Signal Transduction and Cytoskeleton

Adapter Proteins and Signaling Pathways

  • Adapter Proteins
    • Bind to phosphorylated tyrosines on receptor tyrosine kinases (RTKs).
    • Activate downstream signaling pathways by facilitating the interaction with small GTPases like RAS.
RAS and GTPase Function
  • RAS
    • A small GTP-binding protein involved in cell signaling.
    • Overexpressed in some cancers.
    • Activation occurs through a guanine nucleotide exchange factor, leading to the dissociation of GDP and the association with GTP.
    • This activation enables RAS to trigger kinase activity and affect downstream signaling pathways.
Signal Transduction Overview
  • Signal transduction represents the process of relaying signals from outside the cell to the target proteins, ultimately affecting gene expression and protein activity.
  • Key Process:
    • One signaling molecule activates another, leading to a cascade effect.

MAP Kinase Pathway

  • MAP Kinase Cascade
    • Consists of three kinases:
    1. MAP kinase kinase kinase (often referred to as MAPKKK)
    2. MAP kinase kinase (MAPKK)
    3. MAP kinase (MAPK)
    • The cascade activates various downstream proteins altering their functions, including gene transcription.
    • This three kinase structure is due to evolutionary adaptation, allowing complex signaling.

Importance of Crosstalk in Signaling

  • Crosstalk between Pathways:
    • Pathways integrate and coordinate responses through interactions.
    • Can enhance or inhibit signals depending on the combination of activations from multiple receptors.

Roles of Kinases and Phosphatases

  • Kinases
    • Enzymes that phosphorylate proteins, usually activating them.
  • Phosphatases
    • Enzymes that remove phosphate groups, often leading to protein inactivation.
  • Phosphorylation can either activate or inhibit downstream proteins depending on the context within signaling pathways.

Signaling Molecules and Pathways

  • Signal Molecule
    • Commonly referred to as a ligand that binds to its specific receptor, initiating a signaling cascade.
  • Receptor Tyrosine Kinases (RTK) & GPCRs:
    • The two primary types of membrane receptors that activate various intracellular signaling pathways.
AKT Pathway
  • AKT (Protein Kinase B)
    • Activated by a phosphoinositol 3-kinase (PI3K) that is recruited and activated through RTK phosphorylation.
    • AKT phosphorylates target proteins involved in cell survival, metabolism, and growth regulation.
  • Cell Survival vs. Apoptosis
    • Signals that prolong survival through AKT can lead to the inactivation of pro-apoptotic factors (e.g., BAD protein) thereby inhibiting apoptosis.

Differential Effects of Signals

  • Integrating Signals:
    • A cell can integrate signals based on the strength and duration of each signal, thus determining the resultant cellular response.
  • Signals can have antagonistic effects, dictating cellular activities such as growth, protein synthesis, or programmed cell death (apoptosis).

Intracellular Receptors

  • Nuclear Receptors:
    • Bind to hydrophobic signaling molecules like steroid hormones (e.g., cortisol, estrogen).
    • Function inside the cell, often within the nucleus, acting to regulate gene expression once bound to ligands.
    • These receptors may be cytosolic or nuclear and mediate signaling for a wide range of steroid hormones.

Conclusion

  • Signal Transduction Summary:
    • The overall aim of signaling pathways is to regulate either gene expression or protein function leading to diverse cellular responses.
  • Understanding both adapter proteins and specific pathways such as MAP kinase and AKT is essential for grasping cellular communication and regulation.