Cell Signalling

Question 1

What are the 3 stages of cell signaling?

Answer 1

1. Signal reception
2. Signal transduction
3. Cellular response

Question 2

What does signal reception refer to?

Answer 2

The target cell’s detection of an extracellular signal molecule

• A signal is detected when a signal molecule binds to a specific receptor protein located at the cell’s surface (extracellular) or inside the target cell (intracellular).
• Ligand-receptor interaction is highly specific. The signal molecule acts as a ligand, binding to a specific complementary site on the target cell’s receptor to form a ligand-receptor complex.
• This causes the receptor protein to undergo a conformation change. For many receptors this change in confromation directly activates the receptor, enabling it to interact with other molecules in the cell.

Question 3

What is signal transduction?

Answer 3

The process by which a target cell converts an extracellular signal into an intracellular signal that results in a specific cellular response.

• The formation of the activated “ligand-receptor complex” changes the conformation of the receptor protein, initiating the process of transduction.
• Transduction can occur in a single step, as in the case for signalling mediated by a intracellular receptor.
• More often, as in the case of cell surface membrane receptors which are the majority, transduction occurs via a multistep signal transduction pathway consisting of a series of relay molecules.
• transduction may also involve second messengers

Question 4

Describe the role of relay molecules

Answer 4

Relay molecules are usually proteins such as enzymes that operate in a specific sequence.

• Each protein in the pathway typically acts by altering the confromation of and hence activating or inhibiting the protein immediately downstream.
• As the conformational changes are usually brought about by phosphorylation, the relay proteins in a signal transduction pathways are sequentially phosphorylated.
• This forms a phosphorylation cascade that transmits the signal received at the cell surface into the cell.

Question 5

illustrate a cellular response

Answer 5

A signal transduction pathway eventually leads to the regulation of one or more cellular activities.

• The responses may occur in the cytoplasm or may involve action in the nucleus

Question 6

Cytoplasmic response

Answer 6

It involves mainly changes in cell metabolism.

• regulation of enzyme activity such as activation of cytoplasmic enzymes or other proteins
• cytoskeletal arrangement

Question 7

Nuclear response

Answer 7

It involves changes in gene expression such as

• turning specific genes on or off in the nucleus, and hence synthesis of enzymes of other proteins.

Question 8

General role of cell surface membrane receptors (which are transmembrane proteins)

Answer 8

• Hydrophilic or water-soluble molecules are unable to diffuse across the hydrophobic core of the cell membrane
• Thus they bind to specific sites on cell surface receptor proteins
• The binding allows for transmission of the signal into the cell

Question 9

4 main types of cell surfacereceptors

Answer 9

Important

1. G-protein linked receptors
2. Receptor tyrosine kinases
3. Ion channel receptor
4. Integrin receptor

Question 10

What is PK?

Answer 10

It is protein kinase and it transfers phosphate groups from ATP to proteins in a process called phosphorylation.

This turns on the signal transduction pathway

Question 11

What is PP

Answer 11

It is protein phophatase which rapidly remove phosphate groups from proteins in a process called dephosphorylation

This turns off the signal transduction pathway when the initial signal (signal molecule) is no longer present, allowing the kinases to be available for reuse.

Question 12

Sample example of a phosphorylation cascade.

Answer 12

1. A relay molecule activates protein kinase 1
2. Active protein kinase 1 transfers a phosphate from ATP to an inactive molecule of protein kinase 2, thus activating this 2nd kinase.
3. Active protein kinase 2 then catalyses the phosphorylation of protein kinase 3.
4. Finally, active protein kinase 3 phosphorylates a protein that brings about the cell’s response to the signal.
5. Enzymes called protein phosphatases catalyze the removal of phosphate groups from the proteins, making them inactive and available for reuse.

Question 13

Illustrate second messengers

Answer 13

They are non-protein signal molecules that are small, water soluble and can be either ions or molecules

• They serve to transmit the message carried by the extracellular signal molecule - the first messenger - into the target cell’s interior

Question 14

How do second messengers carry out their role

4 points

Answer 14

• Binding of first messengers onto receptors stimulate an increase in the concentration of second messengers
• The small and water-soluble second messengers can readily spread throughout the cytosol by diffusion
• As there is a large variety of relay proteins that are sensitive to the cytosolic concentration of second messengers, binding of second messengers to these proteins can alter the behavior of the relay proteins
• As they often stimulate a variety of cellular activities, second messengers enable cells to mount a large-scale, coordinated response following stimulation by a single extracellular signal molecule.

Question 15

What are GPLRs

Answer 15

GPLRs are cell surface receptors that work with the help of proteins called G proteins which are located in the cytoplasmic side of the cell membrane.

