Cell Signaling II (CH 16)

Question 1

TGFß

Answer 1

transforming growth factor beta

Question 2

TGFß function

Answer 2

important in development/differentiation (i.e. growth arrest); binds cell surface receptors and induces the expression of cell cycle arrest proteins

Question 3

TGFß formation

Answer 3

precursor is secreted from cell and pro domain sequestered by LTBP (latent TGFß binding protein); 2 precursors associate by disulfide bonds; mature domain hetero- or homo-dimer is cleaved; mature TGFß is able to bind TGF receptor

Question 4

TGF receptors

Answer 4

R1, R2, and R3

Question 5

R1

Answer 5

TGF receptor that is inactive in resting state

Question 6

R2

Answer 6

TGF receptor that is constitutively auto-phosphorylated

Question 7

R3

Answer 7

TGF receptor that has no kinase domain

Question 8

R-Smads

Answer 8

activity is regulated by receptors (Smads 1, 2, and 3)

Question 9

Co-Smads

Answer 9

associate with R-Smads (Smad 4)

Question 10

I-Smads

Answer 10

inhibitory Smads (Smad 7)

Question 11

SH2 Domain

Answer 11

Smad domain that binds to phosphorylated tyrosines of other Smads to form a complex

Question 12

Smad complex components

Answer 12

2 R-Smads, 1 Co-Smad (Smad 4), and Importin

Question 13

NLS

Answer 13

nuclear localization signal - binds to importn

Question 14

TGFß Pathway

Answer 14

TGFß binds R2 or R3 at the kinase domain, R2 and R3 recruit R1 to form complex, R2 activates kinase on R1, R1 phosphorylates R-Smad (Smad3), R-Smad unfolds and joins to other R-Smad and Co-Smad at the SH2 Domain, NLS binds to Importin-ß, Imp-ß moves Smad complex into nucleus through nuclear pore (Ran-GTP dependent), Smad complex binds DNA in promotor of TGF responsive genes, initiates transcription, dephospho rylation ends transcroption

Question 15

regulatory mechanisms of TGFß signaling

Answer 15

Ski-Transcriptional Co-Repressor Protein and Inhibitory Smad (Smad 7)

Question 16

Ski-Transcriptional Co-Repressor Protein

Answer 16

Early in TGFß signaling Ski proteins are degraded, levels later increase to repress transcription, Ski (and complex) interacts with Smad4 and deacetylates histone residues in promotor to shut down transcription (by blocking the recruitment of transcription machinery)

Question 17

Inhibitory Smad (Smad 7)

Answer 17

blocks the ability of TGF receptors to phosphorylate R-Smads

Question 18

effect of cancer in TGFß signaling

Answer 18

mutations to receptors or Smads causes loss of growth inhibition; in pancreatic cancer, Smad4 is deleted (DPC) completely halting the TGFß pathway

Question 19

cytokine receptors

Answer 19

transmembrane molecule associated on the cytoplasmic side with JAK family kinases

Question 20

cytokine receptors signal transduction

Answer 20

Ligand binding induces dimerization of subunits. JAKs cross-phosphorylate on activation lip. JAKs bind substrate and ATP efficiently, also phosphorylate tyrosine residues on cytoplasmic tail of receptor. JAK-STAT pathway follows.

Question 21

receptor tyrosine kinases (RTK)

Answer 21

growth factor receptor with inherent kinase domain

Question 22

RTK signal transduction

Answer 22

Ligand binding induces dimerization of subunits. Kinases cross-phosphorylate on activation lip of kinase domain, fully activating kinases. Kinase domains phosphorylate own tyrosines on cytoplasmic side, allowing efficient binding of substrate and ATP. Phosphorylated tyrosine function as docking sites for other proteins. Receptor binds proteins with SH2 or PTB domain at phospho-tyrosine. Tyrosine kinase phosphorylates recruited protein in order to send signals downstream.

Question 23

JAK

Answer 23

just another kinase; tyrosine kinase that is not intrinsic to a receptor; associate with cytokine receptors

Question 24

STAT

Answer 24

signal transducers and activators of transcription; has N-terminal SH2 domain, middle DNA binding domain, and C-terminal tyrosine

Question 25

JAK-STAT pathway

Answer 25

STAT SH2 domain binds phospho-tyrosine, JAK phosphorylates tyrosine residue, STAT releases SH2 domain and dimerizes to reveal NLS. Importin binds NLS and moves STAT dimer into nucleus, binds promotor, and initiates gene transcription.

Question 26

docking proteins

Answer 26

recognize and bind to phospho-tyrosines on receptor, build up protein complex to amplify downstream signaling

Question 27

regulation of JAK signaling

Answer 27

Short-term: SHP1 phophotase binds to docking site, opens, and dephosphorylates JAK.
Long-term: SOCS (supressor of cytokine signaling) protein; has SH2 domain and SOCS box; transduced by activated STATs, SH2 domain binds to phospho-tyr on JAK and receptor, SOCS box recruits E3 ubiquitin ligase to signal degradation of JAK and receptor.

Question 28

Ras

Answer 28

small GTPase bound to membrane through covalently attached isoprenoid lipid group, rests GDP bound

Question 29

Ras activation

Answer 29

Hormone (EGF) binds RTK, inducing dimerization and phosphorylation of tyrosine residues, GRB2 docks to phospho-tyr at SH2 domain, Sos binds GRB2 at SH3 domain and becomes activated, Sos binds Ras, releasing GDP to replace with GTP, Ras is activated and released to interact with downstream effectors.

Question 30

GRB2

Answer 30

adaptor protein with one SH2 domain and two SH3 domains

Question 31

Sos

Answer 31

GEF; proline-rich domain interacts with GRB2 at SH3 domain

Question 32

receptors that activate Ras through GRB2 and Sos

Answer 32

All RTKs and most cytokine receptors

Question 33

Ras MAPK pathway

Answer 33

Active Ras phosphorylates MAP3K Raf, releasing it from protein 14-3-3. Raf phosphorylates MAP2K MEK. MEK phosphorylates MAPK ERK. ERK dimerizes and moves to nucleus to phosphorylate (activate) transcription factors.

Question 34

MAP

Answer 34

mitogen activated protein; induces cell proliferation

Question 35

MAP2K dual specificity

Answer 35

has both serine/threonine kinase domain and tyrosine kinase domain

Question 36

MAPK requirement

Answer 36

requires phosphorylation on both threonine and tyrosine to be fully activated

Question 37

MAPK activation of gene expression

Answer 37

In cytoplasm, MAPK activates kinase p90RSK. p90RSK moves into nucleus and phosphorylates transcription factor SRF (serum response factor).
MAPK dimer moves through nuclear pore and activates transcription factor TCF (ternary complex factor).
TCF and SRF form a complex that binds to promotor region SRE (serum response elements) that promote early response genes. Early response genes (usually transcription factors) go on to initiate transcription of DNA replication and proliferation genes.

Question 38

receptors that can initiate MAPK pathway

Answer 38

RTK, cytokine receptors, and GPCR

Question 39

c-fos

Answer 39

prevalent early response gene; associates with c-Jun to regulate expression of cell cycle progression genes

Question 40

scaffold proteins

Answer 40

hold together multiple proteins to make signaling down pathway faster; prevents cross-talk between signal transduction pathways