Chapter 12: Signal Transduction

Adenylate cyclase

integral protein of the plasma membrane with an active site on the cytoplasmic face. when activated by the alpha subunit of a g-protein it synthesizes cAMP from ATP

integrin

interact with ICAMs to immobilize a leukocyte at a site of inflammation

Describe the steps within the pathway of a ligand gated channel. (Nicotinic-ACh receptor)

1. An action potential is triggered in a neuron 2. ACh is released by a pre-synaptic cell 3. ACh binds to a receptor on a post-synaptic cell 4. Stimulated receptor opens channel 5. Influx of sodium and calcium ions depolarize the membrane 6. Depolarized membrane triggers response

Describe the steps within the JAK-STAT pathway

1. EPO binds to the EPO receptor 2. Activated receptor activates cytoplasmic enzyme JAK 3. JAK phosphorylates the receptor 4. Phosphorylation activates Grb2 leading to MAPK cascade 5. Phosphorylation activates STAT domains

Describe the steps within the pathway of a 7TM receptor (epi-responsive beta-adrenergic receptor)

1. Epi binds to its receptor 2. The activated receptor causes replacement of the GDP bound to the alpha subunit of the g protein to GTP, activating the g protein 3. The alpha subunit moves to adenylyl cyclase and activates it 4. Adenylyl cyclase catalyzes the formation of cAMP 5. cAMP activates protein kinase A 6. Phosphorylation of cellular proteins by PKA causes the cellular response to epi 7. cAMP is degraded by phosphodiesterase, reversing the activation of PKA

Describe the steps within the pathway of FASL and TNF receptors

1. Fas ligand and TNF ligand binds to Fas receptor and TNF-R1 receptor respectively 2. FADD domain binds to fas and TRADD domain binds to TNF-R1 at the "death domains" 3. FADD initiates caspase 8 which acts on the mitochondria 4. Mitochondria release cytochrome c 5. Cytochrome c acts as effector caspases to activate DNase and protein degradation 6. Cell death occurs

Describe the steps within the pathway of a soluble receptor (hormone receptor)

1. Hormone is carried to the target tissue on serum binding proteins and diffuses across the membrane and binds to its specific receptor protein in the nucleus 2. Hormone binding changes the conformation of Rec; it forms homo- or heterodimers with other hormone receptor complexes and binds to specific regulatory regions called HREs in the DNA adjacent to specific genes 3. Binding regulates transcription of the adjacent genes, increasing or decreasing the rate of mRNA formation 4. Altered levels of the hormone-regulated gene product produce the cellular response to the hormone

Describe the steps within the pathway of a receptor enzyme (insulin receptors)

1. Insulin binds receptor and receptor undergoes autophosphorylation in its c-terminus tyr residue 2. Insulin receptor phosphorylates IRS-1 on its tyr residue 3. SH2 domain of Grb2 binds to phosphorylated-tyr of IRS-1. SOS binds to Grb2, then to RAS, causing GDP release and GTP binding to RAS. 4. Activated RAS binds and activates Raf-1 5. Raf-1 phosphorylates MEK on two ser residues, activating it. MEK phosphorylates ERK on a thr and tyr residues, activating it. 6. ERK moves into the nucleus and phosphorylates nuclear transcription factors such as ELK1, activating them. 7. Phosphorylated ELK1 joins SRF to stimulate the transcription and translation of a set of genes needed for cell division

Name 3 of the most common amino acids involved in protein phosphorylation in animal signaling. Name 2 other amino acids involved in protein phosphorylations that are more common in prokaryotes and fungi.

Animal signaling- Ser, Thr, Tyr Prokaryotes- His, Asp

How are heterotrimeric g-proteins activated, inactivated, regulated, etc?

Activation: When a ligand binds to the g-protein coupled receptor, it induces a conformational change that allows the receptor to exchange a GDP for a GTP in the alpha subunit. The alpha subunit with dissociate from the beta and gamma subunits and will activate different signaling cascades and effector proteins. Termination: The alpha subunit will eventually hydrolyze the GTP to GDP and re-associate with the beta and gamma subunits. Terminating the transduced signal

Use an example to describe ligand gated channels

An example of a ligand-gated channel is the nicotinic ACh receptor which is an allosteric protein with two cooperative binding sites. This receptor opens in response to ACh allowing ions to pass into the cell for cell to cell communication at the synapse

Name and describe adhesion receptors.

