Signal transduction is a process where cells translate signals from the extracellular environment into changes inside the cell. The type of signals coming from the extracellular environment vary depending on the cell type and can include chemical signaling molecules from other cells, like hormones, from the cell itself, or changes in the environment, such as pressure, temperature, or light. All cells require signal transduction in order to sense their environment and respond appropriately. Cells in multicellular organisms, like humans, must have signal transduction pathways to coordinate their responses and help the organism maintain homeostasis.
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Signal transduction allows cells to communicate through a three step process. The main three steps include:
Reception is the first step and includes the process where the cell senses a signal in the environment. This is accomplished with proteins on the surface of the cell membrane called receptor proteins. These proteins detect changes, such as chemical signals from other cells, or changes to pressure, light or other sensations. Receptor proteins are transmembrane proteins, meaning they span the cell membrane and thus can detect changes on the extracellular side of the cell and transmit that signal to the inside of the cell. Some receptor proteins may cause a change in ion permeability, phosphorylation of other proteins, or a change in localization in the membrane. For example, growth factor receptors bind to growth hormones in the body and cause changes in the phosphorylation status of the receptor protein inside the cell. This triggers other protein activation in the next step, called transduction.
The next step in the signal transduction pathway is transduction. Here, the signal sensed by the receptor protein is transmitted to the inside of the cell. This may involve activating or deactivating other proteins. Transduction can activate a great number of molecules at once, causing a cascade of activation inside the cell. This helps the singular signal from outside the cell be translated quickly and efficiently into a change inside the cell.
Ultimately, the transduction leads to a response. A response is a change in cellular behavior due to the signal. The following are examples of what a response could include in a cell:
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Thus, the three steps of signal transduction allow for quick and efficient changes in cellular behavior in response to a change in the environment.
Signal transduction depends on multi-step processes, called signal transduction pathways. During transduction, signals can be amplified by increasing the number of proteins being activated. This allows for a more widespread and quicker response.
The signal transduction pathway creates a cascade pattern of activation within the cell. The receptor protein activates other proteins that activate other proteins, forming an activation cascade that results in a change in cell behavior. Although the specific proteins activated are specific for each signal transduction pathway, there are some common activation strategies. For example, many of the proteins in the cascade are kinases. Kinases are proteins that add a phosphate group to another molecule, called phosphorylation. In signal transduction pathways, multiple proteins in the pathway can be kinases, creating an effect called a phosphorylation cascade. Other proteins can remove phosphate groups, called phosphatases. These proteins carry out the process of dephosphorylation which can turn off the signaling cascade.
In between the receptor proteins and the kinases are a type of molecule called a second messenger. Second messengers are named such because they are in between the first step of a signal transduction pathway, the receptor protein, and other proteins further down the cascade. Second messengers are a wide range of molecules. They can be hydrophobic, hydrophilic, and even include different types of molecules such as gases or free radicals. All second messengers are capable of diffusing throughout the cell where they can activate or inhibit their target proteins. They typically also are present in low concentrations in a resting cell, but can quickly be produced or released to carry out the signal transduction cascade. Some examples of second messengers include:
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Relay molecules are any proteins or other molecules that are involved in the signal transduction process. Some of the most commonly used relay molecules are listed in the table below.
| Relay Molecule Type | Function | Examples |
|---|---|---|
| Receptor proteins | Receive signals from the external environment | Insulin receptor, estrogen receptor alpha and beta, wnt receptor |
| Second messengers | Transmit signals from receptors to the internal compartments of the cell | PIP3, nitrous oxide, cAMP |
| Protein kinases | Proteins that phosphorylate other molecules | Protein kinase C, Epidermal growth factor receptor |
| Protein phosphatases | Proteins that dephosphorylate other molecules | Protein phosphatase 2A, protein phosphatase 2C |
| Transcription factors | Proteins that turn on gene expression | p53, beta catenin, STAT 3 |
Signal transduction is the process of cells sensing signals in their environment to change their behavior. The steps of signal transduction are known as a signal transduction pathway. The signal transduction pathway has three main steps:
The process of transduction involves an amplification of the original signal that allows the cell to quickly activate many molecules using fewer steps. The proteins activated in a signaling cascade are often kinases, proteins that add phosphate groups to other molecules, called phosphorylation. Kinases can activate other kinases to create a phosphorylation cascade in the cell. These events ultimately trigger changes in cell behavior, such as motility, gene expression, cell survival, cell death and more.
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The meaning of signal transduction is a process that cells use to sense changes in the environment and communicate with other cells. Signal transduction is both important for unicellular organisms to sense the environment and for multicellular organisms to communicate between cells.
During signal transduction cells receive a signal from the external environment. This is translated into protein activation or deactivation in the cell, which ultimately results in a change in cellular behavior.
There are three main steps in the signal transduction pathway:
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