discusses action of hormones on the nucleotide-regulated system. It analyzes the known regulation of hormone-receptor interaction by the coupling proteins. The analysis of affinity regulation of receptors leads to conclusions that point toward the existence of at least two conformational states of receptors interacting with at least three conformational states or forms of the coupling proteins.

I Introduction

Cellular functions are regulated by hormones. The capacity of a given cell to respond to a given hormone depends on the presence or absence in the cell of a receptive recognition system specific for the hormone, called receptor. It is now well recognized that there are several types of hormone receptors, each triggering a different type of response when occupied by its specific hormone. Receptor types can be subclassified, first on the basis of their location, and second on the basis of the signal transduction mechanism system they use. On the basis of their location receptors are either intracellular or of plasma membrane localization. The former encompass receptors for steroid hormones, vitamin D, and thyroid hormone, the latter encompass receptors for neurotransmitters, peptide and protein hormones, and a variety of regulatory and growth-promoting factors. On the basis of the signal transduction mechanism they use, membrane receptors can be classified into four different subtypes: (1) receptors that regulate cyclic AMP formation; (2) receptors that affect intracellular calcium levels via hydrolysis of phosphatidylinositol phosphates and formation of diacylglycerol and inositol phosphates; (3) receptors that possess tyrosine kinase activity and are assumed to act on cellular metabolism on the basis of this property; and (4) receptors that are ion channels and, on occupancy by their specific ligands, allow flux of specific ions (cations or anions) through the plasma membrane and thereby trigger electrophysiological responses. Examples of the last class of receptors are nicotinic acetylcholine receptors, a cation channel, and receptors of GABA and glutamine which are chloride channels. Examples of tyrosine kinase receptors are insulin, platelet-derived growth factor, and epidermal growth factor receptors. Examples of receptors that affect phosphoinositide hydrolysis are receptors for vasopressin of the VP1-type for TRH, and for catecholamines of the α1-type. Finally, there are a multitude of examples of receptors that affect cyclic AMP formation. In fact, receptors that affect cyclic AMP are subclassified into two subtypes: R receptors, which increase cyclic AMP levels by stimulating the enzyme adenylyl cyclase, and Ri receptors, which decrease cyclic AMP levels by inhibiting the cyclic AMP-forming enzyme. Receptors for catecholamine of the β1- and β2-type, for ACTH, for glucagon, for secretin, for LH, FSH, and TSH, are of the Rs-type. Receptors for catecholamines of the α2-type, for acetylcholine of muscarinic type (M), for opiod peptides, and somatostatin are of the Ri-type. A given cell can have any number of the above receptor types, indeed it can, and often does, have more than one of a given type, such as occurs, for example, with rat fat cells which have at least five different Rs-type receptors (ACTH, β-adrenergic, glucagon, secretin, and LH), two different Ritype receptors (adenosine and PGE), and in addition have receptors for insulin (a tyrosine kinase). Although not yet specifically explored, it is likely that these same cells have receptors of the other classes as well. The receptor complement differs, of course, from cell type to cell type, and from tissue to tissue, and constitutes a complex address system that allows for coordinated but varied regulation of cellular responses and organ homeostasis.

The purpose of this chapter is to discuss in detail the transduction mechanism to which Rs- and Ri-type receptors couple to modulate adenylyl cyclase activity. At the center of this transduction mechanism are two oligomeric coupling proteins called N (or G) proteins. These proteins have the properties of binding as well as hydrolyzing GTP and of regulating hormone affinity for receptors as well as the catalytic activity of the cyclic AMP-forming enzyme. We will approach this complex receptor-coupling protein–adenylyl cyclase system by first reviewing structural and functional aspects that regulate cyclic AMP formation. As part of this, we will discuss the basic structure and regulation of adenylyl cyclase by nucleotides and magnesium, we will speculate on several aspects of the regulation of the activity of the signal transducing proteins. We will then review the actions of hormones on the nucleotide-regulated system. Finally, we will analyze what is known about the regulation of the hormone–receptor interaction by the coupling proteins. This last analysis, i.e., affinity regulation of receptors, leads to some unexpected conclusions that point toward the existence of at least two conformational states of receptors interacting with at least three conformational states or forms of the coupling proteins.

II The Adenylyl Cyclase System: Structure and Regulation in the Absence of Hormonal Influence