Cell Signaling Flashcards
(74 cards)
1
Q
Secreted signaling proteins allow cells to:
A
- sense their environmnt
- influence the behavior/function of other cells
2
Q
The Signaling Pathway includes:
A
- all the proteins and components that transduce the signal to mediate its effects on the cell.
3
Q
What form of communication underlies organ/tissue physiology and homeostasis in adults, and is fundamental to embryonic development?
A
secreted signaling proteins
4
Q
The two types of responses cells can have to signals?
A
- reversible
- change shape or motility
- irrversible
- divide, differentiate, die
5
Q
How do cells sense their environment?
A
- specialized receptor proteins.
- e.g.
- photreceptors (light)
- mechanoreceptors (stress)
- chemical signals (ligands)
- e.g.
6
Q
A ligand is:
A
- a molecule or protein that triggers a signal by binding to a receptor-like protein.
- can activate or silence a response
7
Q
Specificity of receptor/ligand interaction is governed by:
A
- tertiary (3D) structure and non-covalent bonds between amino acid groups
8
Q
Agonists:
A
induce receptor activation
- Any molecule, protein or drug that occupies ligand-binding sites and stimulates receptor activity (either partially or fully).
9
Q
Antagonists:
A
block receptor activation
- Any molecule, protein or drug that occupies ligand-binding sites and exclude agonists but does not stimulate receptor activity.
10
Q
What are two ways antagonists can function?
A
- bind and block active site from ligand
- bind to allosteric site and change active site conformation
11
Q
About 50% of drugs act on:
A
cell receptors
- The remaining 50% act primarily on enzymes.
12
Q
Abnormal cell-cell signaling underlies:
A
- Cancer
- Neurological disorders
- Metabolic disorders
13
Q
The four major classes of receptors:
A
- ion channels
- steroid hormone receptors
- protein kinase receptors
- 7-alpha-helix-receptors
14
Q
Ion channels are classified by:
A
- nature of their gating: ligands, voltage, mechanical or thermal, phosphorylation, lipids
- species of ions passing through
- number of gates
15
Q
Ion Channels (general):
A
- Pore-forming proteins that allow the flow of ions across membranes down an electrochemical gradient
- Present on cell surface and intracellular organelles
16
Q
How do ligand-gated ion channels function:
A
- Binding of ligand opens a channel to allow flow of a specific ion across the membrane, or closes a channel to stop the flow.
17
Q
What receptor system serves as the basis for nerve transmission and muscle contraction?
A
ligand-gated ion channels
18
Q
Cystic fibrosis is a recessive disease caused by:
A
- loss-of-function mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene which is a chloride (Cl-) channel.
- Mutation leads to abnormal salt transport across epithelial resulting in thick mucus build-up in respiratory epithelial cells.
19
Q
Gain-of-function mutations in ion channels are typically associated with _______ inheritance of the disease.
A
dominant inheritance
- you only need one bad gene copy of the receptor for the receptor to negatively interfere with other normal receptors to give rise to disease
20
Q
Tetrodotoxin:
A
- a sodium channel blocker (irreversible)
- blocks action potentials in nerves
- found in pufferfish or fugu
21
Q
Mutations in ion channels can result in:
A
- loss of function
- no ions pass through
- gain of function
- oligomeric groups form
- other normal channels negatively interfered with
22
Q
Five major types of steroid hormones:
A
PGAME
- Progesterone
- Glucocorticoid
- Androgen
- Mineralocorticoid
- Estrogen
23
Q
Steroid hormones bind to receptors located in:
A
cytosol or nucleus
24
Q
Steroid hormone characteristics:
A
- derived from cholesterol
- function by controlling gene expression
- hydrophobic and can cross the cell membrane
25
Steps in Estrogen Signaling Pathway:
1. Estrogen receptor attached to chaperone protein prior to ligand binding.
2. Estrogen binding induces a conformational change in the receptor that causes dissociation from the chaperone protein.
3. Estrogen receptors dimerize and enter nucleus.
4. Dimerized receptors bind to an estrogen response element (ERE, a DNA promoter) that activates gene transcription.
26
Tamoxifen:
* form of endocrine therapy
* a selective estrogen receptor (i.e., competitive) antagonist
27
How does tamoxifen function?