Question 16

Structure of GLPR: Hydrophillic amino acid residues form the interhelical loops and N and C termini

then state its function as a result of this structural adaptation

Answer 16

Enables the extracellular and intracellular domains to be soluble in aqueous medium and also interact with water-soluble ligands and G-protein.

Question 17

Function of GLPR: Enables the membrane-embedded domain to be stabilised and embedded within the membrane bilayer

Hence state the structural adaptation that enables this function

Answer 17

Hydrophobic amino acid residues are primarily found in the seven transmembrane alpha-helices.

Hence, hydrophobic interactions exist between the alpha-helices and also with the hydrophobic fatty acid tails of phospholipids in the membrane bilayer.

Question 18

Structure of GLPR: extracellular domain contains specific amino acids at signal-binding site

Hence state the function this structural adaptation enables

Answer 18

Enables G-protein interaction site to have specific 3D confromation to bind and activate G-protein

Question 19

Structure of GLPR: Intracellular domain contains specific amino acids at signal-binding site

Answer 19

Enables G-protein interaction site to have specific 3D conformation to bind and activate G-protein

Question 20

Function of GPLR: Enables GPLR to initate signal transduction pathways via activation of G-protein

Answer 20

Binding of ligand to GPLR, causes a conformational change in protein, allowing it to interact with G-protein

Question 21

Working principles of G-protein Linked receptor (GLPR)

Answer 21

• The G protein (on the cytoplasmic side of the membrane) mediates the passage of the signal from the membrane surface into the cell interior, by functioning as a molecular switch that is either on or off, depending on whether GDP or GTP is attached.
• When GDP is bound to the G protein, the G protein is inactive.
• The GLPR and G protein work together with another protein referred to as the target protein (or effector), which is usually an enzyme.

Question 22

Illustrate the signal reception section by a GPLR in response to signal molecule binding

Answer 22

The signal molecule (ligand) binds to the extracellular side of the GPLR and causes a change in receptor conformation

Question 23

Illustrate the signal transduction part by a GPLR in response to signal molecule binding

hint 5 points

Answer 23

1. With an increased affinity for the G protein, the cytoplasmic side of the GPLR binds an inactive G protein, causing a GTP to displace the GDP bound to the G protein
2. The G protein is activated
3. The activated G protein dissociates from the GPLR and diffuses along the membrane.
4. The activated G protein binds to a target protein (effector), usually an enzyme, thereby altering target protein activity.
5. Change in target protein activity initiates a cascade of signal transduction event by triggering the next step in the transduction pathway inside the cell, including the:
   - Production of cyclic AMP (cAMP) OR
   - Production of inositol triphosphate and release of calcium ions

IP3, cAMP and Ca2+serve as second messengers in signal transduction

Question 24

The cAMP process

Answer 24

1. An extracellular signal molecule such as epinephrine binds to and activates a GLPR, which activates a specific G protein.
2. The active G protein activates adenylyl cyclase which catalyses the conversion of ATP to cyclic AMP (cAMP). This boosts the concentration of cAMP by 20-fold in a matter of seconds
3. The immediate effect of cAMP is usually the activation of a serine/threonine kinase called protein kinase A. The activated kinase then phosphorylates various other proteins, depending on the cell type.

FYI: The number of cAMP molecules does not persist for long in the absence of the hormone becuase another enzyme, called phophodiesterase, converts the cAMP to AMP.

Question 25

Why can calcium ions be used as a second messenger?

Answer 25

Although cells always contain some Ca2+, the concentration in the cytosol is normally much lower than the concetration outside the cell.

Brief increase to the cytosolic concentration of Ca2+ causes many cellular responses including muscle cell contraction, secretion of certain substances, and cell division

Question 26

How is low cytosolic Ca2+ maintained

4 points

Answer 26

1. Calcium ATPase in the ER membrane sequester Ca2+ from the cytosol into the ER lumen.
2. Calcium ATPase in the plasma membrane actively pump Ca2+ from the cytosol into the extracellular fluid
3. Sodium calcium exchangers in plasma membrane couples export of Ca2+ with the facilitated diffusion of Na+ into the cytosol.
4. Mitochondrial Ca2+ pumps moves Ca2+ into mitochondria

Question 27

Overview of an RTK (receptor tyrosine kinase)

Answer 27

RTK is a major class of cell surface receptors.

• The part of the receptor protein extending into the cytoplasm functions as an enzyme known as tyrosine kinase, which catalyses the transfer of a phosphate group from ATP to the amino acid tyrosine.
• RTK can trigger more than one different signal transduction pathways from a single ligand-binding event.
• RTKs can thus activate several cellular responses and hence display significant functional diversity.

