Structure of a Ran-binding domain complexed with Ran bound to a GTP analogue: implications for nuclear transport

Abstract

The protein Ran is a small GTP-binding protein that binds to two types of effector inside the cell: Ran-binding proteins, which have a role in terminating export processes from the nucleus to the cytoplasm, and importin-β-like molecules that bind cargo proteins during nuclear transport. The Ran-binding domain is a conserved sequence motif found in several proteins that participate in these transport processes. The Ran-binding protein RanBP2 contains four of these domains and constitutes a large part of the cytoplasmic fibrils that extend from the nuclear-pore complex. The structure of Ran bound to a non-hydrolysable GTP analogue (Ran·GppNHp) in complex with the first Ran-binding domain (RanBD1) of human RanBP2 reveals not only that RanBD1 has a pleckstrin-homology domain fold, but also that the switch-I region of Ran·GppNHp resembles the canonical Ras·GppNHp structure and that the carboxy terminus of Ran is wrapped around RanBD1, contacting a basic patch on RanBD1 through its acidic end. This molecular ‘embrace’ enables RanBDs to sequester the Ran carboxy terminus, triggering the dissociation of Ran·GTP from importin-β-related transport factors and facilitating GTP hydrolysis by the GTPase-activating protein ranGAP. Such a mechanism represents a new type of switch mechanism and regulatory protein–protein interaction for a Ras-related protein.

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Figure 1: Representative electron density around switch I in Ran·GppNHp and the conserved WKER motif of RanBD1 (residues 57–60).
Figure 2: The Ran·GTP-analogue structure and conformational change.
Figure 3: Structure of RanBD1.
Figure 4: The Ran·RanBD1 complex structure.
Figure 5: Molecular embrace and the DEDDDL motif.

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Acknowledgements

We thank A. Scherer for help with the original crystallization, R. Schebaum for secretarial assistance, and the staff of the EMBL outstation in Hamburg for help with the data collection. This study was supported by HFSP and the EG (A.W.).

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Correspondence to Alfred Wittinghofer.

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