Ran

How is Ran involved in nuclear transport? Cargo is transported through pores in the nuclear envelope, in association with carriers that bind to pore proteins called nucleoporins. A Ran–GTP gradient across the envelope is created by the asymmetric distribution of the GTPase activating protein known as RanGAP, which is in the cytosol, and of the guanine nucleotide exchange factor RanGEF in the nucleus. Ran–GTP triggers release of imported cargo from import carriers, but promotes loading of export cargo onto export carriers. So, import cargo is dumped in the nucleoplasm and export cargo is collected (see Figure). The reverse happens in the cytoplasm, where RanGAP stimulates hydrolysis of the GTP on Ran bound to the carriers.