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Signal relay pathways

Learn how signals are relayed inside a cell starting from the cell membrane receptor. The chains of molecules that relay intracellular signals are known as intracellular signal transduction pathways.


Once a signaling molecule (ligand) from one cell has bound to a receptor on another cell, is the signaling process complete?
If we're talking about intracellular receptors, which bind their ligand inside of the cell and directly activate genes, the answer may be yes. In most cases, though, the answer is no—not by a long shot! For receptors located on the cell membrane, the signal must be passed on through other molecules in the cell, in a sort of cellular game of "telephone."
The chains of molecules that relay signals inside a cell are known as intracellular signal transduction pathways. Here, we’ll look at the general characteristics of intracellular signal transduction pathways, as well as some relay mechanisms commonly used in these pathways.

Binding initiates a signaling pathway

When a ligand binds to a cell-surface receptor, the receptor’s intracellular domain (part inside the cell) changes in some way. Generally, it takes on a new shape, which may make it active as an enzyme or let it bind other molecules.
The change in the receptor sets off a series of signaling events. For instance, the receptor may turn on another signaling molecule inside of the cell, which in turn activates its own target. This chain reaction can eventually lead to a change in the cell's behavior or characteristics, as shown in the cartoon below.
Because of the directional flow of information, the term upstream is often used to describe molecules and events that come earlier in the relay chain, while downstream may be used to describe those that come later (relative to a particular molecule of interest). For instance, in the diagram, the receptor is downstream of the ligand but upstream of the the proteins in the cytosol. Many signal transduction pathways amplify the initial signal, so that one molecule of ligand can lead to the activation of many molecules of a downstream target.
The molecules that relay a signal are often proteins. However, non-protein molecules like ions and phospholipids can also play important roles.


The cartoon above features a bunch of blobs (signaling molecules) labeled as “on” or “off.” What does it actually mean for a blob to be on or off? Proteins can be activated or inactivated in a variety of ways. However, one of the most common tricks for altering protein activity is the addition of a phosphate group to one or more sites on the protein, a process called phosphorylation.
Phosphate groups can’t be attached to just any part of a protein. Instead, they are typically linked to one of the three amino acids that have hydroxyl (-OH) groups in their side chains: tyrosine, threonine, and serine. The transfer of the phosphate group is catalyzed by an enzyme called a kinase, and cells contain many different kinases that phosphorylate different targets.
Phosphorylation often acts as a switch, but its effects vary among proteins. Sometimes, phosphorylation will make a protein more active (for instance, increasing catalysis or letting it bind to a partner). In other cases, phosphorylation may inactivate the protein or cause it to be broken down.
In general, phosphorylation isn’t permanent. To flip proteins back into their non-phosphorylated state, cells have enzymes called phosphatases, which remove a phosphate group from their targets.

Phosphorylation example: MAPK signaling cascade

To get a better sense of how phosphorylation works, let’s examine a real-life example of a signaling pathway that uses this technique: growth factor signaling. Specifically, we'll look at part of the epidermal growth factor (EGF) pathway that acts through a series of kinases to produce a cellular response.
This diagram shows part of the epidermal growth factor signaling pathway:
Phosphorylation (marked as a P) is important at many stages of this pathway.
  • When growth factor ligands bind to their receptors, the receptors pair up and act as kinases, attaching phosphate groups to one another’s intracellular tails. Read more in the article on receptors and ligands.
  • The activated receptors trigger a series of events (skipped here because they don't involve phosphorylation). These events activate the kinase Raf.
  • Active Raf phosphorylates and activates MEK, which phosphorylates and activates the ERKs.
  • The ERKs phosphorylate and activate a variety of target molecules. These include transcription factors, like c-Myc, as well as cytoplasmic targets. The activated targets promote cell growth and division.
Together, Raf, MEK, and the ERKs make up a three-tiered kinase signaling pathway called a mitogen-activated protein kinase (MAPK) cascade. (A mitogen is a signal that causes cells to undergo mitosis, or divide.) Because they play a central role in promoting cell division, the genes encoding the growth factor receptor, Raf, and c-Myc are all proto-oncogenes, meaning that overactive forms of these proteins are associated with cancer1.
MAP kinase signaling pathways are widespread in biology: they are found in a wide range of organisms, from humans to yeast to plants. The similarity of MAPK cascades in diverse organisms suggests that this pathway emerged early in the evolutionary history of life and was already present in a common ancestor of modern-day animals, plants, and fungi2.

Second messengers

Although proteins are important in signal transduction pathways, other types of molecules can participate as well. Many pathways involve second messengers, small, non-protein molecules that pass along a signal initiated by the binding of a ligand (the “first messenger”) to its receptor.
Second messengers include Ca2+ ions; cyclic AMP (cAMP), a derivative of ATP; and inositol phosphates, which are made from phospholipids.

