Tyrosine Receptors

Home

Cellular Regulation Home

This page is dedicated to the understanding of Tyrosine Receptor signalling pahways that were discussed in class.

Tyrosine Receptors and the Plasma Membrane


These are simple membrane bound receptors whose activity is governed by the phosphorylation of their cytosolic tyrosine residues.

The main role of the Tyrosine Receptor is to cascade a signal of a  change in the external environment to the cell. One of the ways this is accomplished is through the phosphorylation of plasma membrane lipids.

Tyrosine receptors have the ability to act as dimers and phosphorylate each other. This phosphorylation causes a cluster of tyrosine receptors to be present at certain areas of the plasma membrane. It is possible to supress or silence the activities of the Tyrosine receptors by blocking their ability to dimerize (and thus recruit) with other Tyrosine receptors.

The cytosolic component of the Tyrosine Receptor becomes phosphorylated when the extracellular component comes into contact with its specific ligand. The cytosolic phosphorylated tyrosine residue is targeted by SH2 domains of various protiens.

Overabundence or deregulation of HER2 can result in  breast cancer,

Phosphatidylinositol (PI 4,5 P2)
This lipid is found in the plasma membrane of cells. Compared to the other lipids that make the bilayer, Phosphatidylinositol is quite a minor component.

The lipid usually has Carbons 4 and 5 phosphorylated. In this bi-phosphorylated state, the membrane lipid does not interact with any signalling pathway. The lipid must be phosphorylated for a third time to have the ability to join any signalling pathways.

Phosphorylation of PI4,5P2 --> PI3,4,5P3


The main protein that phosphorylates PI4,5P2 is named PI3K (Phosphatidylinositol 3 Kinease). PI3K contains two subunits; P110 and P85, aptly named by their protein molecular weights. PI3K's SH2 domain interacts with the phosphorylated tyrosine residue of the membrane bound receptor and the P110 domain interacts with PI4,5P2 phosphorylating its 3rd carbon.

Phosphorylation of PI4,5P2 creates PI3,4,5,P3 which has signalling capabilites. It is worthy to note that the relative abundance of PI3,4,5P3 is low compared to its unphosphorylated counterpart PI4,5P2. This difference in abundance is important, especialling in signalling- we do not want to have to much of a signal transduction. The production of PI3,4,5P3 is also localized in certain areas of the membrane and detected as a spike after stimulation of Growth Factor.

PH domains
Pleckstrin homology domain is a protein domain that can bind with the phosphatidylinositol lipids and G-protiens.

PH domains play a role in recruiting proteins to different membranes, enabling them to interact with various different sinalling pathways.

PKB (or AKT-1): Protein Kinease B
A serine/threonine specific protien kinease that plays a key role in multiple cellular processes.

PKB contains a PH domain specifcially for binding with the phosopholipids that are present in the lipid-by-layer of the cell.

PKB and PI3,4,5P3 Pathway
The PKB floats freely in the cell. When it comes in close proximity to the PI3,4,5P3 lipid present in the membrane, its PH domain attaches to the

phospholipid. The PH domain located on PKB keeps the protein in its inactive state. PKB can be activated by the phosphoylation of its serine residues.

PDK1 is a protein who has the ability to phosphorylate one of the necessary serine residues that activates PKB. PDK1 also has a PH domain that recgonizes the PI3,4,5,P3 lipid in the by-layer, thus making it locally close to the PKB protien. But  in order for PDK1 to phosphorylate PKB and activate it, a serine residue on PKB must be phosphorylated by mTOR (turc2).
 * PDK1 T308
 * mTOR S473

A protein named mTOR (turc2) targets the serine residue that will allow for the PDK1 and PKB interaction. mTOR is complexed with both RICTOR. (*note figure has Rheb within Turc2 complex when it Rheb is only included in Turc1 complex)

Once mTOR phosphorylates the first serine residue on PKB, PDK1 is then allowed to phosphorylate the final Thr residue in order to activate the PKB protein. Once PKB is fully phosphorylated, it dissassociates from the membrane and freely floats around the cell in signalling pathways.

PKB targets
The PKB protien specifcally targets other proteins that have a secific basiophillic sequence (RxRxxSӨ where Ө is any hydrophobic amino acid). Another important recgonition sequence for PKB is RxRxxSӨP (the difference is the proline amino acid after the hydrophobic amino acid). This sequence is also a target for the protien 14-3-3.

PKB phosphorylates the recgonition sequence which then allows for the interaction and binding of the 14-3-3 protien.

FOXO
One protein which follows this PKB mechanism is the FOXO class of transcription factors. FOXO has been shown to play key role in metabolism of certain organisms. For instance, in C.Elegans, when FOXO is active in the nucleus it illicits a gene change that makes the organism undergo a hibernation metabolic change (a change in metaboism in a situation of low to no food source).



