EMQ 5. Theme: GTPases

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GTPases control the direction and specificity of many cellular process, and mediate many signalling pathways. From the options below, identify the GTPase described in each of the items in the list.

A Arf
B Dynamin
C Dynein
D EF-G
E EF-tu
F Gq
G Gs
H p53
I Ran
J Ras

1. A GTPase involved in transport of proteins into and out of the nucleus through the nuclear pore
Your Answer:
Answer: I, Ran. Ran is a “small GTPase” – that is, a monomeric protein, in contrast to trimeric G proteins such as Gq and Gs. Ran controls the directionality of transport through the nuclear pore. See book page 162.
2. A GTPase that acts to distort a flat membrane into an outward bud and which triggers the formation of a coatamer coat in the generation of coatamer-coated vesicles for vesicular trafficking.
Your Answer:
Answer: A, Arf. See book page 167.
3. A GTPase that becomes GTP-loaded and therefore active under the influence of many mitogenic stimuli, that is, signals that initiate cell division. This GTPase activates MAP kinase kinase kinase (MAPKKK) and phosphatidylinositol 3-kinase among other targets.
Your Answer:
Answer: J, Ras. Ras is a “small GTPase” – that is, a monomeric protein, in contrast to trimeric G proteins such as Gq and Gs. Ras was the first small GTPase to be characterized; the "R" in the names of many other small GTPases such as Ran stands for "Ras-related". See book pages 261 and 265.
4. A GTPase that carries amino acid monomers to the ribosome during protein synthesis. This GTPase forms a complex with aminoacyl tRNAs; after hydrolysing GTP this GTPase leaves the ribosome, allowing peptidyl transferase to catalyze the formation of a peptide bond between the amino acids in the P and A sites.
Your Answer:
Answer: E, EF-tu. EF-tu is a "small GTPase" – that is, a monomeric protein, in contrast to trimeric G proteins such as Gq and Gs. "EF" stands for elongation factor – that is, a protein involved in the elongation of the growing polypeptide chain at the ribosome. See book page 129.
5. A GTPase that severs a membrane bud to create an endocytotic vesicle.
Your Answer:
Answer: B, dynamin. Dynamin is a “small GTPase” – that is, a monomeric protein, in contrast to trimeric G proteins such as Gq and Gs. Dynamin provides the power necessary to force a bud to leave as an independent vesicle. See book page 168. Do not confuse dynamin, a GTPase, with dynein, a molecular motor and ATPase operating on microtubules.
6. A GTPase that, in its GTP-bound state, activates adenylate cyclase which in turn generates the intracellular messenger cAMP.
Your Answer:
Answer: G, Gs. Gs is a trimeric G protein. Many metabotropic cell surface receptors act as guanine nucleotide exchange factors for Gs, causing the Gs to become GTP-loaded and therefore active. See book page 258.
7. A GTPase that, in its GTP-bound state, activates phosphoinositide phospholipase C as part of the mechanism that triggers release of calcium ions from the endoplasmic reticulum.
Your Answer:
Answer: F, Gq. Gq is a trimeric G protein. Many metabotropic cell surface receptors act as guanine nucleotide exchange factors for Gq, causing the Gq to become GTP-loaded and therefore active. See book page 254.
8. A GTPase that, upon hydrolysing GTP, provides the pushing force to move the ribosome three nucleotide residues along messenger RNA.
Your Answer:
Answer: D, EF-G. EF-G is a "small GTPase" – that is, a monomeric protein, in contrast to trimeric G proteins such as Gq and Gs. "EF" stands for elongation factor – that is, a protein involved in the elongation of the growing polypeptide chain at the ribosome. See book page 129.
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