Structural Biochemistry/Cell Signaling Pathways/Receptor Tyrosine Kinases

As we know, kinases is the enzyme that phosphorylate proteins. Serine, tyrosine and threonine kinases are the three most common. Especially, the receptor tyrosine kinases (RTK) play an important role in the cell cycle, cell migration, cell metabolism and many other substantial cell functions. In animal, receptor tyrosine kinases is the membrane receptors that recognize hydrophilic ligands. In plant, instead of tyrosine kinases, serine-threonine kinases, called plant receptor, were used.

How Receptor Tyrosine Kinases are Activated
Receptor tyrosine kinases is a transmembrane structure that anchors into the membrane. The structure consists of ligand-binding domain in the outside and the kinase domain in the inside of the cell. When received the signal, the inside domain adds phosphate group to tyrosine. The ligand-binding domain is made from two receptor complexes that phosphorylate each other in a process called autophosphorylation. RTKs are activated by autophosphorylation. The propagation of the signal in the cytoplasm is triggered. Tyrosine kinase started to phosphorylate intracellular targets. Depending on what response proteins the cell has inside its cytoplasm, the cell would have very different response to the signal.

How the Signal was Interpreted
The signal was transferred from the receptor into the cytoplasm via proteins that bind to phosphorylated tyrosines. Then, those binding-proteins will convert the signal into response inside the cell. Below is the two common classes of proteins that can bind to phosphotyrosines:

1. The Insulin Receptor: Insulin is the hormone that control constant level of blood glucose. It works by binding to RTK and lowering blood glucose. When the blood glucose level is high, the insulin response protein is phosphorylated when it binds to the receptor. Then, this protein binds to additional protein, activating the enzyme glycogen synthase. Glycogen synthase can catalyze the reaction converting glucose to glycogen, making the reaction goes much faster. Therefore, the blood glucose level was maintained.

2. Adapter Proteins: Those proteins function as connection between the receptor and proteins that respond to the signal. A good example of adapter proteins’ functions is the activation of Ras protein. Adapter protein binds to the receptor. Then, it is able to activate Ras protein, which involve in cellular signal transduction.

Mitogen-activated Protein (MAP)- One Important Class of Cytoplasmic Kinases
Mitogen stimulates cell divition by triggering mitosis. It functions by initiating the normal division-control pathways. Kinase cascade (sometimes called phosphorylation cascade) can activate MAP kinase. Kinase cascade is a signaling module that phosphorylates each other. At the end of the process, it initiates MAP kinase (see the mapping below)



In each step, one enzyme can act with many substrates, resulting in the formation of many final products. Consequently, the original signal was amplified through each step. This is also the main function of kinase cascade. How the cell response depends on the MAP kinase.

In order to maximize the function of kinase cascades, a type of proteins, called scafford proteins, rearrange the kinase cascade into a protein complex. The function of each cascade is optimized because of their spatial organization. Even thougha the arrangement helps increase the effect of the cascade significantly and allows segregation of signal modules in different locations, this arrangement also has disadvantage. The amplification of the original signal was decreased since the kinase cascades are not allowed to move freely to bind new substrate.

Groups of MAP kinase

A total of six different forms of MAP kinase have been derived from mammals.

1. Extracelluar-signal regulated kinases (ERK1, ERK2)-ERK1/2 produce long lasting changes in cell signaling. These signals are usually activated in response to proteins or steroids that are capable of stimulating cellular growth (growth factors) and the organic compound phorbol ester also known as tumor promoter. This is important for the human cell because it regulates both cell proliferation and also cell differentiation.

2. c-Jun N terminal kinases (JNKs) also known as Stress Activated Protein Kinase (SAPK). JNK are activated by cytokines, certain ligands for GPCR, agents that interfere with DNA and Protein synthesis, etc. Three of the JNK genes (1,2,3) have been derived from humans. Studies performing knockout of these genes have revealed that these kinase are associated with apoptosis (the cellular signaling leading to the deal of potentially cancerous cells) and immune response.

3. p38 isoforms- similar to JNKs,function as signaling pathways like the other kinase. They are involved in apoptosis like (JNKs) and cell differentiation. Some of the things these isoforms are responsive to are cytokines, stress stimuli, ultraviolet irradiation, heat shock, and osmotic shock.

4. ERK5 (MAPK7)- Further performs in cell proliferation and is activated by both growth factors and stressful stimuli. It has been found that this kinase has a key role in cardiovascular development and neural differentiation. Also, it has unique properties compared to ERK1/2 including its carboxyl-terminal half which has a unique function that is different from ERK1/2.

5. ERK3/4 which also is MAPK6 (ERK3) and MAPK4 (ERK4)- these are considered cytoplasmic proteins that bind, translocate, and finally activate MK5. Furthermore, the stability of ERK3 and 4 are different in that ERK4 is more stable than ERK3.

6. ERK7/8 (MAPK15)- it performs similar to the other pathways in that it is activated by various conditions and molecules leading to signaling. A similarity between ERK3/4 is that it contains a long C terminus as well.

How Receptor Tyrosine Kinases are De-activated
Every receptor must be regulated to make the cell function properly. Too many activations of one receptor may result in the hinder of response to other signal. Therefore, RTKs must be inactivated at some point. The cell uses two mechanisms to deactivate the receptor: dephosphorylation and internalization by endocytosis. Dephosphorylation is the reverse reaction of phosphorylation, making the signal stop being transmitted. Internalization by endocytosis is when the receptor was transferred into the cytoplasm to be degraded.

The Activation of Tyrosine Kinase Receptor, Leading to Cellular Response
1.) Before the binding of the signaling molecule, the receptors act as individual units called monomers. Each of these individual monomer units have an extracellular ligand-binding site, an alpha-helix in the membrane, a ligand binding site, and an intracellular tail with many tyrosine amino acids. The monomers are inactive until reaction with a signaling molecule that binds to its ligand-binding site.

2.) The ligand binding site is susceptible to the binding of various signaling molecules which causes two receptor monomers to work hand-in-hand with each other, forming a complex called a dimer. This process is called dimerization.

3.) The process of dimerization activates the tyrosine kinase region of each individual monomer. Both of these tyrosine kinases adds a phosphate from an ATP molecule to a tyrosine on the tail of the other monomer.

4.) After this process, the receptor is completely activated and it is then recognized by specific relay proteins located within the cell. Each protein binds to a specific phosphorylated tyrosine, leading to a structural alteration which activates the bound protein. Each of these activated proteins trigger a transduction pathway, causing the desired cellular response.