Calcium ions

Calcium ions are a widely used type of second messenger. In most cells, the concentration of calcium ions (Ca2+) in the cytosol is very low, as ion pumps in the plasma membrane continually work to remove it. For signaling purposes, Ca2+ may be stored in compartments such as the endoplasmic reticulum.
In pathways that use calcium ions as a second messenger, upstream signaling events release a ligand that binds to and opens ligand-gated calcium ion channels. These channels open and allow the higher levels of Ca2+ that are present outside the cell (or in intracellular storage compartments) to flow into the cytoplasm, raising the concentration of cytoplasmic Ca2+.
How does the released Ca2+ help pass along the signal? Some proteins in the cell have binding sites for Ca2+ ions, and the released ions attach to these proteins and change their shape (and thus, their activity). The proteins present and the response produced are different in different types of cells. For instance, Ca2+ signaling in the β-cells of the pancreas leads to the release of insulin, while Ca2+ signaling in muscle cells leads to muscle contraction.

Cyclic AMP (cAMP)

Another second messenger used in many different cell types is cyclic adenosine monophosphate (cyclic AMP or cAMP), a small molecule made from ATP. In response to signals, an enzyme called adenylyl cyclase converts ATP into cAMP, removing two phosphates and linking the remaining phosphate to the sugar in a ring shape.
Once generated, cAMP can activate an enzyme called protein kinase A (PKA), enabling it to phosphorylate its targets and pass along the signal. Protein kinase A is found in a variety of types of cells, and it has different target proteins in each. This allows the same cAMP second messenger to produce different responses in different contexts.
cAMP signaling is turned off by enzymes called phosphodiesterases, which break the ring of cAMP and turn it into adenosine monophosphate (AMP).

Inositol phosphates

Although we usually think of plasma membrane phospholipids as structural components of the cell, they can also be important participants in signaling. Phospholipids called phosphatidylinositols can be phosphorylated and snipped in half, releasing two fragments that both act as second messengers.
One lipid in this group that's particularly important in signaling is called PIP2. In response to a signal, an enzyme called phospholipase C cleaves (chops) PIP2 into two fragments, DAG and IP3. These fragments made can both act as second messengers.
DAG stays in the plasma membrane and can activate a target called protein kinase C (PKC), allowing it to phosphorylate its own targets. IP3 diffuses into the cytoplasm and can bind to ligand-gated calcium channels in the endoplasmic reticulum, releasing Ca2+ that continues the signal cascade.

And...it's even more complicated than that!

Signaling pathways can get very complicated very quickly. For instance, the full version of the epidermal growth factor signaling pathway we saw earlier looks like a huge hairball and takes up an entire poster if you try to draw it out! You can see this for yourself in Sal's video on the MAPK pathway.
This complexity arises because pathways can, and often do, interact with other pathways. When pathways interact, they basically allow the cell to perform logic operations and "calculate" the best response to multiple sources of information. For instance, signals from two different pathways may be needed to activate a response, which is like a logical "AND." Alternatively, either of two pathways may trigger the same response, which is like a logical "OR."
Another source of complexity in signaling is that the same signaling molecule may produce different results depending on what molecules are already present in the cell3. For example, the ligand acetylcholine causes opposite effects in skeletal and heart muscle because these cell types produce different kinds of acetylcholine receptors that trigger different pathways4,5,6.
These are just a few examples of the complexities that make signaling pathways challenging, but also fascinating, to study. Cell-cell signaling pathways, especially the epidermal growth factor pathway we saw earlier, are a focus of study for researchers developing new drugs against cancer7,8.

Want to join the conversation?

  • piceratops seed style avatar for user Marta Lanza
    What regulates the distribution of fluid between interstitial and intracellular compartments?
    (7 votes)
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  • piceratops seedling style avatar for user TidoThuan123
    What mechanism ampfily the signal ? and why they have to do that ?
    (5 votes)
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    • female robot grace style avatar for user tyersome
      Many if not all of the steps described in this article can amplify a signal.

      For example each step in the section titled Phosphorylation example: MAPK signaling cascade involves a kinase phosphorylating downstream molecules. For instance, if each time a RAF molecule gets activated it phosphorylates 20 molecules of MEK, then you've amplified the signal by 20 times.

      Why questions are typically difficult in biology, but I'll give a slightly hypothetical example of why this is important. One response a cell needs to divide is an increased rate of protein synthesis, so maybe at the end of the pathway ERK needs to phosphorylate the (up to) 10 million ribosomes to increase their activity. This would take a very long time if only a few ERK molecules were activated! In addition, there are 244 known direct targets identified for ERK in humans§, so even if you only had a few of each of those molecules amplification would still be needed to get a timely response!
      (Note: Those targets often need to modified in multiple locations — for example at least two ribosomal proteins are targets of ERK, one on two different amino acids.)


