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1. When a growth factor binds to a receptor tyrosine kinase, the receptor becomes phosphorylated on tyrosine residues.

a. How does this occur?

b. Why is this phosphorylation important in initiating an intracellular signaling cascade?

c. Why does phosphorylation change the activity of proteins? (What happens to the protein physically when it is phosphorylated?)

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Deanna Hettinger
Deanna HettingerLv2
28 Sep 2019
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Related questions

Select the correct order of events (1-10) for the signal transducon pathway iniated by the EGF receptor. (place the number 11 for those which are not applicable).

_____ MEK is phosphorylated by RAF.
____ The receptor undergoes a conformaonal change.
_____ The second messengers IP3 and DAG are produced.

_____ The cytoplasmic tail of the receptor is phosphorylated.

_____ An adaptor protein binds to the phosphotyrosine residues, recruing SOS and Ras.

_____ MEK is phosphorylated by ERK.

_____ RasGAP stimulates GTPase activity of Ras which returns to the GDP bound state.

_____ ERK is phosphorylated by MEK.

_____ The Galpha subunit dissociates from Gbeta/Ggamma.

_____ ERK enters the nucleus and smulates cell proliferaon.

_____ The receptor dimerizes acvang its tyrosine kinase acvity.

_____ The Galpha subunit binds to Phospholipase C.

____ Epidermal growth factor binds to the receptor.

_____ Ras is phosphorylated by RAF.
____ The Galpha protein releases GDP and binds GTP.

____Glucagon binds to the receptor.

The binding of platelet derived growth factor (PDGF) to the PDGF receptor stimulates phosphorylation of 8 tyrosines in the receptor's cytoplasmic domain. The enzyme phosphatidylinositol 3'kinase (PI 3-kinase) binds to one or more of the phophotyrosines through its SH2 domains and is thereby activated. To identify the activation phosphotyrosines, you synthesize 8 pentapetides that contain the critical tyrosines (at the N-terninus) in their phosphorylated or unphosphorylated forms. You then mix an excess of each of the various pentapeptides with phosphorylated PDGF receptor and PI 3-kinase. Immunoprecipitation of the PDGF receptor will bring down any bound PI 3-kinase, which can be assayed by its ability to add 32P-phosphate to its substrate.

A) Draw a flow diagram showing how this assay works.

B) Do your results support the notion that PI-kinase binds to phosphotyrosines in the activated PDGF receptor? Why or why not?

C) THe amino acid sequences of the PDGF receptor pentapeptide tested above (number accodingly to the position of tyrosine in the PDGF receptor) and peptide segments that are known to bind to PI 3-kinase in other activated receptors are shown below. Which amino acid residues are most important and why?

684 YSNAL YMMMR (FGF receptor)

708 YMDMS YTHMN (insulin receptor)

719 YVPML YEVML (hepatocyte growth factor receptor)

731 YADIE YMDMK (steel factor receptor)

739 YMAPY YVEMR (CSF-1 receptor)

743 YDNYE

746 YEPSA

755 YRATL

QUESTION 1:

(a)Examine the Figure below relating to the S.pombe cellcycle:

1) Describe the function of Cdc25 and Wee1and why deficit of Cdc25 or excess of Wee1 can cause G2arrest

2) b) Describe how deficit of Wee1 or excess ofCdc25 may result in abnormally small cells

(b) Theactivities of Wee1 tyrosine kinase and Cdc25 tyrosine phospatasedetermine the state of phosphorylation of tyrosine 15 in the Cdk1component of mitotic cyclin-Cdk complexes (MPF). When tyrosine 15is phosphorylated, M-Cdk is inactive; when tyrosine 15 is notphosphorylated, M-Cdk is active. Just as the activity of M-Cdkitself is controlled by phosphorylation, so too are the activitiesof Wee1 kinase and Cdc25 phosphatase. The regulation of thesevarious activities can be studied in extracts of frog oocytes. Insuch extracts Wee1 tyrosine kinase is active and Cdc25 tyrosinephosphatase is inactive. As a result M-Cdk is inactive because itsCdk1 component is phosphorylated on tyrosine 15. M-Cdk in theseextracts can be rapidly activated by addition of okadaic acid,which is a potent inhibitor of serine/threonine phosphatases. Usingantibodies specific for each component it is possible to examinetheir phosphorylation states by changes in mobility upon gelelectrophoresis.

(Phosphorylated proteins generally run slower than theirnon phosphorylated counterparts).

1) Based on theresults with okadaic acid, decide whether the active forms of Wee1and Cdc25 are phosphorylated or unphosphorylated. In the figureindicate the phosphorylated forms of Wee 1 and Cdc25 and label thearrows connecting their active and inactive forms to show whichtransitions are controlled by protein kinases and which by proteinphosphatases.

2) Are the proteinkinases and phosphatases that control Wee1 and Cdc25 specific forserine/threonine side chains or for tyrosine side chains? How doyou know?

3) How doesaddition of okadaic acid cause an increase in phosphorylation ofWee1 and Cdc25, but a decrease in phosphorylation ofCdk1?

4) If you assumethat Cdc25 and Wee1 are targets for phosphorylation by activeM-Cdk, can you explain how the appearance of a small amount ofactive M-Cdk would lead to its rapid and completeactivation?

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