An example of adhesion receptors is P-selectin, which interacts with WBCs to keep them rolling along the blood vessel walls rather than free flowing in the plasma. This keeps WBCs poised to react when needed

What is the consequence of an unregulated MAPK pathway?

An unregulated MAPK pathway will result in tumor synthesis as the cells are constantly undergoing cell division.

What is the role of CDKs in the cell cycle? How do cyclins regulate the cell cycle?

CDKs regulate the cell cycle at control points in G0, G1, and G2. CDK-cyclins stimulate phosphorylation cascades after they have been activated for ordered cell division.

How is calmodulin important in signal transduction?

Calmodulin is an intracellular target of the secondary messenger calcium ions. Once calmodulin is bound to calcium ions it acts as part of a calcium signal transduction pathway by modifying its interactions with target proteins such as kinases or phosphatases. Activate calmodulin activates glycogen breakdown through phosphorylase b. It also binds to other proteins, such as the myosin light chain to assist with muscular contraction, the cellular cytoskeleton for amoeboid movement, and neuron exocytosis

Ca2+

Can act as a secondary or tertiary messenger. Many cells respond to stimuli by altering their intracellular calcium concentration. This affects biochemical changes directly or through interaction with calmodulin and other calcium ion binding proteins. Calcium levels themselves are controlled in large part by secondary messengers such as cAMP and IP3, which implicates calcium as a tertiary messenger.

Describe control of signaling by adaptation by giving an example

Continued excitation from chronic insulin over-sensitization results in receptor desensitization, which will decrease glucose uptake in the cell. Leading to a high blood glucose and type II diabetes

Define cytokines as primary messengers and give examples.

Cytokines are small peptides and glycoproteins produced by a wide variety of cells, not confined to glands and received by cells expressing cognate cell surface receptors. These are historically agents of the immune response and may act as autocrine, paracrine, or endocrine. Ex. interleukins, interferons, TNF, haemopoietic growth factors

What are G-proteins?

G proteins are a family of proteins that act as molecular switches inside cells and are involved in transmitting signals from a variety of stimuli outside of a cell to its interior

What are the differences between Gαs, Gαq, Gαi, and Gα t?

Gα s: stimulates adenylate cyclase Gα q: stimulates phospholipase C Gα i: inhibits adenylate cyclase, lowering the levels of cAMP in the cell Gα t: triggers the breakdown of cGMP by phosphodiesterase

Define hormones as primary messengers and give examples.

Hormones are organic chemical messengers produced by discrete glands. They are delivered to the cell via blood. Ex. amines, peptides, steroids, lipids

Why is the response by soluble receptors much slower compared to cell surface receptors?

Hours or days are required for a response because this is the time necessary for the changes in RNA synthesis and subsequent protein synthesis to become evident in altered expression.

MAPK pathway for transcription of genes for mitosis

Insulin IR IR-P IRS-1 Grb2 SOS RAS MEK ERK translocation ELK1 ELK1 + SRF transcription of genes for mitosis

Phosphoinositide 3-kinase pathway

Insulin IR IRS-1 PI3K (PIP2-> PIP3) PKB PDK1 GSK3 GLUT4 cells absorb extracellular glucose and glycogen synthesis is stimulated

Name 3 major cellular changes initiated by insulin

Insulin stimulates mitosis through the MAPK pathway, and insulin stimulates glucose uptake and glycogen synthesis through the phosphoinositide 3-kinase pathway

What are MAPK pathways?

MAP-kinase pathways are interrelated signal transduction cascades that are activated by stimuli such as growth factors, stress, cytokines, and inflammation. These signals are transduced directly or via small g proteins such as ras and rac to multiple tiers of protein kinases that amplify signals and/or regulate each other

Define neurotransmitters as primary messengers and give examples.

Neurotransmitters are chemicals that relay, amplify, and modulate signals between a neuron and another cell. They are most often released from synaptic vesicles by presynaptic cells. Ex. amino acids and monoamines, peptides

What is the role of phospholipase C in signal transduction?

Phospholipase c seperates PIP2 into IP3 and DAG IP3 will activate calcium ion channels on the endoplasmic reticulum and DAG will activated secondary messengers

Explain how unregulated kinase activity can contribute to the development of cancers and tumors.

