Week 5 Study Problems

Question 1

1. How can two different cell types exposed to the same signal respond differently?

Answer 1

-The two cell types can differ in either what intracellular signaling proteins the receptor binds to, or differ in what effector proteins are present to carry out the response.

Also, different genes may encode receptors that share the same binding selectivity (they bind the same signal), but differ in their cytosolic domain, or are even a different class of receptor.

Question 2

What are four general functions of intracellular signaling pathways?

Answer 2

• Relay/transduce
• Amplify
• Integrate and distribute

Primary transduction is also a general function.

Primary transduction is the general function of the receptor.

Question 3

For each way of sending a signal, state whether the signal acts locally or in a distant tissue.

-Paracrine

Answer 3

Paracrine
The signal acts locally. It is secreted by one cell and acts on other cells in the
same tissue or organ.

Question 4

An examples of Paracrine?

Answer 4

Examples are growth factors such as epidermal growth factor, and signals, such as cytokines, that regulate inflammation at the site of an infection.

Question 5

For each way of sending a signal, state whether the signal acts locally or in a distant tissue.

-Endocrine

Answer 5

Endocrine
The signal is secreted into the circulatory system and distributed through out the body so that it acts on cells in distant tissues and organs.

Question 6

Examples of Endocrine.

Answer 6

Examples are insulin and glucagon, secreted from the pancreas into the blood stream.

Question 7

For each way of sending a signal, state whether the signal acts locally or in a distant tissue.

-Neuronal

Answer 7

Neuronal
This type of signaling is specific to neurons. The signal acts very locally. The signal is a neurotransmitter and is released into a gap between the axon terminus and the target cell. The neurotransmitter signal, however, is the end result of another signal that was initiated far away (at the dendrites) that then traveled as an electrical impulse to the axon’s terminus.

Question 8

Example of Neuronal

Answer 8

An example is a neuron regulating the contraction of a muscle fiber.

Question 9

For each way of sending a signal, state whether the signal acts locally or in a distant tissue.

Contact-Dependent

Answer 9

Contact-Dependent This signal is very local as it involves a transmembrane protein, or other cell surface component, on one cell binding to a transmembrane protein on an adjacent cell. This signal is not secreted from a cell, but remains part of the cell’s surface.

Question 10

Example of Contact-Dependent

Answer 10

An example is during development where adjacent cells use contact-dependent signaling to prevent each other from differentiating into the same cell type.

Question 11

What is a molecular switch?

Answer 11

A molecular switch is a signaling protein that switches between an ‘on’ active conformation and an ‘off’ inactive conformation. An incoming signal turns the signaling protein ‘on’, in which state the signaling protein relays the signal, and does so until turned ‘off’.

Question 12

Draw and label a molecular switch that involves a protein kinase and protein phosphatase.

Answer 12

See study guide

Question 13

Draw and label a molecular switch that involves a G-protein.

Answer 13

See study guide

Question 14

What are the three main classes of cell-surface receptors?

Answer 14

• Ligand-gated ion channels
• G-protein-coupled
• Receptors enzyme-coupled receptors (Receptor Tyrosine Kinases being the most common).

Question 15

Describe how ligand binding to a G-protein-coupled receptor activates an intracellular signal.

Answer 15

Ligand binding to a G-protein-coupled receptor (GPCR) causes the receptor to change conformation.

GPCR’s have 7-transmembrane helices that can move relative to each other so can couple ligand binding to a change in the conformation of the GPCR’s cytosolic domain.

The cytosolic domain then has complementarity to the alpha-subunit of trimeric G-proteins.

Binding of the trimeric G-protein to the receptor causes the alpha- subunit to change conformation so that GDP is released and the alpha-subunit disassociates from the beta/gamma unit.

The alpha-subunit rapidly binds to GTP, which causes change in shape so that the alpha-subunit will then bind other proteins to relay/transduce the signal.

Question 16

What are three small, second messengers?

Answer 16

Three common second messengers are Ca2+, inositol trisphosphate (IP3) and cAMP.

