Abstract
The nuclear accumulation of proteins containing nuclear localization signals requires the Ran GTPase and a complex of proteins assembled at the nuclear pore. RanBP1 is a cytosolic Ran-binding protein that inhibits RCC1-stimulated release of GTP from Ran. RanBP1 also promotes the binding of Ran to karyopherin beta (also called importin beta and p97) and is a co-stimulator of RanGAP activity. Yeast karyopherin beta inhibits the GTP hydrolysis by Ran catalyzed by RanGAP. To further define the roles of RanBP1 and karyopherin beta in Ran function, we explored the effects of RanBP1 and karyopherin beta on mammalian proteins known to regulate Ran. Like RanBP1, karyopherin beta prevented the release of GTP from Ran stimulated by RCC1 or EDTA. As with the yeast protein, mammalian karyopherin beta completely blocked RanGAP activity. However, the addition of RanBP1 to this assay partially rescued the inhibited RanGAP activity. Kinetic analysis of the effects on RanGAP activity by karyopherin beta and RanBP1 revealed a combination of competitive and noncompetitive interactions. Solution binding assays confirmed the ability of RanBP1 to associate with Ran and karyopherin beta in a ternary complex, and RanBP1 binding was not competed out by the addition of karyopherin beta. These results demonstrate that RanBP1 and karyopherin beta interact with distinct sites of Ran and suggest that RanBP1 plays an essential role in nuclear transport by permitting RanGAP-mediated hydrolysis of GTP on Ran complexed to karyopherin beta.
Highlights
§ To whom correspondence should be addressed
We further show that the inhibition of Ran-specific GTPase activating protein (RanGAP) by karyopherin  can be reduced by the addition of RanBP1 or a functional Ran binding domains (RanBDs)
We demonstrated previously that Ran interacts with karyopherin  and that this interaction occurs in the presence of RanBP1 [28]
Summary
The murine homolog of RanGAP, Fug1, which exhibits efficient GAP activity toward Ran, was expressed as a GST-fusion protein. The Ran complex was diluted 10-fold into GAP buffer (40 mM Tris-HCl, pH 8.0, 8 mM MgCl2, 80 M bovine serum albumin, and 2 mM GTP) with or without 10 nM GST-Fug1 as the source of RanGAP activity. GST-fusion proteins bound to the glutathione-Sepharose were precipitated by centrifugation and washed twice with RanBP binding buffer.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
