Membrane Receptors and Endocytosis Flashcards
• Cellular signal transduction cassettes
detect, amplify, and integrate external signals Comprised of:
o Specific cell surface membrane receptors
o Effector signaling elements
o Regulatory proteins
• Six steps in signal transduction
- Synthesis of the signaling molecule (ligand)
- Release of the signaling molecule (ligand)
- Transport of signaling molecule to target cell
- Binding of the ligand by a specific receptor protein and change in conformation
- Change in cellular metabolism, function, or development = cellular response
- Removal of ligand, terminating cellular response
- Degradation of ligand
Hormones
• Chemical messengers made by endocrine cells and secreted into the bloodstream
o Affect gene expression and protein synthesis
Autocrine
act on same cell that secreted them
• Paracrine
act on other cells
Hormone types
o Steroids
o Amine (amino acid-derived)
o Peptide (usually lumped together with polypeptides)
o Polypeptide
Steroid hormones
- Immediately diffuse out of endocrine cells into bloodstream
- Lipid soluble (can cross cell membrane)
- Intracellular (cytoplasmic) receptors located inside target cells
- Slower acting/longer half-life than peptide hormones
- (know the general steps of how hormones lead to signal cascades)
Cholesterol precursor
o Corticosteroids
o Androgens
o Estrogens
Amine hormones
- Tyrosine or tryptophan derived
- Stored in endocrine cells until secreted
- Receptor can be located on the cell surface or intracellular
Polypeptide hormones
• Do not immediately enter bloodstream (can be stored in endocrine cell vesicles)
• Water soluble
• Do not readily pass through cell membrane
• Termed ‘first messengers‘
o Bind to external receptors
• Intracellular effects are mediated by “second messengers’ – internal reactions
o Low molecular-weight signaling molecules:
• Cyclic adenosine monophosphate (cAMP) or calcium
‘first messengers
o Bind to external receptors
second messengers’
o Low molecular-weight signaling molecules:
• Cyclic adenosine monophosphate (cAMP) or calcium
G-protein coupled receptors
- Integral membrane proteins (extracellular N-terminus)
- Seven transmembrane-spanning α-helices (ligand binds to pocket)
- Three extracellular and intracellular loops (third intracellular loop recruits G-proteins)
- Intracellular C-terminal tail membrane
- No intrinsic catalytic domains
• G-protein (guanine nucleotide-binding) activates adenylyl cyclase (converts ATP to cAMP)
o Activates protein kinases in the cytosol (signal cascade)
o Phosphodiesterase inactivates cAMP (turns off cell response)
o Specificity conferred by the α-subunit, which contains the GTP-binding site and an intrinsic GTPase activity
• Adenylyl cyclase
o Activated by the action of the α-subunit of the G-protein (Gs)
o Each molecule of bound hormone can stimulate many Gs α-subunits that amplifies the original hormone signal
• Bacterial toxins that target G-proteins:
o Cholera toxin (ADP-ribosylates Gs α-subunit)
o Pertussis toxin (whooping cough) (ADP-ribosylates Gs α-subunit)
o Cholera toxin (ADP-ribosylates Gs α-subunit)
• Increase in cAMP within intestinal epithelial cells leads phosphorylation of Cl- channels and efflux of electrolytes and water (severe diarrhea)
o Pertussis toxin (whooping cough) (ADP-ribosylates Gs α-subunit)
• Enhances cAMP levels that inhibits neutrophil functions
Tyrosine kinase-linked receptors
- No intrinsic enzymatic activity
* Ligand binding forms dimer that activates tyrosine kinases that phosphorylate downstream targets (signal cascade)
Intrinsic enzymatic activity receptors
• Ligand-triggered protein kinases
• Similar to tyrosine-linked receptors (form dimers upon ligand binding)
• Ligand/receptor complex directly acts as a tyrosine kinase (phosphorylates other kinases)
o Phosphorylation activates other protein kinases
