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Chapter 6
Cytoplasmic Signaling Circuitry Programs
Many of the Traits of Cancer
- 5.9 -- 6.2 ~ 6.9 -
Apr 3 & 12, 2007
5.9 The Ras protein functions as a G protein
- The ras oncogene triggers many of the same changes in cells which are transformed by erbB (truncated EGF-R) or sis (PDGF-B).
- Could Ras be found somewhere downstream of erbB and sis ?
- Do the signals emitted by EGF-R and PDGF-R converge on some common molecule ?
(guanosine diphosphate)(guanosine triphosphate)
GRB2: growth factor receptor- bound protein 2
SH2, SH3: Src homolog 2 or 3
Sos: son of sevenless
Ras
growth factor receptor
Regulation of Ras (a GTPase) activity
GTPase-Activating(or Accelerating) Protein
Guanine nucleotide Exchange Factor
e.g., Sos, which catalyzes conversion of inactive GDP-bound Ras to the active GTP-bound form.
(mitogenic signals)
Regulation of Ras (a GTPase) activity
GTPase-Activating(or Accelerating) Protein
Guanine nucleotide Exchange Factor
e.g., Sos, which catalyzes conversion of inactive GDP-bound Ras to the active GTP-bound form.
(mitogenic signals)
(e.g., H-Ras)
Figure 5.31 The Biology of Cancer (© Garland Science 2007)
The structure of the Ras protein
- Two most frequently altered a.a. residues found in human oncoproteins are glycine 12 and glutamine 61, which are closed associated with the γ-phosphate of GTP.
EGF/EGFR-mediated Ras activation
Two classes of GTPase switch proteins (guanine nucleotide-binding proteins) :
1. trimeric (large) G proteins, which directly bind to and activated by receptors, e.g., Smoothened, Frizzled.
2. monomeric (small) G proteins, such as Ras and various Ras- like proteins. Ras doesn’t bind to receptors. It indirectly links to receptors via adapter proteins (such as GRB2) and GEF proteins (such as Sos).
6.2 The Ras protein stands in the middle of a complex signaling cascade
The story of son of sevenless (sos)- the genetics of eye development in Drosophila
Figure 6.5 The Biology of Cancer (© Garland Science 2007)
In the absence of sevenless, the 7th cell in each ommatidium fail
to form.
6.3 Tyrosine phosphorylation controls the location and thereby the actions of many cytoplasmic signaling proteins
Why is phosphorylation critical?
- protein relocalization.
Figure 6.7a The Biology of Cancer (© Garland Science 2007)
Domain structure of Src protein
catalytic SH1 domain
substrate recognition- proline rich
(SH3 domain)
substrate recognition- p-tyrosine (SH2 domain)
Figure 6.8a The Biology of Cancer (© Garland Science 2007)
Structure and function of SH2 domain
act as a modular plug
Figure 6.9 The Biology of Cancer (© Garland Science 2007)
Signal-transducing proteins attach to
phosphorylated receptors
a phosphotyrosine phosphatase (PTP)
Figure 6.10a The Biology of Cancer (© Garland Science 2007)
Signaling molecules with SH2 and SH3 domains
Figure 6.10b The Biology of Cancer (© Garland Science 2007)
Molecular “ligand domains” and “receptor domains” in order to facilitate intermolecular interactions
Table 6.2 The Biology of Cancer (© Garland Science 2007)
Figure 6.12 The Biology of Cancer (© Garland Science 2007)
6.4 SH2 groups explain how growth factors activate Ras
and acquire signaling
Mammalian Ras proteins have been studied in great detail because mutants Ras proteins are associated with many types of human cancer. These mutant proteins, which bind but cannot hydrolyze GTP, are permanently in the “on” state and contribute to neoplastic transformation.
Most oncogenic, constitutively active Ras protein contain a mutation at position 12. Replacement of the normal glycine-12 with other amino acid blocks the functional binding of GAP, and in essence “lock” Ras in the active GTP-bound state.
Figure 6.11 The Biology of Cancer (© Garland Science 2007)
Sidebar 6.3 The SH2 domain of Src has two alternative functions
inactive dephosphorylation of Y527 phosphorylation of Y416 SH2/SH3 bind to p-receptor exposure of the catalytic cleft
6.5 A cascade of kinases forms one of three important signaling pathways downstream of Ras
Figure 6.14 The Biology of Cancer (© Garland Science 2007)
Figure 6.13 The Biology of Cancer (© Garland Science 2007)
The Ras effector loop can bind Raf, PI3K and
Ral-GEF
Figure 6.14 The Biology of Cancer (© Garland Science 2007)
MEK: MAP/Erk kinase
Erk: extracellular signal- regulated kinases 1 and 2 (Erk1/2)
Ras → Raf → MAP (Mitogen-activated protein) kinase pathway
Raf, MEK and Erk1/2 are serine/threonine kinases
phosphotyrosines attract signaling partners → relocalization of partners
phosphorylation of serines/threonines → shift in structure → functional activation
Ras/Raf/MAP Kinase Pathway
dimer
Figure 6.10b The Biology of Cancer (© Garland Science 2007)
Activated MAPK induces gene transcription
transcription factor TCF: ternary complex factor
SRE:serum response element
transcription factor SRF:serum response factor
(early response gene) → enter the cell cycle
Figure 6.15 The Biology of Cancer (© Garland Science 2007)
The Ras-PI3 kinase (PI3K) pathway
(cytoplasm)
Generation of the 2nd messengers – DAG and IP3
Figure 6.16b The Biology of Cancer (© Garland Science 2007)
PI3K converts PIP2 into PIP3
PI3K can bind to phosphotyrosine and Ras.
Figure 6.19a The Biology of Cancer (© Garland Science 2007)
PH domain: pleckstrin homology
PIP2
Docking of PH domains of Akt/PKB to PIP3
PTEN:phosphatase and tensin homolog
Table 6.3 The Biology of Cancer (© Garland Science 2007)
Figure 6.20 The Biology of Cancer (© Garland Science 2007)
Islet cells expressing constitutively active Akt/PKB. The cells are fourfold larger than normal.
Normal islet cells
AKT/PKB enhances the growth of cells
Table 6.4 The Biology of Cancer (© Garland Science 2007)
Sidebar 6.4 Ras is the prototype of a large family of similar proteins
Small G proteins – 35 similarly structured proteins e.g., Ras, Ral, Rac, Ran, Rho…
- All small G proteins operate like a binary switch, using a GTP-GDP-GTP cycle to flip back and forth between an on and an off state.
- Each small G protein has its own specialized guanine nucleotide exchange factor (GEF) to activate it and its own GTPase-activating proteins (GAP)
Figure 6.21 The Biology of Cancer (© Garland Science 2007)
(Ral guanine nucleotide exchange factors)
(Ral-A and Ral-B are Ras-like proteins)
(Rho family proteins)
6.7 A third Ras-regulated pathway acts through Ral, a distinct cousin of Ras
(control of cytoskeleton)
play key roles in the motility that enables cancer cells to invade and metastasize
Figure 6.22 The Biology of Cancer (© Garland Science 2007)
6.8 The Jak-STAT pathway allows signals to be transmitted from the plasma membrane directly to the nucleus cytoki
nes
Figure 6.24a The Biology of Cancer (© Garland Science 2007)
6.9 Cell adhesion receptors emit signals that converge with those released by growth factor receptors
Figure 6.24b The Biology of Cancer (© Garland Science 2007)