Proto-oncogenes have been identified at all levels of the various signal transduction cascades that control cell growth, proliferation, and differentiation. Examples: Oncogenic ERF receptor: tyrosine kinase receptor. results in a defect in binding domain leading to a constant tyr kinase activity Abl-Bcr: chromosomal translocation forms this chimeric gene and fusion protein that alters cellular lcoation of the protein. This protein is not properly regulated and appears in abnormal cell compartments. The overactive kinase phosophorylates tyr, ser, and thr residues of seemingly random proteins. This affects numerous growth-promoting pathways and becomes a pro-survival gene, inhibiting apoptosis and activating proliferation.

calmodulin

acidic protein with four high affinity calcium ion binding sites. when intracellular calcium concentrations rise the binding to calmodulin drives a conformational change in the protein. calmodulin associates with a variety of proteins and in its calcium bound state, modulates their activities. also a regulatory subunit of phosphorylase b kinase of the muscle

7TM receptor/ G-protein coupled receptor

act through heterotrimeric G-proteins. Upon ligand binding, GPCRs catalyze the exchange of GDP for GTP on the alpha subunit, causing the alpha subunit to dissociate from the beta and gamma subunits.

What are the 4 common features of all signal-transducing systems? Describe these.

Specificity- signals fit into the binding site of the proper receptor while others don't Amplification- binding of a signal promotes a cascade of enzymes activating enzymes. A signal will produce a strong response by promoting enzymes and secondary messengers Adaptation- Response needs to be turned off after a period of time, result of a response will inhibit the receptor of a signal. When the signal declines the system becomes sensitive to the signal again Integration- when events are triggered a given cell might receive multiple signals. Those signals will activate their response, but it they activate the same enzyme, the response will be the same as they will integrate.

How are the activities of CDK-cyclin regulated?

The CDK-cyclin complex is short lived and its activity is controlled to a large degree by the ubiquintin-mediated degradation of cyclins. Additional regulation is achieved through the activity of endogenous CKIs and the Ink4 family of proteins that are CDK-cyclin specific.

protein kinase C

activated by DAG and calcium ions DAG and calcium ions coordinate to activate PKC, this activation involves the movement of a PKC away from its location in the substrate-binding region of the enzyme, allowing the enzyme to bind and phosphorylate proteins that contain a PKC sequence. Ser or thr residues embedded in an amino acid sequence are recognized by PKC.

PIP3

When bound to PIP3, protein kinase B is phosphorylated and activated by PDK1. The activated PKB then phosphorylares ser and thr residues in its target proteins, one of which is glycogen synthesis kinase 3

oncogene

a gene that has the potential to cause cancer. in tumor cells they are often mutated and/or expressed at high levels mutations that produce oncogenes are genetically dominant. the defect can be in any of the proteins involved in communicating the divide signal and a tumor will result. some oncogenes encode surface receptors with defective or missing signal binding sites, such that their intrinsic tyr kinase activity is unregulated

protein kinase B

activated by PIP3 and reversible phosphorylation pathway prevents glycogen synthesis in the liver and muscle. PKB also triggers the movement of glucose transporters (GLUT4) from internal vesicles to the plasma membrane, stimulating glucose uptake from the blood.

ligand-gated channel

allow ions to pass through the membrane in response to the binding of a chemical messenger such as a neurotransmitter

integration

allows a system to receive multiple signals and produce a unified response even if the signals are antagonistic

Non-enzyme Non-7TM receptor

aren't enzymes themselves but are enzyme linked examples of enzyme activities associated with the receptor include: tyrosine kinase (BCR, TCR, JAK-STAT) proteases (tumor necrosis factor, FAS receptor) serine/threonine-specific protein kinase histidine kinases (not in humans)

amplification

binding of a signal promotes a cascade of enzymes activation other enzymes

Of the 6 most common secondary messenger molecules in animals, how are cytoplasmic concentrations of each changed to relay signals?

cAMP- concentrations are increased when a G protein activates an adenylate cyclase enzyme, concentrations are decreased by phosphodiesterase cGMP- concentrations are increased when a g protein activates a guanylate cyclase enzyme, concentrations are decreased by phosphodiesterase calcium ions- calcium levels are controlled by secondary messengers such as cAMP and IP3, IP3 will trigger release of calcium from endoplasmic reticulum PIP3- PIP2 is converted into PIP2 using PI3K DAG/IP3- PIP2 is broken int o DAG and IP3 using PLC, DAG will activate secondary messengers like PKC, and IP3 will activate calcium channels