      Does that help?
      (12 votes)
  • leaf green style avatar for user colin
    How exactly does the cell signalling reverse?
    (5 votes)
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    • starky ultimate style avatar for user Greacus
      This differs per pathway. For pathways that release Ca2+ for example, the Ca2+ pumps pump the Ca2+ back into the ER, so there will be less and less Ca2+ in the cytosol.
      Also, in these messenger cascades, often at the end of the cascade a phosphatase is activated, which will deactivate the kinases. If only a very short, small signal is needed, the phosphatases will be activated earlier on in the cascade (depends on the pathway).
      For the ligand/receptor interactions: a ligand often binds it's receptor for a short while, but not very long, so the initial signal won't be active for long.
      Hope this helps you out a bit!
      (9 votes)
  • piceratops seedling style avatar for user Maryania Chau
    Do all signaling pathways simply turn on or turn off enzymes?
    (5 votes)
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    • female robot grace style avatar for user tyersome
      No, they can also regulate the transcription of genes, the translation of proteins, the behavior of structural proteins, vesicle transport within cells, inhibitors of enzymes, and countless other processes. You can probably assume that most processes in a cell are affected in some way by at least one signaling pathway!
      (7 votes)
  • leafers sapling style avatar for user sara
    How does cAMP activate protein kinase A? If it's through phosphorylation, does the cAMP disappear after it has done its job (because it loses its only phosphate)?
    (3 votes)
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    • male robot hal style avatar for user rumaan
      No, not through phosphorylation. Note that the text says that it "activates" PKA and the latter then establishes a phosphorylation cascade.

      PKA is composed of 4 subunits - 2 Regulatory (R) and 2 Catalytic (C) subunits; and as the name suggests only the 2 C subunits have a further signaling role. The R-subunits feature cAMP-binding sites.

      In the resting state when cAMP levels are low, the C-subunits are in a deactivated state due to the R-subunits. What cAMP does is that when it binds at the R sites, it causes a dissociation of the C subunits from this protein ("conformational change") making them now "activated" and thus eliciting downstream responses.

      And yes the cAMP does "disappear" after it has done its job but not quite by losing its phosphate. The enzyme phosphodiesterase converts cyclic-AMP to AMP (by hydrolyzing the 3'C-Phosphate bond) marking the termination of the pathway.

      (5 votes)
  • blobby green style avatar for user rachel.zeggane
    Do signal transduction pathways require a source of energy? Might proteins involved have to be altered or modified?
    (3 votes)
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    • blobby green style avatar for user Mugdha
      There's usually an activation of some sort in the signal transduction pathway to incite a response. So for example the proteins involved might be enzymes that receive substrate from the previous protein/step and are thus activated to function. Hope this helps!
      (3 votes)
  • male robot hal style avatar for user Hunter
    Quick question, can someone explain to me why the G Protein-Coupled receptor goes through the membrane seven times? Is this to increase the surface area in order to interact more with the membrane?
    (3 votes)
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  • blobby green style avatar for user fatuma abubaker
    can someone explain how this works with insulin and glucose? Along with GLUT2 and GLUT4 and their function in all of this? Is GLUT4 relevant for all body cells or just muscle and adipose? So confusing...
    (3 votes)
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  • duskpin seedling style avatar for user pw42663
    How exactly does a phosphorylation cascade lead to a response from the cell? Does a phosphorlized molecule act as a second messenger at the end of the cascade? I'm a little confused on how the pathway actually produces a response from the cell. Thank you!
    (3 votes)
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    • blobby green style avatar for user EC
      Yep! As stated in the article, ligand binding causes a protein kinase to be activated. This protein kinase phosphorylates other proteins in the chain (that's why its called "phosphorylation" cascade). What isn't shown in the diagram is that each kinase phosphorylates the next until one phosphorylates a transcription factor. In most cases, the activated transcription factor then transcribes a section of DNA that codes for a protein in response to the original ligand binding. Thus, the synthesis of this protein is the "response" mentioned.
      Hope this helps!
      (2 votes)
  • blobby green style avatar for user gaby
    How does the signal transduction mechanism works underlying mitosis?
    (2 votes)
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    • leafers ultimate style avatar for user Birddrawer
      I'm not exactly sure what you are asking but I'll do my best to answer. When a cell receives the appropriate growth factor, a kinase is activated. For example, in the article above, MAP Kinase is eventually activated, which phosphorylates different proteins in the cytosol and nucleus. These proteins, once phosphorylated, begin causing reactions that cause the cell to enter the S phase of the cell cycle, where DNA is replicated. Once the cell enters from G1 to S phase, it will inevitably complete the cell cycle through G2 and M, or mitosis, phase.
      (3 votes)