Signal-Transduction Pathways Study Notes

Chapter 13: Signal-Transduction Pathways

Introduction

  • The segment focuses on signal-transduction pathways, crucial for cellular communication and responses to stimuli in biological systems.

  • References the importance of various receiver proteins that transmit signals inside the cell upon interaction with external molecules (ligands).

Dimerized Receptors That Act As or Recruit Kinases

  • Ligand Binding Shifts Quaternary Structure:

    • Ligand binding induces a change in the receptor's structure, leading to receptor dimerization.

Receptor Tyrosine Kinases (RTKs)

  • Definition: Receptor Tyrosine Kinases are a class of membrane receptors that, upon activation by ligand binding and subsequent dimerization, exhibit intrinsic kinase activity.

  • Mechanism:

    • RTKs are initiated when ligands bind, causing two receptor units to dimerize.

    • This dimerization leads to cross-phosphorylation of the tyrosine residues within the receptors, activating their kinase domains.

    • For instance, when Epidermal Growth Factor (EGF) binds to its receptor, it triggers the following cascade:

    • Dimerization of the EGF receptor.

    • Cross-phosphorylation of intracellular RTK domains.

    • Binding of Grb-2 to the phosphorylated receptor domain.

    • Interaction of Sos with Grb-2 allows Ras protein to bind.

    • Activated Ras subsequently activates a cascade of protein kinases that promote cell growth.

  • Example: Illustration of the signaling pathway involving EGF, GDP, GTP, Ras, Grb-2, and Sos which collectively contributes to cellular growth processes.

Dimers that Recruit Kinases

  • Intracellular Domain Interaction:

    • Receptor dimerization brings their intracellular domains into close proximity, allowing for kinase activation.

  • JAK2 Kinases:

    • Associated with intracellular domains of certain receptors (like those activated by growth hormones).

    • Cross-phosphorylation of JAK2 leads to their activation, and subsequently, the phosphorylated JAK2 can phosphorylate additional proteins, propagating the signal derived from the ligand.

  • Example: The Human Growth Hormone utilizes the JAK2 signaling pathway, with its mechanism involving:

    • Hormone-induced dimerization of receptors.

    • JAK2 activation and cross-phosphorylation.

Insulin Signaling Overview

  • Insulin and Receptor Interaction:

    • In the presence of insulin, insulin receptors undergo cross-phosphorylation, resulting in an activated state of the receptor.

  • Signaling Mechanisms:

    • Activation leads to enzymatic amplification reactions involving:

    • Phosphorylation of Insulin Receptor Substrates (IRS) proteins.

    • Interactions with localized phosphoinositide 3-kinase (PI3K).

  • PIP3 Formation:

    • Resulting lipid products include Phosphatidylinositol 3,4,5-trisphosphate (PIP3), which interacts with additional proteins in the signaling pathway.

  • Akt Activation:

    • Active PIP3-dependent protein kinase facilitates the activation of Akt protein kinase, which plays a pivotal role in numerous cellular processes including glucose transport and metabolism.

  • GLUT4 Mechanism:

    • The presence of insulin enhances the fusion of GLUT4 vesicles with the plasma membrane.

    • GLUT4 transporters increase glucose permeability into the cell, enhancing glucose uptake during insulin signaling.