MAPK pathway

chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell of the DNA in the nucleus of the cell. The signal starts when a signaling molecule binds to the receptor on the cell surface and ends when the DNA in the nucleus expresses a protein an produces some change in the cell such as division.

tertiary messengers

changes in cellular concentration of small molecules initiated by a secondary messenger ex. calcium ions, DAG, IP3, and PIP3

secondary messengers

changes in cellular concentration of small, non-protein, water-soluble molecules relay primary message to the other intracellular proteins, while achieving signal transduction ex. cAMP(GMP), calcium ions, IP3, DAG, and PIP3

heterotrimeric g-protein

composed of an alpha, beta, and gamma subunit, these proteins are responsible for triggering proteins during the activation of the signal transduction pathway. They activate or inhibit effector enzymes, which change the concentration of a secondary messenger

protein kinase A

composed of two subunits that dimerize to form a quaternary structure. one subunit plays a regulatory role while the other subunit has kinase activity. when cAMP is made is associates with the regulatory subunit, stabilizing it and releasing the catalytic subunit. PKA catalyzes the phosphorylation of proteins including glycogen phosphorylase b kinase. will target ser and thr residues for phosphorylation in its pathway

Adaptation/desensitization

control signal transduction by modulating the receptor

soluble receptors

diffuse through the membrane and bind to a nuclear receptor protein allowing the receptor to regulate the expression of specific genes

proto-oncogene

encode growth regulating proteins

I-CAM

encodes an intracellular adhesion molecules that is present in low concentrations in the membranes of leukocytes and endothelial cells. Stabilizes cell-cell interactions and facilitates leukocyte transmigration.

CDK/ cyclin-dependent kinases

enzymes that are only active when associated with a kinase. without cyclin CDK folds so that one segment, the T loop, blocks the binding site for protein substates inhibiting protein kinase activity.

Cyclin

family or proteins that activates CDKs by inducing a conformational change in their structure

p-selectins

function as an adhesion molecule on the surgaces of activated endothelial cells, which line the inner surface of blood vessels, and activate platelets. P-selectin plays an essential role in the initial recruitment of leukocytes to the site of injury during inflammation.

adhesion receptor

include a molecular chain of adaptor proteins connecting the adhesion receptors cytoplasmic domains to the cell cytoskeleton

primary messengers

initiate a cellular response by alone are not enough to yield the full response necessary

mechanical receptor

mechanoreceptors are sensory receptors that respond to mechanical pressure or distortion

gated-ion channel

provide a regulated path for the movement of inorganic ions across the plasma membrane in response to various stimuli. These channels may be opened or closed depending on whether the associated receptor has been activated by the binding of its specific ligand or by a change in the membrane electrical potential.

Guanylate cyclase

receptor enzyme that converts GTP into the secondary messenger cGMP after it has been activated by the alpha subunit of a G protein

phospholipase C/ PLC

responsible for catalyzing the reaction that converts PIP2 into DAG and IP3

PI3K

responsible for catalyzing the reaction that converts PIP2 into PIP3

cAMP

secondary messenger produced by adenylate cyclase in response to activation by the alpha subunit of a g-protein. cAMP stimulates cAMP-dependent protein kinase to phosphorylate key target enzymes, changing their activities. cAMP is reduced by cAMP phosphodiesterase

cGMP

secondary messenger produced by guanylate cyclase in response to activation by the alpha subunit of a g-protein. cGMP activates a cytosolic protein kinase that phosphorylates cellular proteins and changes their activities. cGMP is reduced by cGMP phosphodiesterase

specificity

signals fit into the binding site of their specific receptor, others don't

monomeric g-protein/ small g-protein

structure is similar to the alpha subunit of the heterotrimeric g-protein mediate a wide variety of signal transductions ex. RAS and RAC

DAG/ diacylglycerol

synthesized through the breakdown of PIP2 into IP3 and DAG using phospholipase C cooperates with calcium ions in activating protein kinase C, thus acting as a secondary messenger

What is the role of protein kinase A in signal transduction?

when activated by cAMP will activate glycogen breakdown and stop glycogen synthesis, also activates lipolysis modulated by factors that either inhibit or activate adenylate cyclase and cAMP