MCAT Biology : Protein Structure

Study concepts, example questions & explanations for MCAT Biology

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Example Question #21 : Protein Structure

Nuclear transport is a very important concept of study in modern cellular biology. Transport of proteins into the nucleus of an organism requires energy in the form of GTP, which is attached to a protein called Ras-related Nuclear protein (RAN).

RAN is a monomeric G protein found in both the cytosol as well as the nucleus and its phosphorylation state plays an important role in the movement of proteins into and out of the nucleus. Specifically, RAN-GTP and RAN-GDP binds to nuclear import and export receptors and carries them into or out of the nucleus. They also play a role in dropping off cargo that import and export receptors hold onto. RAN's functions are controlled by two other proteins: RAN guanine exchange factor (RAN-GEF) and RAN GTPase activating protein (GAP). RAN-GEF binds a GTP onto RAN, while RAN-GAP hydrolyzes GTP into GDP. As a result, there is a RAN-GTP and RAN-GDP concentration gradient that forms between the cytosol and nucleus.

One of the main roles of RAN is to bind to nuclear import and export receptors and carry them into or out of the nucleus. Given that import and export receptors are proteins, what can we say about the cooperativity displayed by RAN when it comes to binding to import and export proteins? 

Possible Answers:

No cooperativity

Both negative and positive cooperativity

Positive cooperativity

There is not enough information to determine what type of cooperativity exists

Negative cooperativity

Correct answer:

No cooperativity


Binding cooperativity occurs when binding of one substrate increases or decreases the affinity for the other substrates. For cooperativity to work, the protein in question must have multiple subunits, therefore being at least a dimer. RAN is a monomer, and therefore cannot show any cooperativity. 

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