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9.3A: Termination of the Signal Cascade

  • Page ID
    13221
  • Signal cascades convey signals to the cell through the phosphorylation of molecules by kinases.

    LEARNING OBJECTIVES

    Describe the process by which the signal cascade in cell communication is terminated

    KEY TAKEAWAYS

    Key Points

    • The chain of events that conveys the signal through the cell is called a signaling pathway or cascade.
    • Phosphorylation, a major component of signal cascades, adds a phosphate group to proteins, thereby changing their shapes and activating or inactivating the protein.
    • Degrading or removing the ligand so it can no longer access its receptor terminates the signal.
    • Enzymes like phosphotases can remove phosphate groups on proteins during dephosphorylation and reverse the cellular modifications produced by signaling cascades.

    Key Terms

    • signaling cascade: the chain of events that conveys the signal through the cell
    • phosphorylation: the addition of a phosphate group to a compound; often catalyzed by enzymes
    • dephosphorylation: the removal of phosphate groups from a compound; often catalyzed by enzymes

    Termination of the Signal Cascade

    Ligand binding to the receptor allows for signal transduction through the cell. The chain of events that conveys the signal through the cell is called a signaling pathway or cascade. Signaling pathways are often very complex because of the interplay between different proteins. A major component of cell signaling cascades is the phosphorylation of molecules by enzymes known as kinases. Phosphorylation adds a phosphate group to serine, threonine, and tyrosine residues in a protein, changing their shapes, and activating or inactivating the protein.

    image

    Phosphorylation: In protein phosphorylation, a phosphate group is added to residues of the amino acids serine, threonine, and tyrosine.

    The aberrant signaling often seen in tumor cells is proof that the termination of a signal at the appropriate time can be just as important as the initiation of a signal. One method of terminating or stopping a specific signal is to degrade or remove the ligand so that it can no longer access its receptor. One reason that hydrophobic hormones like estrogen and testosterone trigger long-lasting events is because they bind carrier proteins. These proteins allow the insoluble molecules to be soluble in blood, but they also protect the hormones from degradation by circulating enzymes.

    Inside the cell, many different enzymes reverse the cellular modifications that result from signaling cascades. For example, phosphatases are enzymes that remove the phosphate group attached to proteins by kinases in a process called dephosphorylation. Cyclic AMP (cAMP) is degraded into AMP by phosphodiesterase, and the release of calcium stores is reversed by the Ca2+ pumps that are located in the external and internal membranes of the cell.