Question 17

Describe how the Ca2+ second messenger is generated?

Answer 17

Ca2+ is in low amounts in the cytosol, but higher amounts outside the cell or in the endoplasmic reticulum (ER). The electrochemical gradient of Ca2+ favors flow into the cytosol. There are IP3-ligand-gated Ca2+ channels in the ER’s membrane. When IP3 binds to the channel, the channel opens and lots of Ca2+ flows into the cytosol.

Question 18

Describe how the IP3 second messenger is generated?

Answer 18

IP3 is a head group of phospholipids in the cytosolic monolayer of the plasma membrane. The enzyme phospholipase C catalyzes breaking of the covalent bond between IP3 and the glycerol of the phospholipid. The result is free IP3, which diffuses through the cytosol, and diacylglycerol (glycerol plus the two fatty acid tails), which remains in the cytosolic monolayer of the plasma membrane.

Question 19

Describe how the cAMP second messenger is generated?

Answer 19

cAMP is generated by the enzyme adenylyl cyclase, which binds ATP and catalyzes the chemical transformation of ATP into cAMP.

Question 20

State how the second messenger inositol trisphosphate (IP3) can amplify a signal?

Answer 20

For each active phospholipase C enzymes, many IP3 molecules can be released into the cytosol.

Question 21

State how the second messenger cAMP can amplify a signal?

Answer 21

For each active adenylyl cyclase enzymes, many cAMPs can be generated from ATPs.

Question 22

State how the second messenger Ca2+ can amplify a signal?

Answer 22

For each Ca2+ channel that opens, thousands of Ca2+ ions flow into the cytosol.

Question 23

Give an example of a signaling protein that is activated by the Ca2+ messenger.

Answer 23

There are numerous proteins that can bind Ca2+, change conformation and relay the signal. One example is the protein Calmodulin.

Question 24

Give an example of a signaling protein that is activated by the IP3 messenger.

Answer 24

There are numerous proteins that can bind IP3, change conformation and relay the signal. One example is the Ca2+-channel.

Question 25

Give an example of a signaling protein that is activated by the cAMP messenger.

Answer 25

There are numerous proteins that can
bind cAMP, change conformation and
relay the signal. One example is Protein
Kinase A.

Question 26

Describe how ligand binding to a receptor tyrosine kinase activates an intracellular signal?

Answer 26

Receptor tyrosine kinases have a single transmembrane α-helix. The single,
transmembrane α-helix cannot directly couple binding of a ligand to conformation change of the cytosolic domain. Ligands of receptor tyrosine kinases are often dimers. When the ligand binds to one receptor, a second receptor binds to the other half of the ligand. Ligand binding therefore causes the receptor to dimerize, which puts the receptors’ cytosolic domains next to each other.
The receptor’s cytosolic domain is a tyrosine kinase, which can catalyze the transfer of a phosphate, taken from ATP, onto a tyrosine of the other receptor’s cytosolic domain. In other words, the two receptors in the dimer cross-phosphorylate each other. Cytosolic signaling proteins, typically with SH2 domains, then bind to the phosphorylated cytosolic domains and begin to relay the signal.

Question 27

True/False, if false explain why

When a ligand binds to a G-protein coupled receptor, the first intracellular signaling component turned ‘on’ is a monomeric G-protein.

Answer 27

False, a G-protein coupled receptor, after ligand binding, binds to and turns ‘on’ a trimeric G-protein.

Question 28

True/False, if false explain why

When a ligand dissociates from a receptor tyrosine kinase, the receptor changes conformation so that it no longer sends an intracellular signal.

Answer 28

False. After ligand binding, receptor tyrosine kinases form dimers and cross-phosphorylate each other’s cytosolic domains, which relays the signal. Even after ligand disassociates, the cytosolic domains remain phosphorylated until a Protein Phosphatase catalyzes removal.

Question 29

True/False, if false explain why

Ligand binding to a G-protein coupled receptor causes the receptor to change conformation so that its cytosolic domain binds proteins with SH2 domains.

