Membrane Receptors and Endocytosis

Question 1

• Cellular signal transduction cassettes

Answer 1

detect, amplify, and integrate external signals Comprised of:
o Specific cell surface membrane receptors
o Effector signaling elements
o Regulatory proteins

Question 2

• Six steps in signal transduction

Answer 2

1. Synthesis of the signaling molecule (ligand)
2. Release of the signaling molecule (ligand)
3. Transport of signaling molecule to target cell
4. Binding of the ligand by a specific receptor protein and change in conformation
5. Change in cellular metabolism, function, or development = cellular response
6. Removal of ligand, terminating cellular response
7. Degradation of ligand

Question 3

Hormones

Answer 3

• Chemical messengers made by endocrine cells and secreted into the bloodstream
o Affect gene expression and protein synthesis

Question 4

Autocrine

Answer 4

act on same cell that secreted them

Question 5

• Paracrine

Answer 5

act on other cells

Question 6

Hormone types

Answer 6

o Steroids
o Amine (amino acid-derived)
o Peptide (usually lumped together with polypeptides)
o Polypeptide

Question 7

Steroid hormones

Answer 7

• 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)

Question 8

Cholesterol precursor

Answer 8

o Corticosteroids
o Androgens
o Estrogens

Question 9

Amine hormones

Answer 9

• Tyrosine or tryptophan derived
• Stored in endocrine cells until secreted
• Receptor can be located on the cell surface or intracellular

Question 10

Polypeptide hormones

Answer 10

• 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

Question 11

‘first messengers

Answer 11

o Bind to external receptors

Question 12

second messengers’

Answer 12

o Low molecular-weight signaling molecules:

• Cyclic adenosine monophosphate (cAMP) or calcium

Question 13

G-protein coupled receptors

Answer 13

• 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

Question 14

• G-protein (guanine nucleotide-binding) activates adenylyl cyclase (converts ATP to cAMP)

Answer 14

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

Question 15

• Adenylyl cyclase

Answer 15

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

Question 16

• Bacterial toxins that target G-proteins:

Answer 16

o Cholera toxin (ADP-ribosylates Gs α-subunit)

o Pertussis toxin (whooping cough) (ADP-ribosylates Gs α-subunit)

Question 17

o Cholera toxin (ADP-ribosylates Gs α-subunit)

Answer 17

• Increase in cAMP within intestinal epithelial cells leads phosphorylation of Cl- channels and efflux of electrolytes and water (severe diarrhea)

Question 18

o Pertussis toxin (whooping cough) (ADP-ribosylates Gs α-subunit)

Answer 18

• Enhances cAMP levels that inhibits neutrophil functions

Question 19

Tyrosine kinase-linked receptors

Answer 19

• No intrinsic enzymatic activity

* Ligand binding forms dimer that activates tyrosine kinases that phosphorylate downstream targets (signal cascade)

Question 20

Intrinsic enzymatic activity receptors

Answer 20

• 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

Question 21

Ion-channel receptors

Answer 21

• 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

Question 22

Botulinum toxin

Answer 22

Prevents release of acetylcholine neurotransmitter and cleaves proteins involved in docking of neurotransmitter vesicles

Question 23

Calcium and calmodulin

Answer 23

• 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

Question 24

Phosphatidylinositol 4,5-bisphosphate (PIP2)

Answer 24

• 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

Question 25

o Inositol trisphosphate (IP3)

Answer 25

• Water soluble, mobilizes calcium

Question 26

o Diacylglycerol (DAG)

Answer 26

• Anchored in plasma membrane due to hydrophobic fatty acid side chains
• Activates key protein kinase C (PKC) family

Question 27

• Phosphatidylcholine

Answer 27

can be hydrolyzed by other phospholipases to produce other lipid second messengers:

Question 28

• Phosphatidylcholine 2nd messengers

Answer 28

o Different species of DAG (generated by PLC)
o Phosphatidic acid (generated by PLD)
o Arachidonic acid (generated by PLA2)

Question 29

Transcytosis

Answer 29

• Movement of receptors to a different membrane from the one in which it was endocytosed

Question 30

Arachidonic acid and prostaglandins

Answer 30

• Key inflammatory and pain mediator
• Precursor of eicosanoids
• Modulate smooth muscle contraction, platelet aggregation, gastric acid secretion, and salt and water balance

Question 31

eicosanoids

Answer 31

o Prostaglandins, prostacyclins, thromboxanes, and leukotrienes
o Act like hormones and signal via G-protein coupled receptors

Question 32

• Arachidonic acid conversion to prostaglandins involves

Answer 32

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

Question 33

• Cyclooxygenase inhibitors

Answer 33

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)

Question 34

• Arachidonic acid also converted into

Answer 34

leukotrienes by lipoxygenases

Question 35

Receptor-independent signaling

Answer 35

• 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

Question 36

Endocytosis

Answer 36

• Packaging of extracellular materials in vesicles at the cell surface
• Requires ATP
• Three types:
-pinocytosis, phagocytosis, receptor mediated endocytosis.

Question 37

o Pinocytosis

Answer 37

drinking”
• Nonspecific absorbtion of extracellular fluids
• Membrane caves in, then pinches off into the cytoplasm as pinocytotic vesicle

Question 38

o Phagocytosis

Answer 38

“eating”

• Activated by attachment to pathogen-associated molecular patterns (PAMPS)

Question 39

o Receptor-mediated endocytosis

Answer 39

• Selective
• Formation of vesicles (containing receptors) at surface of membrane
• Clathrin coated vesicles (forms polyhedral lattice)
• Example: Iron transport via transferring receptor