Lecture 16: Signalling through enzyme-linked cell surface receptors

Question 1

Receptor tyrosine kinases

Answer 1

• Receptors for a variety of growth factors and hormones • Epidermal growth factor (EGF), platelet derived growth factor (PDGF), fibroblast growth factor (FGF), hepatocyte growth factor (HGF), insulin, nerve growth factor (NGF) etc.

Question 2

Receptor tyrosine kinase activation

Answer 2

• Extracellular ligand(s) bind • Receptor oligomerizes – Usually dimerization • Receptor oligomerization causes enzymatic activation of kinase – Usually autophosphorylation • Autophosphorylated receptor transmits downstream signals – Usually by phosphorylating other protein

Question 3

General receptor tyrosine kinase activation

Answer 3

• Ligand is a dimer • Ligand will only bind to a dimer interface (EGF, not shown) • Ligand is oligomerized by association with other molecules

Question 4

Protein interaction “modules” involved in kinase signal transduction cascades

Answer 4

• SH2 domain • Src Homology 2 • Phosphotyrosine binding domain • Flanking amino acids help determine specificity • PTB domain • Phosphotyrosine binding domain • SH3 domain • Src Homology 3 • Binds proline rich motifs • PH domain • Pleckstrin Homology domain • Recognizes and associated with membrane lipid • PI 3,4,5 trisphosphate (PIP3)

Question 5

Activation of Ras

Answer 5

• Small GTP-binding protein • Important for broadcasting many signaling pathways • Many receptor tyrosine kinases modulate the activity of Ras

Question 6

MAP-kinases

Answer 6

• Mitogen Activated Protein Kinase – A mitogen is a signal that tells a cell to divide, compared to a growth factor which mainly stimulates cell growth • Form a signaling module – MAP-kinase-kinase-kinase (Raf-1) – MAP kinase (MEK) – MAP kinase • Prolong the signal from growth factor receptors to promote longer term effects like gene changes in gene expression

Question 7

Recruitment of signaling molecules to PIP3

Answer 7

• The production of specific inositol lipids by PI 3-kinase generates specific binding sites for other signaling molecules • Proteins containing a “PH-domain” can bind to these sites

Question 8

Tyrosine kinases associated receptors

Answer 8

• This class depends on cytoplasmic tyrosine kinases for their activity • The largest family of cytoplasmic tyrosine kinases is the Src family, but there are many others (Janus kinase-Jak, Focal Adhesion Kinase-FAK)

Question 9

Receptor serine/threonine kinases

Answer 9

• Transforming Growth Factor β (TGFβ) signaling • Other TGFβ-like ligands include the activins and BMPs (bone morphogenetic proteins) • The type-I receptor also has an associated inhibitory factor that is dissociated by the activated type-II homodimer

Question 10

Examples of signaling by regulated proteolysis

Answer 10

Wnt and Notch signalling

Question 11

Wnt signalling

Answer 11

Question 12

Notch interaction with ligands is regulated by ___

Answer 12

glycosylation

Question 13

Other examples of regulated intermembraneous proteolysis

Answer 13

• Cholesterol sensing

– Sterol Regulatory Element Binding Protein (SREBP) contains 2 TMD that are selectively cut in response to levels of cholesterol in the membrane

• Alzheimer’s disease

– Amyloid Precursor Protein (APP) is cut within its TMD to generate the β-amyloid peptide that is aggregated in the extracellular space of the brain in Alzheimer’s patients

• Housekeeping functions

– The transmembrane stubs that are left by signal sequence cleavage are degraded by cleavage within the TMD