Nov 12th - Lecture 30 ~ Enzyme-coupled receptors

enzyme-coupled receptors either have enzymatic function or recruit proteins that do

• many enzyme-coupled receptors have intrinsic catalytic domains that are activated upon ligand induced dimerization

  • Some lack catalytic function, but rather recruit enzymes

• Many enzyme coupled receptors have kinase functions that promote signaling complex assembly through site specific phosphorylation

  • Receptor tyrosine kinases (RTK) and receptor serine/threonine kinases are common

  • RTK phosphorylates tyrosine amino acids in proteins

    • RTK's regulate diverse functions

RTK autophosphorylation upon ligan-induced dimerization

• receptor tyrosine kinases have kinase domains that specifically phosphorylate tyrosine residues

• Ligands for RTKs are typically dimers, and, upon binding, they induce receptor dimerization

• Dimerization brings the kinase domains into close proximity to tyrosine residues — autophosphorylation

  • phosphorylation of the receptors creates binding sites for signaling complex assembly

• In the absence of ligand, RTKs dimerize at a low level, but ligand-independent signaling is possible if alterations in regulatory DNA sequences result in high receptor expression

phosphorylation of RTKs generates docking sites for intracellular signaling proteins

• creates a site where other proteins can interact with the RTK to get activated

  • examples — PI 3 kinase, GAPs, GEFs, and PLC-gamma

Ras is a monomeric (small) G protein activated by many different RTKs

• binding of an activated RTK by Grb2 recruits Sos (a Ras-GEF) that exchanges GDP on inactive Ras with GTP

• Ras is a monomeric G protein (distinct from the trimeric G proteins of GPCR signaling)

  • activated Ras passes the signal forward until its bound GTP is hydrolyzed

Ras initiates a MAP kinase phosphorylation cascade

• Mitogen-Activated Protein (MAP) Kinase pathways, as the name suggests, are intracellular signaling processes driven by ATP-dependent phosphorylation that promote cell division

  • a mitogen is any signal that results in cell proliferation

• there are multiple MAPK pathways, but the general structure is depicted:

  • a top-level MAP kinase kinase kinase phosphorylates a MAP kinase kinase

  • the MAPKK phosphorylates a MAPK which phosphorylates effector proteins

• Since each activated kinase activates another kinase, this form of signal transduction is called a phosphorylation cascade

  • No phosphorylation cascade is linear, each kinase has multiple targets

• The specific members of the MAPK pathway initiated by Ras

  are as follows:

  • MAPKKK = Raf (Rapidly Accelerated Fibrosarcoma)

  • MAPKK = MEK (MAPK/ERK Kinase)

  • MAPK = ERK (Extracellular signal-Regulated Kinase)

MAP kinase pathways are modular and different combinations produce different effects

• there are many different MAPKs, so to prevent cross-talk, different MAPKs are held together by scaffold proteins

Rho family G proteins link extracellular signaling to the cytoskeleton

phosphorylation of RTKs generates docking sites for intracellular signaling proteins

• an example of PI 3-kinase enzyme-coupled receptors is Akt & PIP

Summary / Key Concepts

• Enzyme-coupled receptors are either enzymes themselves or recruit enzymes as part of a signaling complex

• The largest group of enzyme-coupled receptors are the receptor tyrosine kinases (RTKs)

  • RTKs are activated by ligand-induced dimerization and phosphorylation of tyrosine residues – signal complex proteins bind these phosphotyrosines

• RTKs have variable ligand-binding extracellular domains and intracellular kinase domains – phosphorylation occurs on c-terminal tails and within “kinase insert” regions

• the specifics an RTK’s signaling depend on the enzymes recruited. RTKs that recruit a signal through Ras and MAPK pathways

  • RTKs that recruit PI3K signal through PI(3,4,5)P 3 and Akt

  • PI3K produces the PI(3,4,5)P 3 that allows binding of AKT and the enzyme that activates it

• MAPK pathways contain different kinase modules

  • different combinations result in different responses

  • scaffolds can assemble MAPK pathways and limit crosstalk between different receptors

  • MAP Kinases promote cell division

    • Akt promotes cell survival and growth (via Tor)

    • Rho family GTPases interact with cell surface receptors to control the cytoskeleton

• When neuron growth cones find their target, ephrin signaling through Rho leads to termination of actin polymerization, myosin-based contraction, and growth cone collapse – this creates a synapse