Final answer:
The nuclear import receptor binds to GTP-Ran, as it is involved in the nuclear transport process, whereas MAP kinase does not directly interact with GTP-Ran.
Step-by-step explanation:
The correct answer to what would bind to GTP-Ran is the nuclear import receptor. GTP-Ran is part of the Ran GTPase system that is crucial for the nuclear transport of proteins and RNA. GTP-bound Ran (GTP-Ran) associates with the nuclear transport receptors (importins and exportins) that facilitate the transport of cargo molecules across the nuclear envelope.
By interacting with GTP-Ran, the nuclear import receptors change their conformation, enabling the cargo to be released into the nucleus. This is distinct from the action of MAP kinase or the MAP-kinase cascade, which are involved in cellular signaling pathways, but do not directly interact with GTP-Ran. It's worth noting that the RAS protein, involved in signal transduction and cell division, can bind GTP but the context of the question specifies GTP-Ran, which points to the nuclear import mechanism.