Cell Signaling Flashcards
Secreted signaling proteins allow cells to:
- sense their environmnt
- influence the behavior/function of other cells
The Signaling Pathway includes:
- all the proteins and components that transduce the signal to mediate its effects on the cell.
What form of communication underlies organ/tissue physiology and homeostasis in adults, and is fundamental to embryonic development?
secreted signaling proteins
The two types of responses cells can have to signals?
- reversible
- change shape or motility
- irrversible
- divide, differentiate, die
How do cells sense their environment?
- specialized receptor proteins.
- e.g.
- photreceptors (light)
- mechanoreceptors (stress)
- chemical signals (ligands)
- e.g.
A ligand is:
- a molecule or protein that triggers a signal by binding to a receptor-like protein.
- can activate or silence a response
Specificity of receptor/ligand interaction is governed by:
- tertiary (3D) structure and non-covalent bonds between amino acid groups
Agonists:
induce receptor activation
- Any molecule, protein or drug that occupies ligand-binding sites and stimulates receptor activity (either partially or fully).
Antagonists:
block receptor activation
- Any molecule, protein or drug that occupies ligand-binding sites and exclude agonists but does not stimulate receptor activity.
What are two ways antagonists can function?
- bind and block active site from ligand
- bind to allosteric site and change active site conformation
About 50% of drugs act on:
cell receptors
- The remaining 50% act primarily on enzymes.
Abnormal cell-cell signaling underlies:
- Cancer
- Neurological disorders
- Metabolic disorders
The four major classes of receptors:
- ion channels
- steroid hormone receptors
- protein kinase receptors
- 7-alpha-helix-receptors
Ion channels are classified by:
- nature of their gating: ligands, voltage, mechanical or thermal, phosphorylation, lipids
- species of ions passing through
- number of gates
Ion Channels (general):
- Pore-forming proteins that allow the flow of ions across membranes down an electrochemical gradient
- Present on cell surface and intracellular organelles
How do ligand-gated ion channels function:
- Binding of ligand opens a channel to allow flow of a specific ion across the membrane, or closes a channel to stop the flow.
What receptor system serves as the basis for nerve transmission and muscle contraction?
ligand-gated ion channels
Cystic fibrosis is a recessive disease caused by:
- 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.
Gain-of-function mutations in ion channels are typically associated with _______ inheritance of the disease.
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
Tetrodotoxin:
- a sodium channel blocker (irreversible)
- blocks action potentials in nerves
- found in pufferfish or fugu
Mutations in ion channels can result in:
- loss of function
- no ions pass through
- gain of function
- oligomeric groups form
- other normal channels negatively interfered with
Five major types of steroid hormones:
PGAME
- Progesterone
- Glucocorticoid
- Androgen
- Mineralocorticoid
- Estrogen
Steroid hormones bind to receptors located in:
cytosol or nucleus
Steroid hormone characteristics:
- derived from cholesterol
- function by controlling gene expression
- hydrophobic and can cross the cell membrane
Steps in Estrogen Signaling Pathway:
- Estrogen receptor attached to chaperone protein prior to ligand binding.
- Estrogen binding induces a conformational change in the receptor that causes dissociation from the chaperone protein.
- Estrogen receptors dimerize and enter nucleus.
- Dimerized receptors bind to an estrogen response element (ERE, a DNA promoter) that activates gene transcription.
Tamoxifen:
- form of endocrine therapy
- a selective estrogen receptor (i.e., competitive) antagonist
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
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.
Phosphatase:
- enzyme that removes phosphate groups
The three domains of protein kinase receptors:
- extracellular domain that binds the ligand
- trans-membrane domain
- cytoplasmic domain that has kinase activity or binds a protein-kinase protein
The two major subfamilies of protein kinase receptors:
- tyrosine kinases
- serine/threonine kinases
DEFINED BY THE AMINO ACID PHOSPHORYLATED
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)
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
Grb (G-protein receptor binding):
- Binds to phosphorylated receptors.
- Contains an SH2-domain that recognizes tyr-P.
SoS (GEF):
“Son of Sevenless”
- Binds to Grb and activates small G-proteins such as Ras.
