Week 2 - Cell Signaling

Overview of Cellular Signaling

  • Cell-to-cell communication is crucial for multicellular organisms.

  • Universal mechanisms of cellular regulation have been discovered by biologists.

Signal Transduction

  • Signal-Transduction Pathway: Steps converting a signal on a cell's surface into a cellular response.

  • Pathway similarities indicate ancestral signaling molecules evolved in prokaryotes and were later adopted by eukaryotes.

Types of Signaling

Local Signaling

  • Cells communicate using chemical messengers.

  • Animal and plant cells can directly connect via cell junctions.

  • Local regulators are messenger molecules traveling short distances.

Long-Distance Signaling

  • Involves hormones traveling in blood for communication between distant cells.

Three Stages of Cell Signaling

  1. Reception: Binding of a signal molecule (ligand) to receptor protein, causing a conformational change.

  2. Transduction: Cascades of molecular interactions relaying signals to target molecules.

  3. Response: Results in cellular activities regulation or gene transcription.

Reception Mechanisms

  • Intracellular Receptors: Found in cytosol/nucleus; activated by small hydrophobic chemical messengers (e.g., steroid hormones).

  • Plasma Membrane Receptors: Water-soluble signal molecules bind to receptors in the membrane, which include:

    • G-protein-linked receptors

    • Receptor tyrosine kinases

    • Ion channel receptors

G-Protein-Linked Receptors

  • Utilize a G protein as an on/off switch. Activation leads to various cellular responses.

Receptor Tyrosine Kinases

  • Attach phosphates to tyrosines and can trigger multiple signal transduction pathways.

Ion Channel Receptors

  • Change shape to allow specific ions (such as Na+ or Ca2+) to pass through.

Transduction Pathways

  • Typically involve multiple steps allowing signal amplification and regulatory opportunities.

  • Phosphorylation cascades turn activities on/off.

Second Messengers

  • Small molecules (e.g., cyclic AMP, calcium ions) acting as intermediaries in signal transduction pathways.

Cellular Responses

  • Response may lead to enzyme activity regulation or changes in transcription in the nucleus, affecting cellular behavior and function.

Signal Amplification and Specificity

  • Multistep pathways amplify signals, with each activated product outnumbering precursors.

  • Specificity arises from a cell's unique collection of proteins, affecting how different cells respond to the same signal.

Efficiency of Signaling

  • Scaffolding proteins increase signaling efficiency by organizing multiple relay proteins.

Termination of the Signal

  • Inactivation mechanisms revert receptors to an inactive state after the signal molecules detach.