Cell Signaling: Mechanism developed by cells to monitor and respond to changes in their environment.
Unicellular Organisms: Use cell signaling to influence the behavior of other unicellular organisms.
Multicellular Organisms: Cells communicate and function as a unit through cell signaling.
Mechanism of Cell Signaling
Communication occurs via molecules present on the cell surface or secreted into the environment.
Ligand: An extracellular signaling molecule that begins the signaling pathway by binding to a receptor.
Receptor: A protein on the cell that senses the presence of the ligand in the environment.
Intracellular Signaling Proteins: Activated proteins that continue the signaling pathway inside the cell.
Effector Proteins: Mediate specific outcomes such as alterations in metabolism, gene expression, or cell shape/movement.
Types of Signaling Molecules
Membrane-Bound Ligands: Require direct cell contact for detection (e.g., immune response development).
Secreted Ligands: Can be detected by the secreting cell itself (autocrine signaling), nearby cells (paracrine signaling), or distant cells via bloodstream (endocrine signaling).
Synaptic Signaling: Specialized paracrine signaling involving neurons at a synapse.
Receptors
Cell Surface Receptors: Recognize hydrophilic ligands that cannot pass through the plasma membrane.
Intracellular Receptors: Bind ligands that can cross the plasma membrane and are located inside the cell, often in the nucleus or cytoplasm.
Transmembrane Receptors: Include channel-linked receptors, G-protein-coupled receptors, and enzyme-coupled receptors.
Protein-Protein Interactions and Domains
Domains: Parts of proteins that facilitate interactions, can function independently.
SH2 Domain: Binds to phosphorylated tyrosines.
Plextron Homology Domain: Binds phosphorylated lipids on the plasma membrane.
SH3 Domain: Binds proline-rich sequences.
Scaffold & Adapter Proteins: Help form signaling complexes by binding other proteins.
Regulation of Protein Activity
Post-Translational Modifications: Phosphorylation and ubiquitination as molecular switches.
Kinases and Phosphatases: Enzymes that add/remove phosphate groups to toggle protein activity.
Ubiquitination: Tags proteins for degradation or forms scaffolds for protein interaction.
GTP/GDP Binding: GTP-bound proteins are active, GDP-bound are inactive.
Signal Transduction Pathways
Second Messengers: Small molecules like cAMP, DAG, IP3, and calcium ions that propagate and amplify signals.
Examples of signaling cascades:
Insulin Pathway: Converts glucose to glycogen through activation of glycogen synthase.
Cytokine Signaling: Activates STAT proteins for immune responses.
Adrenaline Signaling: Uses cAMP as a second messenger to activate gene transcription.
Conclusion
Summary of key concepts: types of signaling, recruitment of proteins, regulation of protein activity, role of second messengers, and examples of signaling pathways.