A number of mutations have been described in G proteins that have profound effec

Question

A number of mutations have been described in G proteins that have profound effects on their activity. For example, some mutations greatly increase the affinity of the G protein for GDP making it very difficult to be exchanged for GTP. Other mutations prevent the hydrolysis of GTP to GDP. For each of the following choices, indicate which of the following mutations is responsible: High GDP affinity RAS mutant, Hydrolysis-deficient RAS mutant, or Neither.

a. Hydrolysis-deficient RAS mutant

b. Neither

c. High GDP affinity RAS mutant

1. Continual activation of the MAP kinase pathway even in the absence of signaling molecule
2. Failure to activate MAP kinase pathway following ligand binding to receptor kinase
3. Failure of receptor kinase to dimerize in the presence of ligand
4. Phosphorylation of receptor kinase when bound by ligand
5. Change in the expression of genes in the absence of the appropriate signaling molecule
6. Target genes are not expressed even in the presence of signaling molecule
7. Increase in cytosolic kinase activity
8. No change in cytosolic kinase activity as a result of ligand binding
9. No change in membrane potential even in the presence of a signaling molecule

10. A continuously activated receptor in the absence of ligand

Explanation / Answer

1. Continual activation of the MAP kinase pathway even in the absence of signaling molecule- Hydrolysis-deficient RAS mutant

2. Failure to activate MAP kinase pathway following ligand binding to receptor kinase- High GDP affinity RAS mutant

3. Failure of receptor kinase to dimerize in the presence of ligand- Neither

4. Phosphorylation of receptor kinase when bound by ligand- Neither

5. Change in the expression of genes in the absence of the appropriate signaling molecule- Hydrolysis-deficient RAS mutant

6. Target genes are not expressed even in the presence of signaling molecule- High GDP affinity RAS mutant

7. Increase in cytosolic kinase activity- Hydrolysis-deficient RAS mutant

8. No change in cytosolic kinase activity as a result of ligand binding- High GDP affinity RAS mutant

9. No change in membrane potential even in the presence of a signaling molecule- Neither

10. A continuously activated receptor in the absence of ligand- Hydrolysis-deficient RAS mutant

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