Lecture Notes: Receptors

Further reading:
G protein-coupled receptors:
Helmreich, E and Hofmann, K-P (1996) Structure and function of proteins in G-protein-coupled signal transfer. Biochim Biophys Acta 1286, 285-322.
Tyrosine kinase receptors:
Moghal, N and Sternberg, P W (1999) Multiple positive and negative regulators of signalling by the EGF-receptor. Curr. Opin. Cell Biol. 11,
190-196. (Glasgow University library)

Extracellular signals are detected by receptors. Cellular responses to a particular signal are dependent on receptor specificity which determines

• Ligand binding
• Agonists
• Antagonists
• Effector activity

Receptor affinity is measured as Kd:

The actions of extracellular signalling molecules are mediated through two types of receptor protein, cell surface (integral membrane proteins) and intracellular receptor proteins.

1. lipophilic hormones (eg steroids, retinoids) -  intracellular receptors
2. lipophilic hormones (eg prostaglandins) - membrane receptors
3. water soluble compounds (range from small charged biogenic amines to large peptide hormones) - membrane receptors

• intramolecular interactions
• intermolecular interactions
• docking sites for ligands
• docking sites for adapter and G proteins
• docking sites for kinases/phosphatases
• docking sites for scaffolding proteins
• DNA binding domains
• domains involved in dimerisation/oligomerisation
• domains involved in targeting the molecule to particular subcellular compartments

1. ligand-gated ion channels (ionotropic receptors)
• mediate fast responses, affecting ion fluxes and membrane potential
2. G protein-coupled receptors
• mediate slow transmission and affect metabolic processes
3. receptors containing intrinsic enzyme activity
• guanylate cyclase (ANP receptors)
• tyrosine kinase (growth factor and insulin receptors)
4. receptors associated with tyrosine kinases
• cytokine receptors
• interferon receptors
• growth factor receptors

Lodish 4th ed., Fig. 20-3b

• multiple subunit proteins which form cation or anion channels
• channel properties are determined by the composition of subunit isoforms
• agonist affinity
• ion permeabilities
• conductance properties
• activation and deactivation times
• desensitisation kinetics
• activity can be regulated by varying composition of different subunits

Two main families

1. nicotinic receptor family (include nACh receptors, GABA_A, GABA_C, glycine receptors and the 5-HT₃ receptor)
2. glutamate receptor family (several different receptor types classified into NMDA receptors and non-NMDA receptors)

Heteropentameric structure of the nAChR: 2 a subunits + 1 b + 1 g + 1 d

Three dimensional structure

Lodish 4th ed., Fig. 21-38a

Glutamate receptor subunits differ in membrane topology, the second hydrophobic domain is thought to loop into the plasma membrane, but doesn't cross it:

The hydrophobic loop II is part of the lining of the ion channel.

G protein-coupled receptors (GPCRs)

• Largest family of membrane receptors (>1000 different receptors)
• Respond to a wide range of extracellular signals (peptide hormones, biogenic amines, lipophilic hormones, as well as sensory stimuli - light, odorants)
• Several distinct groups with similar sequence within group but very little similarity between GPCR groups
• Main GPCR groups made up by the adrenergic/rhodopsin-like receptors (group I), the secretin receptor-like receptors (group II) and the metabotropic glutamate and GABA receptors (group III)

• From mutational studies of different GPCRs, it has been determined that intracellular loop 3 (C3), along with other intracellular domains, is important for interactions with the G protein partner.
• The 3D structure of GPCRs is thought to be a barrel-like structure in the membrane, similar to the 7 transmembrane domain protein, bacteriorhodopsin, whose structure has been determined by X-ray crystallography.

Signal transduction through an analogous series of molecular events involving cycle of interactions with ligand and G protein partner

• Ligand binding induces conformational changes to receptor, exposing G protein activating domains
• Induces exchange of GDP for GTP and activation of heterotrimeric G protein
• Activated G protein dissociates, a and bg subunits regulate activity of various effectors
• Intrinsic GTPase activity in a subunit hydrolyses GTP, returning to inactive state and promotes reassociation with bg subunits

• acetylcholine activates 5 muscarinic receptor subtypes
• M1, M3 and M5 subtypes couple to Gq/11
• M2 and M4 subtypes couple to Gi/o
• serotonin (5-HT) activates 7 classes of serotonergic receptors
• Type I couple to Gi/o
• Type 2 couple to Gq/11
• Types 4, 6, and 7 couple to Gs
• epinephrine (adrenaline) and norepinephrine (noradrenaline) activate three classes of adrenergic receptors
• a₁ receptors couple to Gq/11
• a₂ receptors couple to Gi/o
• b receptors couple to Gs

• means of amplifying signal
• G proteins can act as integrators of inputs from several stimuli (convergence)
• G proteins can regulate more than one effector (divergence)

• each receptor capable of activating up to a hundred G proteins
• each G protein activating an effector which synthesises several second messenger molecules

Interaction among multiple components allows a DYNAMIC range of responses to stimuli.

1. Many GPCRs can activate multiple G proteins

1. Ligands may also activate more than one receptor type
• GPCRs - enables activation of different second messenger pathways dependent on coupling of receptor subtype
• Activate receptors belonging to two classes: GPCRs and ionotropic receptors - enables either slow or fast-mediated responses

1. receptors associated with cytosolic kinases
2. receptor tyrosine kinases
3. receptor serine kinases

• includes receptors for several growth factors (EGF, PDGF, FGF etc.) and insulin
• over 50 different members
• classified into at least 14 different subgroups based on their structural organisation

1. single transmembrane spanning proteins which dimerise when ligand binds
• platelet derived growth factor (PDGF) receptor
• epidermal growth factor (EGF) receptor
2. covalently linked dimers (insulin receptor, IGF-1 receptor)

Lodish 4th ed., Fig. 20-21

• Ligand binding and receptor dimerisation required for autophosphorylation of tyrosine residues (pY) in the cytosolic domain. Receptor pY participate in the recruitment of cytoplasmic molecules into receptor-based signaling complexes

Lodish 4th ed., Fig. 20-23

Protein modular domains

• protein domains involved in binding pY
• SH2 domains (src homology)
• PTB domains (phosphotyrosine binding)
• other domains involved in protein-protein interactions
• SH3 domains (src homology)
• PH domains (pleckstrin homology)

Major signalling partners for RTKs

• Phospholipase C (PLC)g
• PI3 kinase
• Ras (indirectly activated by RTKs)

• enzymes that control phospholipid metabolism
• Ras-like GTPases
• protein kinases and protein kinase signalling cascades such as the MAP kinase pathway
• transcription factors
• polypeptides that regulate cytoskeletal architecture and cell adhesion

Specificity is determined by individual protein modular domains involved in protein-protein interactions

• interact with specific sequence motifs present only in certain binding partners
• can be regulated by phosphorylation and/or conformational changes

Lecture notes last updated 16/2/2002

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