4.2: Introduction to Signal Transduction

Last lesson, we were introduced to the signal transduction pathway. We learned about the general concepts and terms that are related to signal movement. Topic 4.2 delves deeper into the specifics of transduction, providing examples of each step of the process.

Vocab List

Written Explanation

Signal Transduction Recap:

Diagram of the signal transduction pathway

Types of Receptors:

Receptor proteins can be located either inside the cell, or on the outer membrane surface. Intracellular (inside the cell) receptors typically bind small hydrophobic ligands known as steroid hormones.

Diagram of an intracellular receptor

Cell surface receptors are integral proteins that typically bind to hydrophobic/charged ligands. Their extracellular surface is what receives the signal, and the intracellular surface is what transfers the message to the cell.

Ligand-gated ion channels

When this receptor binds to its ligand, the receptor opens a gate, allowing ions to enter/exit the cell. The charge and concentration of the ions can continue the signal transduction pathway or elicit a cellular response.

 Diagram of a ligand-gated ion channel

G-protein coupled receptors

This receptor transmits its signal to a G-protein (it is coupled to it). Then, the G-protein phosphorylates a GDP, turning it to GTP and releasing it to the cell to continue the pathway.

 Diagram of a G-protein coupled receptor

Receptor tyrosine kinases

This receptor is linked to an enzyme which phosphorylates tyrosine. The activated tyrosine releases some activated proteins to continue the pathway.

 Diagram of a receptor tyrosine kinases
Some examples of cell surface receptors

Types of Transduction:

Now that the cell has received a signal, it needs to move and modify it to its destination. This can take many different forms in different cells and for different types of signals, but a few of the important ones are listed below.

When a cell receives a signal, it may be very weak, or be far from its destination of causing a cellular response. To amplify the signal, a transduction pathway may use a phosphorylation cascade. In a phosphorylation cascade, the initial reception results in the activation of a kinase, by phosphorylating it (adding a phosphate group to it). Then, that activated kinase activates another few kinases, cascading through the cell, and amplifying the signal. Eventually, the activated kinases will fully transfer the signal and trigger a response. When the response occurs, cells usually use the enzyme phosphatase to deactivate the kinases.

Diagram of phosphorylation cascade

Another way to pass the message from reception to destination is with second messengers. These small, non-protein molecules are released by a receptor and transduct the message.

Calcium ions (Ca2+)

Released by ligand-gated ion channels, these ions can bind to proteins and trigger a response.

 Diagram of calcium ion transduction

Cyclic adenosine monophosphate (cyclic AMP / cAMP)

This messenger is created by an enzyme called adenylyl cyclase. Once generated, it can activate protein kinase A (PKA)

 Diagram of cAMP transduction
Two types of second messengers