Unit: Cell communication and cell cycle
Chapter: Signal transduction
Reference: Components of signal transduction, Steps in the signal transduction pathway, Importance of signal transduction
Learning objectives
- To understand the steps involved in signal transduction pathway
- To learn about the importance of signal transduction
Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events (protein phosphorylation catalysed by protein kinases) involving second messengers, which ultimately results in a cellular response
Components of signal transduction
Both plants and animals, have similar transduction components.
- Signal molecules/ Ligands/ Extracellular messengers/ primary messengers
- Receptors
- Second messengers/effector molecules
- Downstream response elements
Signal Transduction involves
• Sensing of signal by the receptor
• Transfer of the information from the signal from one biochemical form to another (transduction) to amplify the impact of signal leading to a cellular response.
• Signal transduction pathway consists of protein kinases and protein phosphatases whose catalytic actions change the conformations, and thus the activities, of the proteins they modify
Membrane receptors function by binding the signal molecule (ligand) and causing the production of a second signal (also known as a second messenger) that then causes a cellular response. These types of receptors transmit information from the extracellular environment to the inside of the cell by changing shape or by joining with another protein once a specific ligand binds to it. Examples of membrane receptors include G Protein-Coupled Receptors and Receptor Tyrosine Kinases.
Intracellular receptors are found inside the cell, either in the cytoplasm or in the nucleus of the target cell (the cell receiving the signal). Chemical messengers that are hydrophobic or very small (steroid hormones for example) can pass through the plasma membrane without assistance and bind these intracellular receptors. Once bound and activated by the signal molecule, the activated receptor can initiate a cellular response, such as a change in gene expression.
Steps in the signal transduction pathway
- Enzymes that transfer phosphate groups from ATP to a protein are called protein kinases. Many of the relay molecules in a signal transduction pathway are protein kinases and often act on other protein kinases in the pathway.
- Often this creates a phosphorylation cascade, where one enzyme phosphorylates another, which then phosphorylates another protein, causing a chain reaction.
- Also important to the phosphorylation cascade are a group of proteins known as protein phosphatases. Protein phosphatases are enzymes that can rapidly remove phosphate groups from proteins (dephosphorylation) and thus inactivate protein kinases.
- Protein phosphatases are the "off switch" in the signal transduction pathway. Turning the signal transduction pathway off when the signal is no longer present is important to ensure that the cellular response is regulated appropriately.
- Dephosphorylation also makes protein kinases available for reuse and enables the cell to respond again when another signal is received.
- Kinases are not the only tools used by cells in signal transduction. Small, nonprotein, water-soluble molecules or ions called second messengers (the ligand that binds the receptor is the first messenger) can also relay signals received by receptors on the cell surface to target molecules in the cytoplasm or the nucleus.
- Examples of second messengers include cyclic AMP (cAMP) and calcium ions.
- Cell signaling ultimately leads to the regulation of one or more cellular activities. Regulation of gene expression (turning transcription of specific genes on or off) is a common outcome of cell signaling.
- A signaling pathway may also regulate the activity of a protein, for example opening or closing an ion channel in the plasma membrane or promoting a change in cell metabolism such as catalyzing the breakdown of glycogen.
- Signaling pathways can also lead to important cellular events such as cell division or apoptosis (programmed cell death).
Importance of signal transduction
- The signals are then passed from one molecule to another inside the cell, which results in a specific cell response, such as cell division or cell death.
- Signal transduction is important for cells to grow and work normally.
- Cells that have abnormal signaling molecules may become cancer cells
Solved examples
Example 1. How are receptors classified?
- Kinase Activity b) Ligands c) Structure d) All of the above
Solution 1: d. Receptors are classified based on kinase activity, ligands, and structure.
Example 2. Which receptor family is penetrated through the membrane and has intrinsic enzymatic activity?
a) Kinases b) GPCR c) Intracellular receptors d) All of the above
Solution 2: a. Kinases is penetrated through the membrane and has intrinsic enzymatic activity.
Summary
- Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalysed by protein kinases, which ultimately results in a cellular response.
- Signaling receptors can be single-span plasma membrane receptors associated with tyrosine or serine/threonine kinase activities, proteins with seven transmembrane domains, or intracellular receptors.
- Signal transduction is important for cells to grow and work normally.
- The two mechanisms of signal transduction: generation of second messengers, and receptor phosphorylation.