Cell Signalling (ch15) Flashcards
What is an example of a unicellular organism responding to physical and chemical changes in its environment?
Saccharomyces cerevisia take on the “Schmoo” shape in response to chemical mating signal.
What is a gap junction?
A narrow, water-filled channel which directly connects adjacent cells.
What components form a gap junction?
6 connexins from each cell (12 total) form a tube called a connexon.
What differentiates a heteromeric connexon from a homomeric connexon?
Heteromeric: connexins are of different types
Homomeric: connexins are all the same type
What differentiates the structure of a homotypic gap junction from a heterotypic gap junction?
Homotypic: all connexins forming the connexons are of the same type
Heterotypic: connexins which form the connexons are of different types
What is a gap junction called in a plant cell?
A plasmodesmata.
List some examples of signal molecules.
- Proteins
- Peptides
- Amino Acids
- Nucleotides
- Steroids
- Retinoids
- Fatty Acid Derivatives
- Gases
In what 3 ways are extracellular signal molecules exposed to the extracellular space?
- Exocytosis
- Diffusion
- Attached to extracellular side of plasma membrane
What are examples of ways in which cells respond specifically to combinations of signals?
ABC might tell a cell to survive, while ABCD triggers division, ABCE causes differentiation, and no signal at all triggers apoptosis.
What are the 2 main types of receptors involved in cell-signalling?
- Cell-surface receptors
2. Intracellular receptors
How many distinct nuclear receptors do humans have?
48.
What is an “orphan” receptor.
A receptor whose function remains a mystery.
What are the 3 classes of cell-surface receptors?
- Ion-channel-coupled receptors
- G-protein-coupled receptors
- Enzyme-coupled receptors
What are the 4 types of cell signalling?
- Contact-dependent (juxtacrine)
- Paracrine (/autocrine)
- Synaptic
- Endocrine
What is the “fast” way cells react to signalling? What is the “slow” way?
Fast: alter protein function
Slow: alter protein synthesis
How can two different cells have different responses to the same signal ligand?
By having different receptors, signal cascade proteins, effector proteins, or by activating different genes.
What effect does acetylcholine have on a skeletal muscle cell?
Triggers contraction.
What effect does acetylcholine have on a heart pacemaker cell?
Reduces the rate of firing.
What effect does acetylcholine have on a salivary gland cell?
Triggers secretion.
How is it possible for 2 cells of the same type to respond differently to the same ligand?
Because of morphogen gradients which have varied concentrations of inducing or inhibiting factors.
Describe the 1º and 2º response when cells react to a signal by altering protein synthesis.
1º: synthesized protein stimulates response
2º: synthesized protein is transcription factor
What are some examples of small intracellular mediators/2º messengers?
Calcium, cAMP.
What are 3 major characteristics of small intracellular mediators/2º messengers?
- Generated in large numbers
- Diffuse away from source
- Alter protein conformation
What are 3 major characteristics of large intracellular signalling proteins/1º messengers?
- Are molecular switches
- Generate 2º messengers
OR - Activate next signal/effector protein
How is a proteins activation state modified by phosphorylation? What enzymes are involved?
Activation: phosphorylation by protein kinase
Deactivation: de-phosphorylation by protein phosphatase
How many protein kinases exist in the human genome? What about protein phosphatases?
Kinases: 520
Phosphatases: 150
What differentiates signalling by phosphorylation from signalling by GTP-binding?
Phosphorylation: protein is directly phosphorylated
GTP-binding: protein binds to GTP, which is phosphorylated
What 2 types of proteins are activated by GTP-binding?
- Large trimeric GTP-binding proteins (G-proteins)
2. Monomeric GTP-binding proteins
How do scaffold proteins facilitate signal transduction?
It holds the intracellular signal proteins so that they are in position to activate downstream messengers. Decreases cross-talk.
What is a signalling complex? How do they associate with receptors?
Receptor itself can act as a scaffold protein by allowing signalling proteins to bind to phosphorylation sites on the activated receptor.
What is a “transient” signalling complex?
A complex which is rapidly assembled upon activation but which is immediately disassembled.
What is an “all-or-none” signal response?
Exactly how it sounds. If the threshold isn’t met no activation occurs.
What is a “hyperbolic” signal response?
The signal response is initially strong but loses intensity over time.
What is a “sigmoidal” signal response?
The signal response is slow at first, then accelerates rapidly before eventually losing intensity.
In what 5 ways can desensitization occur?
- Receptor sequestration
- Receptor down-regulation
- Receptor inactivation
- Signal protein inactivation
- Production of inhibitory protein
How many types of G-Protein coupled receptors do humans have?
> 800 (150 orphan).
How many types of G-Protein coupled receptors do mice have that are responsible for smell alone?
~1000.
Describe the structure of a G-Protein coupled receptor.
7 transmembrane domains with an extracellular binding site.
Describe the structure of a G-Protein. Which of these structures binds GDP/GTP?
Heterotrimeric, with α, β, and γ subunits. The α subunit binds GDP/GTP.
What percentage of drugs target G-Protein coupled receptors?
~50%.
What protein stimulates the production of cAMP when activated?
Adenylyl cyclase.
What is required for cAMP to produce a rapid response?
Rapid synthesis and rapid breakdown.
Do all G-Protein coupled receptors and G-Proteins stimulate adenylyl cyclase?
No. Some can have an inhibitory effect on adenylyl cyclase.
What are some examples of hormonal responses which are mediated by the G-Protein stimulated increase in cAMP?
Increased concentration of:
- TSH
- ACTH
- LH
- Adrenaline
- Parathormone
- Glucagon
- Vasopressin
Outline the pathway through which G-Protein coupled receptors have downstream non-nuclear effects via cAMP.
