The group imaged single import complexes (ICs)—importin(s) and cargo—interacting with NPCs. Trajectories of the ICs within the pore indicate rapid, random movement forward and backward.
One transport model proposes an affinity gradient across the pore between ICs and pore components, but the back and forth movements do not support this model. Rather, the data suggest that molecules move randomly within the pore and exit either side. “If the IC enters the pore and reexits the same side,” says Musser, “no net energy went in, so you've lost nothing. But if the IC gets out the other side, it can be dissociated by RanGTP, and you've [achieved] transport.”
The distribution of the time an IC resides within a pore suggests there is only one rate-limiting step during import. When RanGTP was depleted, ICs lingered longer in the pore, so RanGTP-mediated dissociation is probably this limiting step. Since dissociation is expected to occur on the nucleoplasmic side, the IC may need to land close enough to the pore's edge to encounter RanGTP and exit. ▪