Please explain your answer. No credit if you do not explain your answer. 37. Wha
ID: 11375 • Letter: P
Question
Please explain your answer. No credit if you do not explain your answer.37. What is the basis for the observation that tubulin dimers can accumulate in GTP-bound state but once
assembled, the GTP becomes hydrolyzed?
A) Stathmin prevents GTP hydrolysis
B) The dimers are accessible to GTP exchange factors but the assembled microtubule is not.
C) MAPs stimulate GTPase activity of tubulin
D) GTPase activity is intrinsic to tubulin but depends on interactions with assembled subunits
E) Accessory proteins prevent hydrolysis of GTP-tubulin dimer
F) None of the above.
Explanation / Answer
Please explain your answer. No credit if you do not explain your answer.
37. What is the basis for the observation that tubulin dimers can accumulate in GTP-bound state but once
assembled, the GTP becomes hydrolyzed?
A) Stathmin prevents GTP hydrolysis
B) The dimers are accessible to GTP exchange factors but the assembled microtubule is not.
C) MAPs stimulate GTPase activity of tubulin
D) GTPase activity is intrinsic to tubulin but depends on interactions with assembled subunits
E) Accessory proteins prevent hydrolysis of GTP-tubulin dimer
F) None of the above.
The ability of importin to interact with various proteins (the "subunits" referenced in D) of the NPC (e.g., nucleoporins)enables it to transport its cargo into the nucleus. This directionality is determined by a steep gradient of a small GTPase calledRan across the nuclear envelope, where the concentration of the GTP-bound form is greatest in the nucleus. This RanGTP gradient is maintained by a nuclear Ran guanine nucleotide exchange factor (RanGEF) and a cytoplasmic Ran GTPase activating protein (RanGAP) and is believed to power all nucleocytoplasmictransport processes. Upon entry into the nucleus, RanGTP bindsto importin and induces a conformational change that ultimately results in the release of the NLS cargo.
Bottom line? interactions are decisive :)