GEFS: Guanine nucleotide exchange factors:
- Activate G-proteins by catalyzing the exchange of GDP (inactive) for GTP (active).
GAPS: GTPase activating proteins:
- Promote inactivation of G-proteins by stimulating conversion of GTP (active) to GDP (inactive) via hydrolysis.
G-proteins:
- Bind guanine nucleotides and act as a molecular switch during signaling.
The Ras nucleotide exchange reaction:
- Inactive Ras-GDP binds SoS-GEF.
- SoS-GEF stimulates inactive Ras-GDP to active Ras-GTP.
- Ras-GTPase activity stimulated by GTPase-activating proteins.
- Active Ras-GTP converted to inactive Ras-GDP.
G-proteins (such as Ras) have intrinsic:
- GTPase activity
- hydroylze GTP → GDP
- i.e. ras is a GTPase
ras family:
- small G-proteins
- involved in receptor signaling and cell division
What pathway is downstream of ras?
Ras-MAPK Pathway
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.
MAPK =
mitogen activated protein kinase
- MAPK is only one of many downstream proteins activated by ras-GTP.
Ras can be activated by:
multiple ligand/receptor types
Ras pathways can influence all of these cell activities:
- Cell growth & division
- Cytoskeleton
- Cell adhesion
- Membrane traffic
- Anti-apoptosis
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.
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.
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.
Noonan Syndrome:
- mutation in the PTPN11 gene which encodes SHP2.
- results in a gain-of-function phenotype which leads to hyperactive Ras.
The most abundant class of receptors is:
7-alpha-helix receptors
- 800 in the human genome
- 450 are odorant receptors
Most receptor-targeted drugs are directed at what class of receptors?
7-alpha-helix receptors
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.
Steps from when a ligand binds to a 7-alpha-helix receptor to downstream effects:
- ligand binds to 7-alpha-helix receptor
- 7-alpha-helix receptor hydrolyzes GDP → GTP
- beta and gamma subunits of large G-protein dissociate from the ligand-bound receptors and regulate gene activity downstream
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
The three main subclasses of large G-protein alpha subunits:
- Gs-alpha
- Gi-alpha
- Gq-alpha
Gs-alpha:
- subunit of large G-protein alpha
- activates PKA
Gi-alpha:
- subunit of large G-protein alpha
- inhibits PKA
Gq-alpha:
- subunit of large G-protein alpha
- activates PKC
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.
Desensitization:
- terminates signaling even in the presence of the ligand
Ligand-bound β-adrenergic receptor is inactivated by:
- a protein kinase (BARK) and β-arrestin
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
Receptor-mediated calcium influx triggers many events in cells, such as:
- Muscle contraction
- Regulated secretion
- Cell division
Gq-proteins activate protein kinase C (PKC) via:
Ca2+ and phospholipase-C (PLC)
Steps in Gq-protein activation of protein kinase C (PKC):
- Gq-α-GTP activates phospholipase C (PLC).
- PLC hydrolyzes membrane PiP2 into iP3 and diacyl glycerol (DAG).
- iP3 triggers release of Ca2+ from smoothER into cytoplasm.
- DAG and Ca2+ activate protein kinase C (PKC).
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
How is protein kinase C (PKC) activated by Ca2+ and DAG?
- PKC C1 domains bind DAG.
- In the presence of Ca++ the C2 domain binds phosphatidyl serine (PS).
- This removes the pseudosubstrate from the active site and
- PKC becomes active; cleaved proteolytically to become constitutively active
Most kinases are normally inactive because they are:
- inhibited – receptor activation frees them from inhibition…this occurs in distinct ways for different kinases
Calmodulin:
- a calcium-binding protein that regulates the activity of many proteins
- binds four Ca2+ molecules
Calmodulin is closely related to:
- the calcium-binding subunit of the protein troponin which regulates muscle contraction
Mechanism of calmodulin-activated protein kinase (CAMK) activation:
- CAMK – active site blocked by an inhibitory domain.
- Ca2+/calmodulin binds to inhibitory domain of CAMK. Active site of CAMK becomes accessible.
CAMKII is an important mediator of:
- learning and memory
- has been implicated in AD
- Other CAMK proteins are involved in cancer and musculoskeletal diseases.
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.