Question 28

What does each RTK comprise of

Answer 28

1. An extracellular signal-binding site
2. An alpha-helix spanning the membrane
3. An intracellular tail containing multiple tyrosines and tyrosine kinase domain

Question 29

How does signal reception of an RTK work

4 points

Answer 29

1. The signal molecule (ligand) binds to a subunit of the RTK, resulting in receptor aggregation and dimerisation.
2. Dimerisation leads to the activation of the tyrosine kinase activity of the receptor, resulting in autophosphorylation / cross phosphorylation
3. Each tyrosine kinase domain adds phosphate from an ATP molecule to a tyrosine on the tail of its own or the other polypeptide subunit
4. RTK is fully activated.

Question 30

Cellular response segment of GPLR mediation

Answer 30

The last activated molecule in the transduction pathway triggers a cellular (cytoplasmic or nucleus) response.

*NOT exactly cellular response but it is after it*

• The intrinsic GTPase activity of the G protein soon hydrolyses its bound GTP to GDP, so that the G protein is inactive again. The signal molecule has also dissociated from the GPLR.
• The inactive G protein leaves the enzyme, whcih returns to its orginal inactive state. The G protein is now available for reuse.

Question 31

Illustrate how does signal transduction of RTK occur

3 points but first 2 are connected

Answer 31

• The activated RTK binds cytoplasmic relay proteins, thereby altering their activity, localisation or ability to interact with other intracellular signalling proteins.
• Each relay protein recognises and binds to a specific phosphorylated tyrosine. The bound relay protein becomes activated, in many cases by undergoing a conformational change.
• Each activated relay protein triggers a transduction pathway, thus initiating a cascade of signal transduction events inside the cell.

Question 32

Describe cellular response by RTK

Answer 32

The last activated molecule in each transduction pathway triggers a cellular (cytoplasmic or nucleus) reponse.

Not related but
Rmb: multiple transduction pathways can be triggered by the activation of one RTK, as each phophorylated tyrosine is recognised by a different relay protein.

Consequently several different cellular responses are generated.

Question 33

What are the advantages of multistep pathways

Answer 33

1. Signal amplification
2. Provide more opportunities for coordination and regulation
3. Contribute to the spcificity of the response

Question 34

Key features of signal amplification

Answer 34

Signal amplification is the process of enhancing signal strength as the signal is relayed through a transduction pathway. As a consequence, the response is amplified.

Key features are as follows:

1. at each catalytic step in the cascade, the number of activated prodcuts is greater than in the peceding step.
2. A small number of extracellular signal molecules is sufficient to elicit a cellular response
3. Response of the target cell is large, as a large number of activated molecules is produced at the end of the signalling cascade.

Question 35

Hoe is this amplification effect possible

Answer 35

1. The presenece of multiple steps in the transduction pathway.
2. Persistence of protein in the pathway in the active form long enough to process numerous molecules of substrate before they become inactive again

These factors result in a small number of first messengers binding to cell surface receptors leading to hundred of millions of glucose molecules from glycogen.

Question 36

Regulation of cell signalling overview

Answer 36

• For a cell of a multicellular organism to remain sensitive to and continually respond to incoming signals, each molecular change in its signaling pathways must only last a short time.
• Thus, the key to a cell’s continuing receptiveness to signals is the reversibility of the changes that the signal produce.
• This entails signal termination, whereby the signal response is terminated by processes which return the receptor and each of the components of the signal transduction pathway to their inactive state.

Question 37

Stages of cell signalling that can serve as regulation points

Answer 37

At reception and transduction. Mechanisms resulting in signal termination that are covered include:

1. Protein phosphatase activity: protein phosphatases catalyse the dephosphorylation and inactivation of protein kinases/other relay protein, impeding the transduction pathway downstream of affected proteins.
2. Intrinsic GTPase activity of G protein: GTPase rapidly catalyses hydrolysis of bound GTP to GDP, inactivating G-protein
3. Phosphodiesterase activity: enzyme catalyses the conversion of cAMP to AMP, thus decreasing the concentration of cAMP

Question 38

Complete summary of specificity of cell signalling

Answer 38

• A cell responds in specific ways to the specific signals or combination of signals it receives.
• The specific combination of signalling proteins, including receptors, relay proteins in transduction pathways and proteins needed to carry out the response, in a particular cell type gives the cell great specificity in both the signals it detects and the responses it produces.
• Thus two cells that respond differently to the same signal differ in one or more of the proteins that receive, transduce and/or respond to the signal, so that different pathways are activated by the same signal in different cells.