Allosteric control of the exportin CRM1 unraveled by crystal structure analysis.
Bottom Line: Moreover, during the last decade, CRM1 has become a more and more appreciated target for anti-cancer drugs.Hence, detailed understanding of the flexibility, the regulatory features and the positive binding cooperativity between CRM1, Ran and cargo is a prerequisite for the development of highly effective drugs.Here we review recent structural advances in the characterization of CRM1 and CRM1-containing complexes with a special emphasis on X-ray crystallographic studies.
Affiliation: Abteilung für Molekulare Strukturbiologie, Institut für Mikrobiologie und Genetik, Göttinger Zentrum für Molekulare Biowissenschaften, Georg-August-Universität Göttingen, Germany.Show MeSH
Mentions: Generally, importin β type nuclear transport receptors share a relatively low sequence homology of 10%–20%. The highest degree of sequence conservation between CRM1 and other importin β type transport receptors has been detected in the first three HEAT repeats. This N-terminal region has been termed the CRIME domain (CRM1-importin β etc.) 35,85 and is strictly required for the association with RanGTP (Figs3 and 4). The CRIME domain binds to the switch II region of Ran, one of the two GTP-triggered switches common to small GTPases – hence nuclear transport receptors are able to sense the loading state of Ran 86,87. Another important region involved in RanGTP binding is the acidic loop or HEAT9 loop (Fig.4), a stretch of 26 residues of CRM1 forming a β-hairpin (Figs2B and 3) 50. In complex with cargo and/or RanGTP, the acidic loop adopts a seatbelt-like conformation, which locks Ran in a position intricately interacting with the CRIME domain of CRM1. In this conformation the acidic loop tip touches HEAT repeats 14 and 15 located on the opposing side of the CRM1 toroid (Fig.3B,D) 50, thus increasing the rigidity of CRM1 in complexes 73. In contrast, in the structure of CRM1 in complex with RanGTP and the Ran binding protein RanBP1 (Fig.3E) the acidic loop is shifted from its seatbelt lock to a position that triggers RanGTP and cargo release 72. An amazing and unique property of CRM1 is the atypical arrangement of HEAT repeat 21 and especially the orientation of helix 21B, for which two major conformations have been found (Fig.3). In complex with SPN1 and RanGTP or RanGTP alone, the B-helix of HEAT repeat 21 is in a stacking arrangement to the other HEAT repeats (e.g. HEAT repeat 20), but in an inverted orientation facing the convex outer side of the toroid (Fig.3B,D) 50,72. In contrast, in the crystal structures of free CRM1 or bound to SPN1, the B-helix of HEAT repeat 21 spans the central channel of CRM1 and interacts with a region in the vicinity of the base of the acidic loop contacting the back side of the NES cleft (Fig.3A,C) 51,74,75. This again indicates the high structural flexibility of CRM1 and is further highlighted by the fact that not all N-terminally located HEAT repeats (CRIME domain) could be modeled in the Ran-free CRM1-SPN1 structure due the lack of electron density (Fig.3C and Table1). Moreover, the orientation of helix 21B has implications on the distance between the N- and C-terminal HEAT repeats. While in the parallel orientation (e.g. in the ternary CRM1-RanGTP-SPN1 complex) the N- and C-terminal regions form a tight and intricate interaction pattern (Fig.3B,D), the conformation of free CRM1 lacks such interactions entirely (Figs2B and 3A) 50,51,72,74,75. An additional feature located C-terminally adjacent to the helix 21B is a stretch of acidic residues named C-terminal acidic tail (Fig.2B). The electrostatic interaction of this stretch and basic residues on the helix B of HEAT repeat 12 in proximity to the NES cleft have been shown to play a pivotal role in NES binding and release 88.
Affiliation: Abteilung für Molekulare Strukturbiologie, Institut für Mikrobiologie und Genetik, Göttinger Zentrum für Molekulare Biowissenschaften, Georg-August-Universität Göttingen, Germany.