PKB phosphorylates FOXO which causes the protien to be recgonized and bound by 14-3-3. The binding of the two protiens causes the inactivation of FOXO making it leave the nucleus and allowing for growing conditions in the cell to resume. For instance, if the cell is under low insulin conditions, there will be low PKB-FOXO interactions. Thus FOXO remains in the nucleus acting as a transcription factor that causes the cell to undergo hibernation condiitions. When insulin levels rise, PKB-FOXO interactions increase and FOXO is bound to 14-3-3 causing it to leave the nucleus. Thus genes are no longer being targeted for a hibernation response and the cell is allowed to undergo growth.

14-3-3 promoting export from the nucleus
Protiens that need to be imported into the cell nucleus have a specific string of amino acids called a Nuclear Localization Sequence (NLS). The NLS is targeted by nuclear transport protiens such as Importin. Transport of a nucleus bound protein by Importin is done in a cyclic fashion. First, Importin recgonizes the NLS domain on the target portein for transport. The Importin-substrate complex is then shuttled into the nucleus via nuclear pores. Once inside the nucleus, Importin dissociates with its cargo and associates with RAN to be shuttled out of the nucleus for further use. Ran binds to Importin when it is in the GTP bound state. When RAN and Importin are shuttled out of the nucleus, it interacts with GAP to turn it into its GDP bound form thus dissociating with Importin. In the nucleus, RAN interacts with a GEF turning it back to its GTP bound state and the cycle continues.

14-3-3 competition with Importin
If FOXO is in the nucleus it will always bind to DNA, so in order to silence its effects it must be removed from the nucleus. When FOXO is phosphylated by PKB it will bind to 14-3-3. 14-3-3 nullifies the effect of the NLS domain of FOXO (competing with the binding of Importin).

There are cancer models associated with the knock out of FOXO. Causing a knockout of FOXO leads to tumorigenesis since FOXO is a transcription factor that casues cell growth to be stagnant. PKB can then also be considered an oncogene.

PTEN and SHIP
The levels of PI3,4,5P3 (PIP3) and its di-phosphorylated cousin PI45P2 (PIP2) in the plasma membrane are kept in check by the phosphotase PTEN.

PTEN (Phophatase and tensin Homolog) is a protein that acts as a tumor supressor through the action of its phophatase protein product. PTEN has the specific function of changing PIP3 back into PIP2 thus lowering its signalling capabilites.

The SHIP protein does a similar function as PTEN expect that it changes PIP3 into PI34P2 (where the phophate is removed from the 5th carbon as opposed to the 3rd carbon). This PIP3 modification lipid has a hypomorphc function to PIP3 (i.e. it can still signal with PKB but not as efficiently as PIP3).

It has been shown that human cancers usually have a high rate of mutation in their PTEN proteins. Thus, PTEN is a strong tumor supressor.

TORC1 & 2 and S6K
TORC1 and TORC2 are two important complexes of proteins that are used in metabolic signalling in cells. TORC2 consists of the follwing protein subunits; TORC2, RICTOR and mTOR. TORC2 phosphorylates PKB preparing it to be phosphorylated a second time by PDK to become active.

TORC1, which is similar to TORC2, contains the following protein subunits; mTOR, RAPTOR, FKB38, and RHEB. TORC1 specifically phosphorylates the Ribosomal protein S6K. S6K is important in the control of protein translation in the cell.



The subunits of TORC1 have important properties when it comes to the control of TORC1 interaction with its S6K target. RHEB is a small G-protein that when its in GTP bound form promotes the interaction between TORC1 and S6K. This promotion is done through the inhibition of the inhibitor subunit FKB38. FKB38 specifically inhibits the interaction of TORC1 with S6K.

The TORC1 complex is activated/inhibited depending on the amount of nutrients available in the cell. For instance, if the cell is experienceing levels of low nutrition (ex low levels of glucose) then the TORC1 complex is silenced (since we do not want protein translation). TORC1's activity is controlled by the TSC1/TSC2 protein complex.

TSC1/TSC2 Complex and Nutrition levels
The TSC1/TSC2 complex is a GAP that specficially inhibits the activity of RHEB in the TORC1 complex (esentially silencing the TORC1 complex and downstram protein translation). TSC1/TSC2 activity is controlled by many different factors that correlate to the amount of nutrients/energy available in the cell.

For instance, TSC1/TSC2 activity is increased by the presence of increased levels of AMP (the mono-phospho of ATP), meaning that there is a low level of nutrients available. TSC1/TSC2 activity is repressed by the presence of certain amino acids and PKB meaning that there is sufficient nutrients in the cell available for protein translation.

Hemimegalencephaly
Mutation causing overgrowth of one hemisphere in the brain and in some cases the skin cells. These mutants usually effect the AKT pathway to cause constitive activation thereby leading to high levels of S6 phosphorylation within the blood. High levels of S6 will lead to ribosome biosynthesis and protein translation nescessary for cell growth. Hemimegalencephaly leads to multiple seziures a day and can be treated in some by surgically seperating the corpus callosum to allow the brain to reform neural network and/or drugs. Requires further studying because in children it was a sporatic mutant at the same location for 4 children. This is coorlative data but if done in a mouse model could be used to make causational link.
 * PI3K mutation
 * AKT mutation