* tamoxifen metabolized into hydroxytamoxifen (H-tam) in breast tissue
* H-tam binds to the ER and prevents binding of estrogen
* ER/H-tam complex functions represses estrogen target genes via recruitment of transcriptional co-repressors
28
What is a kinase?
**KINASES PHOSPHORYLATE**
* A protein or protein domain with enzymatic activity that transfers phosphate groups from high energy donor molecules such as ATP to specific target molecules.
29
Phosphatase:
* enzyme that removes phosphate groups
30
The three domains of protein kinase receptors:
1. extracellular domain that binds the ligand
2. trans-membrane domain
3. cytoplasmic domain that has kinase activity or binds a protein-kinase protein
31
The two major subfamilies of protein kinase receptors:
* tyrosine kinases
* serine/threonine kinases
**DEFINED BY THE AMINO ACID PHOSPHORYLATED**
32
Steps in protein kinase receptor activation:
* Ligand binds to extracellular domain of each receptor subunit.
* Subunits dimerize, bringing intracellular domains into proximity with each other.
* Intracellular domains phosphorylate and activate each other.
* Additional cytosolic proteins bind phosphorylated receptors (are “recruited” to them)
33
Phosphorylation of protein kinase receptors is:
* reversible (by phosphatases)
* can turn a hydrophobic portion of a protein into a hydrophilic one
* facilitates receptor binding to other proteins/molecules
34
Grb (G-protein receptor binding):
* Binds to phosphorylated receptors.
* Contains an SH2-domain that recognizes tyr-P.
35
SoS (GEF):
"Son of Sevenless"
* Binds to Grb and activates small G-proteins such as Ras.
36
GEFS: Guanine nucleotide exchange factors:
* Activate G-proteins by catalyzing the exchange of GDP (inactive) for GTP (active).
37
GAPS: GTPase activating proteins:
* Promote inactivation of G-proteins by stimulating conversion of GTP (active) to GDP (inactive) via hydrolysis.
38
G-proteins:
* Bind guanine nucleotides and act as a molecular switch during signaling.
39
The Ras nucleotide exchange reaction:
1. Inactive Ras-GDP binds SoS-GEF.
2. SoS-GEF stimulates inactive Ras-GDP to active Ras-GTP.
3. Ras-GTPase activity stimulated by GTPase-activating proteins.
4. Active Ras-GTP converted to inactive Ras-GDP.
40
G-proteins (such as Ras) have intrinsic:
* GTPase activity
* hydroylze GTP → GDP
* i.e. ras is a GTPase
41
ras family:
* small G-proteins
* involved in receptor signaling and cell division
42
What pathway is downstream of ras?
**Ras-MAPK Pathway**
43
Ras-MAPK pathway:
* ras is activated by protein kinase receptor dimerization.
* ras-GTP leads to phosphorylation cascade that ultimately leads to MAP kinase activity.
* MAP kinase activates transcription factors in the nucleus, stimulating gene expression of proteins involved in cell division.
44
MAPK =
mitogen activated protein kinase
* MAPK is only one of many downstream proteins activated by ras-GTP.
45
Ras can be activated by:
multiple ligand/receptor types
46
Ras pathways can influence all of these cell activities:
* Cell growth & division
* Cytoskeleton
* Cell adhesion
* Membrane traffic
* Anti-apoptosis
47
Ras and cancer:
* Ras is one of the most frequently altered proteins in human tumors.
* Oncogenic mutations in ras turn all of its downstream pathways by putting ras in a constitutively active state.
48
Most oncogenic mutations in ras are:
* amino acid substitutions at just two positions: gly-12/gly-13 or gln-61.
* These mutations affect structure of ras and abolish its ability to hydrolyze GTP, so it is always in the active state.
49
Neurofibromatosis Type-1:
* mutation in the NF1 gene.
* encodes neurofibromin-1, a Ras-GAP.
* GAPs catalyze the conversion of GTP to GDP.
* Mutations in GAP genes result in overactive Ras.
50
Noonan Syndrome:
* mutation in the PTPN11 gene which encodes SHP2.
* results in a gain-of-function phenotype which leads to hyperactive Ras.
