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Cell Biology Lecture 223.21: G-Linked Protein Receptors

Cell Signaling

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G-Linked Protein Receptors Largest family of cell surface receptors

7-pass transmembrane protein numbers present vary by organism (700 known in humans)

Each type mediates responses to different ligands

proteins, modified amino acids, fatty acids

Same ligand can activate several G protein receptors Each G-protein receptor interacts with a specific G-Protein

Cell Signaling - GTP-binding protein

To turn on protein’s signalling activity: 1. release bound GDP 2. bind GTP (nucleotide exchange)

Resultant signal may be: 1. stimulatory (Gs) or

2. inhibitory (Gi) To turn off signaling activity:

hydrolysis of GTP by intrinsic GTPase causes inactivation

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G-Linked Protein Receptors & G-proteins

G-protein: at least 20 types in humans trimeric complex (for specifics of G-protein attachment to cell membrane, review Lecture 13: Membrane proteins)

G-protein activation

To turn on G-protein activity: 1. Receptor binds signal

molecule 2. G-protein

a. binds activated receptor b. releases bound GDP c. bind GTP

Depending on the G-protein the resultant activity may be: 1. stimulatory (Gs) or

2. inhibitory (Gi) Resultant activity may be from

1. α subunit 2. βγ complex 3a. both α and βγ, together 3b. both α and βγ, separately

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G-protein: βγ complex

e.g. βγ complex activates target here, a K+ channel

To turn off signalling activity: 1. hydrolysis of GTP

by GTPase activity intrinsic to α subunit causes inactivation of α

2. reassociation of (inactive) α with βγ inactivates βγ

Some G proteins directly regulate ion channels

G-protein: α subunit

α  subunit activates target here, i.e., an enzyme (adenylyl cyclase) To turn off signalling activity: 1. hydrolysis of GTP

by GTPase activity intrinsic to α subunit causes inactivation (usually within seconds)

2. reassociation of (inactive) α with βγ inactivates βγ

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G-protein: α subunit

RAPID Caffeine Inhibits

G-protein & Receptors: Adrenaline case study

The same extracellular signal (e.g. adrenaline, the “fight or flight” hormone) can initiate different responses in different cell types:

1.  Heart cell increased [cAMP] à increased heart rate

2.  Muscle cell increased [cAMP] à increased glycogen breakdown

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G-protein & Receptors: Adrenaline case study

The same extracellular signal (e.g. adrenaline, the “fight or flight” hormone) can initiate opposite responses mediated by different G-proteins and receptors:

1.  adrenaline binds to β-adrenergic receptor activates stimulatory G-protein (Gs) Gs turn adenylate cyclase ON result: increased cAMP concentration

2.  adrenaline binds to α2-adrenergic receptor activates inhibitory G-protein (Gi) Gi turns adenylate cyclase OFF result: decreased cAMP concentration

Note: - the β and γ subunits of Gs and Gi are identical-they differ only in the α subunit

How does cAMP influence downstream events in the cell?

Protein Kinase A (PKA): regulatory + enzymatic subunits regulatory subunits inhibit kinase activity binding of cAMP dissociates regulatory subunits dissociation activates kinase activity enzyme phosphorylates its target proteins to cause different effects

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G-Linked Protein Receptors

Adrenalin rapid response e.g. 1 Adrenaline-mediated

cAMP response in skeletal muscle PKA activation result: 1. glycogen breakdown (power boost) 2. stop glycogen synthesis (not shown)

e.g. 2 Adrenaline-mediated

cAMP response in heart muscle result: increase heart rate

G-Linked Protein Receptors

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G-Linked Protein Receptors

G-Linked Protein Receptors and cAMP

Adrenalin slow response Adrenaline-mediated cAMP response in hypothalamus (endocrine cells) PKA activation result: 1. Activate transcriptional

regulator 2. Turn on subset of genes,

including somatostatin (which controls hormone release)

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G-Linked Protein Receptors

Cell Biology Lecture 223.21: G-Linked Protein Receptors

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G-protein Coupled to Ion Channel G-protein Coupled to Enzyme Adenylyl Cyclase

G-Protein Linked Receptors and Ca 2+

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Ca2+ responsive proteins: Calmodulin (CaM for Ca2+ modulated protein) & CaM Kinase

CaM (calcium modulated protein, or Calmodulin) regulates serine and threonine kinases

-i.e., myosin light chain kinase (activates smooth muscle contraction) -i.e., CaM Kinase II in nerve synapse (alters neurotransmitters secretion)

G-Linked Protein Receptors

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G-protein signalling

G-Linked Protein Receptors and cAMP

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Ca+ responsive proteins: Calmodulin

G-Linked Protein Receptors