MODULE 28 PDF - Cellular Signaling

MODULE 28 Catalytic Receptors 1. Receptors with intrinsic protein tyrosine kinase activity: receptor tyrosine kinases (RTKs): catalyze the transfer of the γ phosphate of ATP to hydroxyl groups of tyrosines on target proteins control cell cycle, migration, metabolism, survival, proliferation,...

MODULE 28 Catalytic Receptors 1. Receptors with intrinsic protein tyrosine kinase activity: receptor tyrosine kinases (RTKs): catalyze the transfer of the γ phosphate of ATP to hydroxyl groups of tyrosines on target proteins control cell cycle, migration, metabolism, survival, proliferation, and differentiation 2. Receptors associated with enzymes: do not contain an intrinsic protein tyrosine kinase activity their cytoplasmic domains interact with non-receptor tyrosine kinases 3. Receptor threonine/serine kinases (RTSK): heterodimeric functional units with intrinsic serine/threonine protein kinase activity 4. Receptor tyrosine phosphatases (RTP): lack cognate ligands might be triggered by events such as cell-to-cell contact 5. Guanylyl cyclases of the natriuretic peptide receptor family Receptor Tyrosine Kinases (RTKs) ligands: growth factors, some hormones most receptors are formed from two or more single transmembrane proteins that associate before or after ligand binding each receptor chain has three domains: a ligand-binding domain, an α-helical transmembrane domain, a cytoplasmic domain with tyrosine kinase activity ligand binding activates the tyrosine kinase in each receptor to phosphorylate tyrosine residues on another receptor chain (autophosphorylation) Tyr phosphorylation triggers the assembly of an intracellular signaling complex on the receptor cytoplasmic domains adaptor (docking) proteins with various domains dock on phosphotyrosines of the receptors RTKs A. the dimeric ligand PDGF crosslinks two receptors together B. the monomer FGF forms multimers by binding to heparan sulfate proteoglycans on the target cell surface or in ECM and crosslinks adjacent receptors C. the insulin receptors are tetramers before ligand binding; ligands induce a rearrangement of the receptor chains so that the two kinase domains come close together; the activated receptor autophosphorylates its kinase domains, which then phosphorylate the docking protein IRS-1 on multiple tyrosines, creating multiple docking sites MODULE 28 Docking Proteins for RTKs recruit signaling proteins to the activated receptors typically, contain a membrane-targeting region (through myristoylation, transmembrane domains, palmitoylation, PH domains) contain multiple Tyr phosphorylation sites that are binding sites for SH2 domains of signaling proteins examples of docking proteins: GAB, insulin receptor substrate 1 (IRS1) Protein Modules on Signaling Proteins Some SH2 domain-containing proteins have enzymatic activities such as kinase activity (e.g., Src kinases) or phosphatase activity (PTP). Proteins with only interaction domains (e.g., SH2 or SH3) link different proteins; e.g., Grb2 links cell surface receptors to the Ras/MAP kinase signaling. Such proteins are called adaptor proteins (docking proteins are also adaptor proteins). Activation by Membrane Translocation Some RTKs activate PI-3 kinase that generates phosphatidyl-inositol (3,4,5)-trisphosphate [PIP3], a second messenger. Kinases PDK1 and PKB/AKT have PH domains that bind phosphorylated inositol phospholipids and translocate to the membrane. The second messenger PIP3 activates PDK1, which, in turn, activates PKB/AKT through phosphorylation. Activation by a Conformational Change e.g., binding of p85, the regulatory subunit of PI-3 kinase, to phosphotyrosines in the RTK called PDGFR or in Insulin Receptor Substrate 1 (IRS1) causes conformational changes in p85 that enhance PI-3 kinase activity the binding of PI-3 kinase to phosphotyrosines also allows for its translocation to the cell membrane where its substrate PIP2 is found PI-3 kinase phosphorylates PIP2 to PIP3, and this creates a docking site for other proteins (see the previous slide) Activation by Tyrosine Phosphorylation example: phospholipase C (PLCγ) binds to phosphotyrosines of some activated RTKs through its SH2 domains; as a result, PLCγ is tyrosine-phosphorylated and activated the activation of PLCγ is also dependent on the products of PI-3 kinase; thus, PLCγ binds PIP3, and the second messengers diacylglycerol and Ins(1,4,5)P3 are generated from PIP2 PI3-Kinase Signaling RTKs and other receptors activate PI3-kinase that phosphorylates inositol phospholipids PIP2 and PIP3 can serve as docking sites for signaling proteins PIP2 is also cleaved by phospholipase C to soluble IP3 and membrane-bound diacylglycerol; IP3 releases Ca2+ from the sER; diacylglycerol with Ca2+ and phosphatidylserine activate protein kinase C (PKC) If PIP2 and PIP3 are not cleaved; they remain in the plasma membrane until they are dephosphorylated by phosphatases; mutations inactivating phosphatase PTEN prolong signaling by PI3-kinase and promote cancer MODULE 28 The PI3-Kinase Stimulates Cell Survival and Growth PI3-kinase signals cell survival by activating AKT/PKB After binding to PIP3, AKT/PKB alters conformation and is activated through phosphorylation by PDK1 Active AKT/PKB returns to the cytoplasm and phosphorylates proteins (e.g., BAD, a pro-apoptotic protein, inactive when phosphorylated); PKB/AKT also inhibits other cell death activators RAS Signaling Ras is activated in response to ligands of various RTKs (e.g., EGFR and others) Upon EGF binding, EGFR forms a dimer Downstream signaling proceeds through pathways such as Raf-MEK-ERK (all three are kinases!) and PI3K-Akt- mTOR Both branches of Ras signaling are involved in cell proliferation, apoptosis, migration, fate, and differentiation RAS Signaling Pathways: the RAF-MEK-ERK Pathway Also called mitogen-activated protein kinase (MAPK) pathway e.g., EGF binds to EGFR, EGFR dimerizes, activates, and recruits a GEF (SOS) via adaptor proteins (SHC and Grb2) SOS activates RAS to RAS-GTP, which activates kinase RAF; RAF phosphorylates/activates mitogen-activated protein kinase kinase 1 and 2 (MEK1/2) that activates ERK1/2 ERK1/2 moves to the nucleus to activate transcription factors Putting It Together There are five classes of catalytic receptors: 1. Receptor tyrosine kinases 2. Tyrosine kinase-associated receptors 3. Receptor serine/threonine kinases 4. Transmembrane guanylyl cyclases 5. Tyrosine phosphatases Signaling complexes are often formed through the modular binding domains in signaling proteins Ligand binding to RTKs induces receptor autophosphorylation, creating docking sites for proteins with SH2 or PTB domains; some proteins are adaptors that couple receptors to the small GTPase Ras or other signaling molecules PI-3 kinase generates PIP2 and PIP3, which serve as docking sites, as well as secondary messengers RAS activates at least three pathways (please know the RAF-MEK-ERK signaling kinase pathway)