Cell Signaling II (CH 16) Flashcards
TGFß
transforming growth factor beta
TGFß function
important in development/differentiation (i.e. growth arrest); binds cell surface receptors and induces the expression of cell cycle arrest proteins
TGFß formation
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
TGF receptors
R1, R2, and R3
R1
TGF receptor that is inactive in resting state
R2
TGF receptor that is constitutively auto-phosphorylated
R3
TGF receptor that has no kinase domain
R-Smads
activity is regulated by receptors (Smads 1, 2, and 3)
Co-Smads
associate with R-Smads (Smad 4)
I-Smads
inhibitory Smads (Smad 7)
SH2 Domain
Smad domain that binds to phosphorylated tyrosines of other Smads to form a complex
Smad complex components
2 R-Smads, 1 Co-Smad (Smad 4), and Importin
NLS
nuclear localization signal - binds to importn
TGFß Pathway
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
regulatory mechanisms of TGFß signaling
Ski-Transcriptional Co-Repressor Protein and Inhibitory Smad (Smad 7)
Ski-Transcriptional Co-Repressor Protein
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)
Inhibitory Smad (Smad 7)
blocks the ability of TGF receptors to phosphorylate R-Smads
effect of cancer in TGFß signaling
mutations to receptors or Smads causes loss of growth inhibition; in pancreatic cancer, Smad4 is deleted (DPC) completely halting the TGFß pathway
cytokine receptors
transmembrane molecule associated on the cytoplasmic side with JAK family kinases
cytokine receptors signal transduction
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.
receptor tyrosine kinases (RTK)
growth factor receptor with inherent kinase domain
RTK signal transduction
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.
JAK
just another kinase; tyrosine kinase that is not intrinsic to a receptor; associate with cytokine receptors
STAT
signal transducers and activators of transcription; has N-terminal SH2 domain, middle DNA binding domain, and C-terminal tyrosine
JAK-STAT pathway
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.
docking proteins
recognize and bind to phospho-tyrosines on receptor, build up protein complex to amplify downstream signaling
regulation of JAK signaling
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.
Ras
small GTPase bound to membrane through covalently attached isoprenoid lipid group, rests GDP bound
Ras activation
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.
GRB2
adaptor protein with one SH2 domain and two SH3 domains
Sos
GEF; proline-rich domain interacts with GRB2 at SH3 domain
receptors that activate Ras through GRB2 and Sos
All RTKs and most cytokine receptors
Ras MAPK pathway
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.
MAP
mitogen activated protein; induces cell proliferation
MAP2K dual specificity
has both serine/threonine kinase domain and tyrosine kinase domain
MAPK requirement
requires phosphorylation on both threonine and tyrosine to be fully activated
MAPK activation of gene expression
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.
receptors that can initiate MAPK pathway
RTK, cytokine receptors, and GPCR
c-fos
prevalent early response gene; associates with c-Jun to regulate expression of cell cycle progression genes
scaffold proteins
hold together multiple proteins to make signaling down pathway faster; prevents cross-talk between signal transduction pathways
