Cell Signaling (Lecture 9)

Question 1

Types of cell signaling

Answer 1

• Same cell–> autocrine
• Direct contact to an adjoining cell–> eg, gap junctions (Ca2+) or membrane-enclosed ligands (antigen presentation)
• cell is close proximity–> paracrine
• far-away cell –> endocrine

Question 2

Complexity and Flexibility of Cell Signaling

Answer 2

1. Same receptor can stimulate different pathways in different cells
2. Within a cell, a receptor can stimulate multiple pathways
3. Signal strength can be altered along pathway
4. Signals from multiple receptors can be integrated to produce a cellular response

Question 3

Intracellular receptors

Answer 3

• In cytoplasm or nucleus
• Often inactive by inhibitory, heat shock, or chaperone protein
• Bind to small hydrophobic ligands that travel through bld and pass through membrane
  
  • When bound, blocking protein is displaced
  • Complex can bind directly to DNA + cause gene expression change
• Ie, estrogen binds to estrogen receptor

Question 4

Cell-surface receptors

Answer 4

enzyme-coupled receptors & 7-helix transmembrane receptors

(RTKs) (GPCRs)

• Also includes ligand-gated ion channels

Both signal with two signaling molecule types:

2nd messengers and signaling proteins

Question 5

Second messengers

vs

Signaling Proteins

Answer 5

Question 6

SH2 (Src homology 2) domain

recognition motif

Answer 6

Binds phosphotyrosine

Question 7

SH3 (Src homology 3) domain

recognition motif

Answer 7

Binds proline-rich sequences

Question 8

PTB domain

recognition motif

Answer 8

(Phosphotyrosine binding)

Binds phosphotyrosine

Question 9

PH (plexitrin homology) domain

recognition motif

Answer 9

Binds lipids (eg, phosphatidylinositol bi-and triphosphates)

Question 10

Ways signaling proteins can act as molecular switches

Answer 10

1. Allosteric regulation: conformational change in a protein d/t binding another molecule
2. Phosphorylation of an animo acid (Tyr, Ser, Thr) by a protein kinase
   * Dephosphorylation by a phosphatase
3. Phosphorylation w/ GEF (exchanging bound GDP–> GTP)
   * Loss of phosphorylation: GTP hydrolysis by a GTPase or GTPase Activating Protein (GAP)

Question 11

Scaffold proteins

Answer 11

• Bind several signaling proteins to bring into close proximity
• Association with a scaffold protein/post translational modification can localize signaling to a part fof the cell
  
  • Ie, lipid groups, prenylation / fatty acid acylation links proteins to the interal surface of the plasma membrane

Question 12

Initial Steps of Receptor Tyrosine Kinase Signaling

Answer 12

• RTKs are single pass transmembrane proteins with an extracellular ligand-binding domain and an intracellular protein kinase domain*
  1. Ligand binding: ligands can be monomers or dimers
• Causes conformational changes
• Can also cause dimerization, activating kinase and leading to transphosphorylation

2. Tyrosine residues in the intracellular domain are phosphorylated, creating docking sites for SH2 and PTB domain proteins (this activates downstream signaling pathways)
   * Because RTKs have several P-Tyr residues, it can interact with different signaling proteins at the same time

Question 13

MAPK-ERK Signaling Pathway

(Mitogen Activated Protein Kinase)

(Extracellular Signal-Regulated Kinase)

Answer 13

• One of 2 major signaling pathways stimulated by EGF
• Every MAPK signaling pathway consists of 3 parts: MAP kinase kinase kinase, MAP kinase kinase, MAP kinase

1. EGF binds to = activates its receptor, EGFR
2. GRB2 binds to the activated receptor at SH2 domain, plasma membrane recruits SOS
3. SOS (a GEF) facilitates binding of GTP to Ras, activating it (Ras-GTP active; Ras-GDP inactive)
4. Tethered to cell membrane, Ras-GTP phosphorylates/activates Raf (MAP kinase kinase kinase)
5. Raf phosphorylates/activates MEK (MAPK/Erk kinase), which phosphorylates/activates ERK (MAP kinase)
6. ERK has targets in cytoplasm and can translocate into the nucleus, where it phosphorylates TFs, leading to transcription of target genes

Question 14

Termination of the MAPK-ERK pathway

Answer 14

• Spontaneous hydrolysis of GTP to GDP inactivates Ras
• Protein phosphatases (Tyr and Ser/Thr) dephosphorylate and deactivate every component of the signaling pathway
• Internalization of the receptor (via endocytosis) and sorting to lysosomes for degradation

Question 15

Signaling of G-Protein Coupled Receptors (GPCRs)

Answer 15

• Largest family of receptors; 7 pass transmembrane proteins that transduce extracellular signals by activation of heterotrimeric GTP-binding protein (G protein)
  
  • consist of three subunits: ß, alpha (several isoforms) gamma

1. In absence of a ligand, some GPCRs associate with the G protein, others need receptor activation.
   * alpha subunit is bound to GTP
2. Ligand binds, GPCR interacts with G (3rd intracell loop)

• ßy subunits complex at membrane (C terminus)
• alpha subunit (loosely associated with ßy) anchors at N

3. GPCR works as GEF, exchanges GDP for GTP at alpha subunit
4. G protein dissociates from receptor (also alpha subunit from ßy complex). Both subunit + complex can bind to downstream targets (effectors) and activate signaling pathways

Eventually, the alpha subunit hydrolyzes GTP by RGS (Reg G protein sigaling; a GAP for alpha subunit)

• alpha subunit and ßy re-join, signaling terminates