SEPARATE DOMAINS OF THE RAN GTPASE INTERACT WITH DIFFERENT FACTORS TOREGULATE NUCLEAR-PROTEIN IMPORT AND RNA PROCESSING

The small Ras-related GTP binding and hydrolyzing protein Ran has been implicated in a variety of processes, including cell cycle progressio n, DNA synthesis, RNA processing, and nuclear-cytosolic trafficking of both RNA and proteins. Like other small GTPases, Ran appears to funct ion as a switch: Ran-GTP and Ran-GDP levels are regulated both by guan ine nucleotide exchange factors and GTPase activating proteins, and Ra n-GTP and Ran-GDP interact differentially with one or more effecters. One such putative effector, Ran-binding protein 1 (RanBP1), interacts selectively with Ran-GTP. Ran proteins contain a diagnostic short, aci dic, carbonyl-terminal domain, DEDDDL, which, at least in the case of human Ran, is required for its role in cell cycle regulation. We show here that this domain is required for the interaction between Ran and RanBP1 but not for the interaction between Ran and a Ran guanine nucle otide exchange factor or between Ran and a Ran GTPase activating prote in. In addition, Ran lacking this carboxyl-terminal domain functions n ormally in an in vitro nuclear protein import assay. We also show that RanBP1 interacts with the mammalian homolog of yeast protein RNAL, a protein involved in RNA transport and processing. These results are co nsistent with the hypothesis that Ran functions directly in at least t wo pathways, one, dependent on RanBP1, that affects cell cycle progres sion and RNA export, and another, independent of RanBP1, that affects nuclear protein import.