51
The most abundant class of receptors is:
7-alpha-helix receptors
* 800 in the human genome
* 450 are odorant receptors
52
Most receptor-targeted drugs are directed at what class of receptors?
7-alpha-helix receptors
53
7-alpha-helix receptors are coupled to:
* trimeric large G-proteins whose alpha subunits contain ras-like domains.
* Ligand binding causes exchange of GDP by GTP and activation of the G-protein, initiating signaling cascade.
54
Steps from when a ligand binds to a 7-alpha-helix receptor to downstream effects:
1. ligand binds to 7-alpha-helix receptor
2. 7-alpha-helix receptor hydrolyzes GDP → GTP
3. beta and gamma subunits of large G-protein dissociate from the ligand-bound receptors and regulate gene activity downstream
55
In 7-alpha-helix receptors, what acts as the GEF that exchanges GDP for GTP, thus activating the pathway?
the 7-alpha-helix receptor itself
56
The three main subclasses of large G-protein alpha subunits:
* Gs-alpha
* Gi-alpha
* Gq-alpha
57
Gs-alpha:
* subunit of large G-protein alpha
* activates PKA
58
Gi-alpha:
* subunit of large G-protein alpha
* inhibits PKA
59
Gq-alpha:
* subunit of large G-protein alpha
* activates PKC
60
Steps in β-adrenergic receptor desensitization:
* β-adrenergic receptor: 7-α-helix receptor
* binds the adrenal-gland hormone epinephrine and the neurotransmitter norepinephrine.
* Ligand-bound receptor becomes substrate for β-adrenergic receptor kinase (BARK).
* Phosphorylated receptor is bound by β-arrestin - blocks interaction with Gs.
61
Desensitization:
* terminates signaling even in the presence of the ligand
62
Ligand-bound β-adrenergic receptor is inactivated by:
* a protein kinase (BARK) and β-arrestin
63
Gs-α/Gi-α regulate Ca2+ cell levels via:
* Adenylyl Cyclase (AC) and PKA.
* AC activates cAMP, which activates PKA
* PKA opens calcium channels
* calcium rushes into cell
64
Receptor-mediated calcium influx triggers many events in cells, such as:
* Muscle contraction
* Regulated secretion
* Cell division
65
Gq-proteins activate protein kinase C (PKC) via:
Ca2+ and phospholipase-C (PLC)
66
Steps in Gq-protein activation of protein kinase C (PKC):
1. Gq-α-GTP activates phospholipase C (PLC).
2. PLC hydrolyzes membrane PiP2 into iP3 and diacyl glycerol (DAG).
3. iP3 triggers release of Ca2+ from smoothER into cytoplasm.
4. DAG and Ca2+ activate protein kinase C (PKC).
67
How is protein kinase C inactivated?
* its active site is filled with an N-terminal pseudosubstrate peptide, held in place by its C1 and C2 domains
68
How is protein kinase C (PKC) activated by Ca2+ and DAG?
1. PKC C1 domains bind DAG.
2. In the presence of Ca++ the C2 domain binds phosphatidyl serine (PS).
* This removes the pseudosubstrate from the active site and
3. PKC becomes active; cleaved proteolytically to become constitutively active
69
Most kinases are normally inactive because they are:
* inhibited – receptor activation frees them from inhibition...this occurs in distinct ways for different kinases
70
Calmodulin:
* a calcium-binding protein that regulates the activity of many proteins
* binds four Ca2+ molecules
71
Calmodulin is closely related to:
* the calcium-binding subunit of the protein troponin which regulates muscle contraction
72
Mechanism of calmodulin-activated protein kinase (CAMK) activation:
1. CAMK – active site blocked by an inhibitory domain.
2. Ca2+/calmodulin binds to inhibitory domain of CAMK. Active site of CAMK becomes accessible.
73
CAMKII is an important mediator of:
* learning and memory
* has been implicated in AD
* Other CAMK proteins are involved in cancer and musculoskeletal diseases.
74
The ras protein family includes:
* **ras** family: receptor signaling and cell division.
* **rab** family: traffic of membrane vesicles.
* **ran** family: nucleus/cytoplasm traffic.
* **rac** family: actin cytoskeleton.