Answer 29

False. The cytosolic domain of G-protein coupled receptors bind to trimeric G-proteins. Proteins with SH2 domains bind to proteins that have phosphorylated tyrosine, not to G-protein coupled receptors.

Question 30

True/False, if false explain why

Inositol 1,4,5-trisphosphate (IP3) stimulates an increase in cytosolic Ca2+ that alone is sufficient to activate protein kinase C.

Answer 30

False. IP3 does bind ligand-gated Ca2+ ion channels, causing them to open and flood the cytosol with Ca2+, and Protein Kinase C does need to bind Ca2+ to be active, but Protein Kinase C also needs to bind diacylglycerol to change to its ‘on’ conformation.

Question 31

True/False, if false explain why

After being activated, the trimeric G-protein α-subunit diffuses throughout the cytosol.

Answer 31

False, the α-subunit is covalently modified with a fatty acid that anchors the G-protein at the cytosolic surface of the plasma membrane, so it does not diffuse through out the cytosol.

Question 32

True/False, if false explain why

Ca2+ ion channels, but not Ca2+-ATPasepumps, are required for Ca2+ signaling.

Answer 32

False, opening of Ca2+ channels results in an increase in the amount of Ca2+ in the cytosol, which transmits a signal, but Ca2+ signaling is dynamic and involves oscillations in the cytosolic amount of Ca2+.. To create an oscillation (an increase then a decrease then an increase then a decrease . . . ) Ca2+ pumps are required to remove the Ca2+ from the cytosol (either into the lumen of the endoplasmic reticulum or out of the cell), so that another pulse of Ca2+ can happen.

Question 33

In a series of experiments, genes that code for mutant forms of an RTK are introduced into cells. The cells also express their own normal form of the receptor from their normal gene, although the mutant genes are constructed so that the mutant RTK is expressed at considerably higher concentration than the normal RTK. What would be the consequences of introducing a mutant gene that codes for an RTK (A) lacking its extracellular domain, or (B) lacking its intracellular domain?

Answer 33

An RTK mutant that lacks its extracellular domain cannot bind ligand so will not interfere with the normal RTK binding ligand and sending an intracellular signal.
An RTK mutant that lacks its intracellular domain can still bind ligand and form dimers. Because there is more mutant RTK than normal, almost all the dimers that form will at least have one, if not both, lacking the intracellular kinase domains. Because the kinase domains are lacking, cross-phosphorylation will not happen so that the signal will not be transmitted.

Question 34

What are the similarities and differences between the reactions that lead to the activation of trimeric G proteins and the reactions that lead to the activation of Ras?

Answer 34

Activation of both trimeric G-proteins and monomeric G-proteins, like Ras, involve binding to a protein (Guanine Exchange Factor; GEF) that causes the G-protein to release GDP. The G-protein then rapidly binds GTP and changes conformation to the ‘on’ state.
In the case of trimeric G-proteins, the GEF is a receptor that couples ligand binding to activating the G-protein. In contrast, monomeric G-proteins are not activated by receptors but by a cytosolic GEF, which is itself activated by a receptor or one or two steps away from a receptor.

Question 35

Why do you suppose cells use Ca2+ (which is kept by Ca2+ pumps at a cytosolic concentration of 10-7M) for intracellular signaling and not another ion such as Na+ (which is kept by the Na+ pump at a cytosolic concentration of 10-3M).

Answer 35

The amount of Ca2+ in the cytosol is 10,000 times less than Na+. The very low amount of Ca2+ permits a relatively small influx of Ca2+, by way of ion channels, to result in a large change (for example, a ten-fold change) that can alter the activity of many proteins that bind Ca2+, or bind Calmodulin, which is a protein that first binds Ca2+. To get the same ten-fold change in Na+ concentration would require a relatively large influx of Na+.

Question 36

What is an example on how two different cell types, exposed to the SAME signal, but still respond differently because different genes may encode receptors that share the same binding selectivity but differ in their cytosolic domain?

Answer 36

Example: acetylcholine (a neurotransmitter) binds to a ligand-gated ion channel, but also a G-protein coupled receptor.