BISC 130 - Chapter 9: Cell Communication

• Ligands

  • Released from "signaling cells"

  • They bind to receptor proteins on "target cells"

• There are 4 types of signaling mechanisms

  1. Autocrine

     1. Ligand binds to receptor in signaling cell.

  2. Direct Signaling

     1. Ligand travels from cyptoplasm to cyptoplasm from signaling cell to target cell

  3. Paracrine

     1. Ligand binds to nearby cells

     2. Rapidly degraded ligand ensures signal is local.

1. Endocrine

   1. Ligand binds to receptors on distant cells

   2. Stable ligand ensures it can travel across the body

   3. Ligand just refers to any signaling molecule. Ex: Hormones, neurotransmitters.

2 Types of Receptors

1. Intracellular Receptor

   • Receptor protein in the cyptoplasm of the cell

• Ligands for these receptors must be small and hydrophobic in order to diffuse across the membrane

  • Ex: Steroid hormones

1. Cell Surface Receptor

   • Receptor protein on the surface of the cell, anchored in the plasma membrane

• These receptors must possess 3 distinct domains.

  • Extracellular Domain

    • Exposed to the outside of the cell

  • Intracellular Domain

    • The part that binds ligand

  • Transmembrane Domain

    • Part of the protein that spans the membrane

  • Cyptoplasmic Domain

    • Exposed to cyptoplasm

• Ligands for these receptors are water-soluble

  • Ex: Small molecules or proteins

Signal Transduction

• Binding in ligand leads to change in shape of receptor

  • It behaves differently as a result (examples listed below are how proteins behave differently when binded to ligand)

    • Ex: An intracellular receptor may enter the nucleus, cause genes to be turned on/off.

    • Ex: This (binding in ligand) may lead to the release/creation of second messengers.

    • Ex: A cell surface receptor may phosphorylate proteins. --> Explained on side-notes below.

• Phosphorylation

  • The modification of protein by the addition of a phosphate group(s) from ATP.

    • Catalyzed by a class of enzymes called kinases.

• Dephosphorylation:

  • The removal of a Phosphate group(s) from a protein

    • Catalyzed by a class of enzymes called Phosphatases.

    • If soemthing ends in 'ase', it is an enzyme.

• *The function of a protein can be switched on/off depending on its phosphorylation state.

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• Signal Transduction can ultimately result in:

  • Altered cell behavior

  • Cell growth/division

  • Apoptosis - programmed cell death. The cell destroys itself when infected or grows out of control.

• Single-celled yeasts secrete a "mating factor" ligand to find other yeast cells to mate with.

• Some bacteria can communicate

  • Ex: Quorum Sensing

    • Some bacteria can form structures called biofilms.

• But this requires high cell density to form.

  • Would waste energy to try to form with few bacteria.

• Release ligands called "autoinducers" to communicate cell density to other bacteria.

  • Low ligand density: ligand diffuses but no response

  • High ligand density: signaling pathway activated. There is a formation of biofilm.

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