THE C-TERMINUS OF THE NUCLEAR RAN/TC4 GTPASE STABILIZES THE GDP-BOUND STATE AND MEDIATES INTERACTIONS WITH RCC1, RAN-GAP, AND HTF9A/RANBP1

Ran/TC4 is a member of the Ras superfamily of GTPases. It is unusual in being predominantly nuclear and because it possesses an acidic -DEDDDL sequence instead of a consensus prenylation domain at the C terminus. Ran is required for nuclear protein import rind cell cycle progression, and has been implicated in mRNA processing and export and DNA replication. The inhibition of cell cycle progression by a dominant gain-of-function mutant of Ran has been shown to be abrogated by removal of the -DEDDDL sequence, suggesting that this domain is essential for Ran function. We demonstrate here that the -DEDDDL sequence stabilizes GDP ; binding to Ran, and that the domain is required for high affinity interaction with a Ran-binding protein, HTF9A/RanBP1. RTFSA functions as a co-stimulator of Ran-GAP (GTPase activating protein) activity on wild-type Ran, but in the absence of the acidic C terminus of Ran, HTF9A behaves as a Ran-GAP inhibitor. An antibody directed against the C-terminal region preferentially recognizes the GTP;bound form of Ran, suggesting that this domain undergoes a nucleotide-dependent conformational change. The results suggest that the acidic C-terminal domain is important in modulating the interaction of Ran with regulatory factors, and implicate Iran-binding proteins in mediating the effects of Ran on cell cycle progression.