Ion-channel receptors
• Ligand binding changes confirmation of receptor
o Allows specific ions (sodium, potassium) to flow through channel
• Bacterial toxin that affects ion-channels (not directly)
o Botulinum toxin
• Prevents release of acetylcholine neurotransmitter and cleaves proteins involved in docking of neurotransmitter vesicles
Botulinum toxin
Prevents release of acetylcholine neurotransmitter and cleaves proteins involved in docking of neurotransmitter vesicles
Calcium and calmodulin
• Cells maintain steep intracellular (100 nM)/ extracellular (1 mM) Ca2+ concentration gradient that enables rapid changes in Ca2+ concentration via hormone ligation
• Calcium binds to calmodulin protein inducing conformational change
o Two globular domains joined by a long α-helix
• Calcium/calmodulin complex binds to and modifies target proteins (kinases) that initiate signal cascade
Phosphatidylinositol 4,5-bisphosphate (PIP2)
• Second messenger responsible for calcium mobilization
• Hydrolyzed by a PIP2-specific phospholipase C (PLC), to generate two second messengers:
o Inositol trisphosphate (IP3) and o Diacylglycerol (DAG
o Inositol trisphosphate (IP3)
• Water soluble, mobilizes calcium
o Diacylglycerol (DAG)
- Anchored in plasma membrane due to hydrophobic fatty acid side chains
- Activates key protein kinase C (PKC) family
• Phosphatidylcholine
can be hydrolyzed by other phospholipases to produce other lipid second messengers:
• Phosphatidylcholine 2nd messengers
o Different species of DAG (generated by PLC)
o Phosphatidic acid (generated by PLD)
o Arachidonic acid (generated by PLA2)
Transcytosis
• Movement of receptors to a different membrane from the one in which it was endocytosed
Arachidonic acid and prostaglandins
- Key inflammatory and pain mediator
- Precursor of eicosanoids
- Modulate smooth muscle contraction, platelet aggregation, gastric acid secretion, and salt and water balance
eicosanoids
o Prostaglandins, prostacyclins, thromboxanes, and leukotrienes
o Act like hormones and signal via G-protein coupled receptors
• Arachidonic acid conversion to prostaglandins involves
cyclooxygenase isoforms
o COX-1 (constitutive) and COX-2 (response to inflammatory mediators)
o Stimulate inflammation, regulate blood flow to organs such as the kidney, control ion transport across membranes, modulate synaptic transmission, and induce sleep
• Cyclooxygenase inhibitors
o Nonsteroidal anti-inflammatory drugs (NSAIDs)
• Aspirin and Ibuprofen
o Block cyclooxygenase conversion to prostaglandins.
o Aspirin irreversibly inactivates both forms
o Selective inhibitors of COX-2 (celecoxib) are effective treatments for inflammatory conditions (rheumatoid arthritis)
• Arachidonic acid also converted into
leukotrienes by lipoxygenases
Receptor-independent signaling
• Low molecular-weight signaling molecules (e.g., nitric oxide (NO)) that cross plasma membrane and directly modulate the activity of the catalytic domains of transmembrane receptors or cytoplasmic signal transducing enzymes
• NO stimulates guanylate cyclase (generates cGMP (relaxes blood vessels))
o Angina symptoms treated with glyceryl trinitrate, which is converted to NO
Endocytosis
• Packaging of extracellular materials in vesicles at the cell surface
• Requires ATP
• Three types:
-pinocytosis, phagocytosis, receptor mediated endocytosis.
o Pinocytosis
drinking”
• Nonspecific absorbtion of extracellular fluids
• Membrane caves in, then pinches off into the cytoplasm as pinocytotic vesicle
o Phagocytosis
“eating”
• Activated by attachment to pathogen-associated molecular patterns (PAMPS)
o Receptor-mediated endocytosis
- Selective
- Formation of vesicles (containing receptors) at surface of membrane
- Clathrin coated vesicles (forms polyhedral lattice)
- Example: Iron transport via transferring receptor