GPCR > G-Protein > AC > cAMP > PKA > downstream effects (ion channels, GTPases, target proteins, etc)
Outline the pathway through which G-Protein coupled receptors have downstream nuclear effects via cAMP.
GPCR > G-Protein > AC > cAMP > PKA > (nucleus) > CREB > gene transcription
What are the major results of phospholipase C activation by G-Protein-mediated increase in [cAMP]?
- Stimulates IP₃ to trigger release of Ca2+ from the ER
2. Stimulates diacylglycerol to activate PKC
What protein acts as an intracellular mediator of calcium? How does it interact with calcium?
Calmodulin. Has NH₂ and COOH domains which each bind 2 calcium ions.
How does calmodulin activate other proteins?
Can wrap around target proteins to activate them but only when bound to 4 calcium ions.
What is the structure of the calcium/calmodulin-dependent kinase (CaM-kinase)? What is its purpose?
Two stacked rings with 6 kinase domains. Somehow involved in memory and learning in vertebrates.
How can G-Proteins directly regulate ion channels?
Inhibitory G-Proteins can have an α subunit which inhibits adenylyl cyclase while the βγ subunit opens potassium channels. (example)
How can G-Proteins indirectly regulate ion channels?
By moderating the level of cAMP, which is itself able to open or close ion channels. (example: sodium channels)
In what 3 ways can G-Protein coupled receptors be desensitized?
- Receptor inactivation
- Receptor sequestration
- Receptor downregulation
How do arrestin and GPCR kinase (GRK) interact with G-Protein coupled receptors?
GRK phosphorylates the GPCR across multiple sites, allowing arrestin to bind and prevent further signalling.
What are the 6 types of enzyme-coupled cell-surface receptors?
- Receptor tyrosine kinases (RTKs)
- Tyrosine-kinase associated receptors (cytokine receptors)
- Receptor serine/threonine kinases
- Histidine-kinase-associated receptors
- Receptor guanylyl cyclases
- Receptorlike tyrosine phosphatases
What do all the other receptor tyrosine kinases have in common that insulin receptor (IGF1) does not share?
All the TRKs must dimerize and autophosphorylate each other to activate while the IGF1 receptor is already a dimer.
How is the EGF receptor tryrosine kinase (RTK) different from the other RTKs?
EGF also dimerizes but instead of both RTKs autophosphorylating, one “receiver” does all the work while the “activator” gets phosphorylated for free.
During insulin receptor (IGF1) (receptor tryrosine kinase) activation, what is the function of IRS1?
A scaffold protein which binds to the receptor and has multiple phosphorylation sites to allow binding of other proteins.
What other proteins are associated with IRS1 (an insulin receptor IGF1 scaffold protein).
Grb2, an adaptor protein which binds Sos and another scaffold protein.
Describe the structure of Grb2, an adaptor protein involved in insulin receptor IGF1 activation.
One SH2 domain which binds to IRS1 (docking protein) and two SH3 domains which bind to Sos and scaffold proteins.
What is the function of GTPase Ras? What is it stimulated by? What are its downstream effects?
To mediate receptor tyrosine kinase signalling. Stimulated by Ras-GEF (Sos). Activates MAPKKK (Raf) > MAPKK (Mek) > MAPK (Erk) > altered protein activity and gene expression.
How do you avoid cross-talk between parallel MAP kinase modules in the cell?
Employing scaffold proteins (which may or may not have their own kinase domains).
Why do we need the MAPK pathway?
Because the Ras signal only persists for 3 minutes.
How can receptor tyrosine kinases (RTKs) influence membrane-bound phosphoinositides (PIPs)?
By activating Phosphoinositide 3-kinase which can phosphorylate PIPs and change their binding characteristics with other proteins.
Outline the tyrosine kinase (RTK) and phosphoinositide 3-kinase (PI 3-kinase) pathway.
RTK > PI 3-Kinase > PI(4,5)P₂ becomes PI(3,4,5)P₃ > PDK1 > Akt > mTOR > active Akt > Bad
In the tyrosine kinase (RTK) and phosphoinositide 3-kinase (PI 3-kinase) pathway, what is the function of Bad?
Bad inactivates apoptosis-inhibitory protein. When phosphorylated by Akt, Bad is inactivated and apoptosis-inhibitory protein can function.
What is an example of a tyrosine-kinase associated receptor?
A cytokine receptor or Janus kinase.
What is the downstream effect of a Janus kinase (JAK)? What protein is activated by JAK directly? Where does this protein go?
To alter gene transcription. Activates STAT1 which is translocated to the nucleus as a dimer.
How many protein tyrosine phosphatases do humans have?
~100.
What is the purpose of Notch signalling?
During development, involved in cell differentiation. Signals surrounding cells not to differentiate.
Is the activation of a Notch receptor reversible?
No. Differentiation is a permanent process.
Where are Notch ligands expressed in the body? Where are the receptors?
Notch ligands are expressed on the surface of signalling cells and must come into direct contact with receptors on the surface of target cells.
What protein do SH2 domains bind to?
Short amino acid sequences containing a phosphotyrosine.
Why does CaM-kinase 2 have prolonged activity after activation?
It traps bound calcium/calmodulin until cytosolic levels are normal and it must be dephosphorylated to be inactivated.
Besides the cytoskeleton, what type of proteins make good drug targets?
Scaffold proteins.
What kind of transporter is an acetylcholine receptor at a neuromuscular junction?
An acetylcholine-gated cation channel. Allows Na⁺ into the cell.
What activates a G-protein during cell signalling?
A G-protein coupled receptor (GPCR).
What kind of signalling pathway is activated in response to an odorant?
A GPCR activates a G-protein with downstream effects.
What is remarkable about the Notch signalling pathway?
It is the simplest intracellular pathway which is activated at the membrane and has effects on the nucleus.
