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Receptors Overview

Notes

Receptors Overview

Sections

RECEPTORS OVERVIEW

Response speeds

  • Fast
  • Slow

Signaling pathways

  • Intracellular
  • Membrane-bound

RESPONSE SPEEDS

Fast responses

  • Due to altered protein function
  • Occur in milliseconds to minutes

Slow responses

  • Due to change in transcription and protein synthesis
  • Occur in minutes to hours

SIGNALING PATHWAYS

Intracellular receptors

  • Hydrophobic signaling molecules can travel through the plasma membrane (hormones)
  • Receptors inside the cell
  • Hormone/receptor complex travels to nucleus to bind DNA and promote a response

Membrane-bound receptors

  • Receptor bound to membrane with binding site accessible to the extracellular space
  • Ligand binds to receptors
  • This causes receptor activation which activates further proteins within the cell
  • Activated proteins transduce signal eventually leading to either altered gene expression or altered cellular metabolism

RECEPTOR CLASSES

G-protein coupled receptors

  • Receptor traverses the plasma membrane
  • Ligand binds to receptor
  • Receptor causes G-protein to bind GTP and become active
  • Active G-protein induces further responses within the cell

Receptor tyrosine kinases

  • Receptor traverses the plasma membrane
  • Ligand binds to receptor
  • Receptor dimerizes and phosphorylates itself
  • Intracellular proteins are activated by binding the phosphorylated receptor
  • Activated intracellular proteins induce further response within the cell

Ligand-gated ion channels

  • Channel traverses the membrane in a closed conformation
  • Ligand binds to channel
  • Channel opens allowing ions to pass through
  • Ions initiate intracellular response

Full-Length Text

  • In this tutorial, we will explore generic cellular signaling pathways as well as take a look at three classes of membrane-bound receptors.
  • To begin, start a table to understand some key concepts about receptors.
  • Denote that we'll address:
    • Response speeds: fast and slow.
    • Signaling pathways: intracellular and membrane-bound
    • Receptor classes:
      1. G-protein coupled receptors
      2. Receptor tyrosine kinases (a subset of enzyme-coupled receptors)
      3. Ligand-gated ion channels.

Let's explore response speeds.

  • Note that a response to a signal can be either fast or slow.
  • Note that fast responses are due to altered protein function and can occur in milliseconds to minutes.
  • Note that slow responses are due to a change in transcription and protein synthesis and can occur in minutes to hours.
  • The receptors used by target cells are either located inside the cell (intracellular) or are membrane-bound.

Let's explore the differences in the signaling pathways of these two receptor types, beginning with intracellular receptors.

  • Draw the plasma membrane of the target cell.
  • Within the cell, draw the intracellular receptor, which can be found in the cytoplasm or nucleus of cells.
  • Now draw the hydrophobic signal molecule outside the cell; the hydrophobicity of the molecule is what allows it to pass through the plasma membrane.
    • Indicate that these types of molecules are typically hormones.
  • Indicate that the hormone is able to pass through the plasma membrane and bind directly to its receptor.
  • Now draw the nucleus with some DNA.
  • Indicate that the hormone/receptor complex now travels to the nucleus where it binds to specific genes in the DNA to promote a response (most often the transcription and translation of the gene product).

Now let's look at the membrane-bound receptor signaling pathway, which can get very complex with multiple stages. Here, we'll keep it simple and general.

  • Draw the receptor traversing the plasma membrane.
  • Now draw the signal molecule binding the receptor, which changes the receptor and initiates the transduction phase of the response.
  • To show transduction, draw three different relay proteins within the cell and indicate that the receptor activates one protein, which in turn activates another protein, which in turn activates the next protein, like a relay race.
    • During this phase the signal may be altered, modulated, distributed or amplified by other proteins.
  • Now indicate that when the final protein is activated, it travels to the nucleus and binds DNA to alter gene expression.
  • Or indicate that it stays in the cytoplasm to alter the cell's metabolism in some way.
  • Now let's explore three common classes of receptors:
    • G-protein coupled receptors
    • Receptor tyrosine kinases (a subset of enzyme-coupled receptors)
    • Ligand-gated ion channels.

Start with G-protein coupled receptors.

  • Draw a plasma membrane.
  • Label extracellular and cytosol.
  • Show a receptor traverse the plasma membrane.
  • Show an enzyme attached to the plasma membrane.
  • Attached to the receptor, draw the G-protein with GDP bound to it.
    • Indicate that this means the G-protein is inactive.
  • Now show the signal molecule bind the receptor.
  • Indicate that this causes GTP to bind the G-protein, kicking out the GDP and activating the protein.
  • Re-draw the plasma membrane.
  • Draw the receptor with the bound signal molecule and the enzyme.
  • Now draw the activated G-protein binding to the enzyme.
  • Indicate that the now activated enzyme transduces the signal, which eventually results in a cellular response.
  • For receptor tyrosine kinases, draw a plasma membrane.
  • Now draw two receptors.
  • Draw the signal molecules binding to the receptors.
  • Indicate that this causes the receptors to dimerize (bind together).
  • Re-draw the plasma membrane.
  • Now, show the dimerized receptors bound to the signal molecules.
  • Draw a phosphate attached to each receptor; dimerization allows the receptors to phosphorylate one another.
  • Now, draw two intracellular relay proteins bound to the phosphate.
  • Indicate that these relay proteins are activated by this binding and go on to promote a cellular response.
  • Finally, for ligand-gated ion channels, draw the plasma membrane.
  • Draw ions outside the membrane.
  • Draw the channel protein traversing the membrane, but indicate that it is closed.
  • Draw a ligand and indicate that it binds to the channel.
  • Now redraw the plasma membrane and channel, with the ligand bound.
  • Indicate that the channel is now open.
  • Draw the ions and indicate they can now travel through the channel and initiate a response within the cell.

This concludes the diagram overviewing receptors.

UNIT CITATIONS:

  1. Campbell, N. A. & Reece, J. B. Biology, 7th ed. (Pearson Benjamin Cummings, 2005).
  1. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Molecular Biology of the Cell, 5th ed. (Garland Science, 2008).
  1. Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Essential Cell Biology, 3rd ed. (Garland Science, 2010).
  1. Marieb, E. N. & Hoehn, K. Human Anatomy & Physiology, 10th ed. (Pearson, 2016).
  1. Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., Ploegh, H. & Matsudaira, P. Molecular Cell Biology, 6th ed. (W. H. Freeman and Company, 2008).