Nucleo cytoplasmic trafficking
b. CRM1 dependent export
The exportin CRM1 (chromosome region maintenance 1) is the most versatile nuclear export receptor as so far more than 250 different proteins have been identified binding to it (Read More: NESdb | NESbase). Initial understanding of the binding mode where deduced from the first full export complex structure of SPN1, RanGTP and CRM1 (Fig.1). The crystal structure revealed striking differences to other karyopherins crystallized as CRM1 itself forms a toroid like structure with the N- and C-terminus in close contact. Ran binds to the center of this doughnut shaped ring and interacts with both termini and a structural feature, the acidic loop, inserted between the two helices forming HEAT repeat 9. Moreover SPN1, the cargo, is bound to the outer surface of CRM1 and does not interact with RanGTP, like in other structures. The structural rearrangements of CRM1 when switching from the empty to the Cargo and RanGTP-bound state must therefore achieve structural changes at two distant sites. Besides changing from a relaxed conformation to a strained conformation causing tight interaction with RanGTP, the binding region for SPN1 has to be altered. Especially changes within the NES-binding cleft (Fig. 2) required for binding of the first residues of SPN1 seem to be of great importance.
Fig.1. CRM1 in complex with RanGTP and SPN1.
Top side view with CRM1 in green, RanGTP in Red and Snurportin1 in purple.
Investigations using the same complex of CRM1, RanGTP and SPN1but exchanging the NES sequence of SPN1 revealed that the spacing between the hydrophobic residues may vary (Read More), but they have to adapt to the fixed distribution pattern of the hydrophobic pockets within the NES-binding cleft.
Further crystal structures of empty CMR1 gave insight into the structural rearrangements of CRM1 (Fig. 3) and revealed an additional interesting feature of CRM1, namely the positional changes of the B-helix of HEAT repeat 21 (Fig. 3). Whereas it is in a parallel orientation with respect to the other HEAT helices in the compact conformation of CRM1, it adopts a crossing orientation in the extended conformation. Together with the acidic loop this helix plays an important role in the cooperative binding mode of CRM1, as they both interact with the backside of the NES-binding cleft, thus modulating its accessibility.
Fig. 3. Structural changes in CRM1.
CRM1 in the empty form adapts an extended conformation with the N-and C-terminal regions quite distant and the C-terminal helix in a crossing and the acidic loop in a flipped back conformation (left panel). In the cargo bound form CRM N-and C-terminal closely interact and the C-terminal helix is in a parallel orientation at the outside of the molecule, whereas the acidic loop in a seatbelt conformation fixing RanGTP on CRM1 (right panel).
Taken together all the structures available enable the generation of a complete nucleocytoplasmic shuttling cycle of CRM1 based on structural models (Fig. 4).
Fig. 4. CRM1 transport cycle.
Schematic overview of the individual steps indicating the conformational changes during a CRM1 export cycle. The steps depicted highlight the different states of CRM1 (grey) with respect to the overall shape as well as the positions of the acidic loop (blue), the C-terminal helix (HEAT helix 21B, red) and the conformation of the NES-binding cleft (orange) during the transport cycle. See also main text.