3. (6pts each) Suppose the following GTPases are mutated so that they permanentl

Question

3. (6pts each) Suppose the following GTPases are mutated so that they permanently bind to GDP. Please predict the location where the stated cargo proteins would accumulate, and provide a brief explanation (2 sentences max). A. GTPase: Ran, Cargo: Protein with NLS sequence Cargo protein location with mutant Ran Explanation B. GTPase: Sar1, Cargo: Secreted protein Cargo protein location with mutant Sar Explanation - C. GTPase: Dynamin, Cargo: Low Density Lipoprotein (LDL) particle Cargo protein location with mutant dynamin - Explanation - D. GTPase: ARF1, Cargo: ER Resident protein with KDEL sequence Cargo protein location with mutant ARF1 Explanation -

Explanation / Answer

A. Cytosol

explanation: This is because of the GTP cycle of Ran; due to the distribution of the different enzymes across the nuclear membrane- like a GAP that hydrolyses GTP bound to Ran is available in the cytosol. When in the cytosol GAP would hydrolyze GTP bound ran, Ran-GDP would be produced. In the case of mutant Ran, the GDP produced doesnt dissociate from it in the cytosol and the complex of Ran-GDp accumulates in the cytosol

B. cytoplasm:

SAr1 GDP present in the cytosol interacts weth theGEF in the ER membrane to undergo a change from GDP to GTP. It is after this exchange SAr1 is able to insert into the ER membrane to pick up the cargo. If the bound GDP is not exchanged it accumulates in the cytosol.

C) Clathrin coated vesicles containing LDL bound to LDL-receptors (a cell surface receptor) are internalized in clathrin/AP2 coated vesicles. These vesicles from the plasma membrane move to endosome by the help of a cytosolic protein, dynamin. Dynamin catalyzed hydrolysis of GTP leads to release of the vesicle from the donor membrane.

If dynamin is permanently bound to GDP, then the vesicle will not be released from the plasma membrane and the cargo protein will remain in the clathrin coated vesicles.

D) KDEL sequence is present at C-terninal sequence ER- resident proteins and acts as a sorting signal which is recognized by the KDEL receptor on the ER and cis golgi. The sequence ables to retrieve soluble proteins of the ER. ARF1 is a ADP ribosylation factor that helps in the retrograde transport from the golgi to ER, though COPI vesicles.

If ARF1 is mutated then the cargo protein with KDEL sequence will remain in the golgi complex.

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