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Interaction of nuclear 
import receptors with FG repeats. 
Left: Nuclear import receptors contain 
various low-affinity FG repeat–binding 
sites on their surface. This facilitates 
their initial recruitment to NPCs because 
of interactions with FG repeats found 
on the cytosolic fibrils of the NPCs. The 
interior of the NPC is filled with a mesh 
of FG repeat–containing proteins whose 
weak interactions with each other restrict 
nonspecific diffusion of proteins and other 
macromolecules through the pore. Right: 
Cargo receptors can rapidly partition into 
the FG repeat mesh by interacting with 
the FG repeats and locally melting the 
mesh. This partitioning into and out of the 
mesh substantially accelerates diffusion of 
the cargo receptor (and its bound cargo) 
through the NPC. Proteins without surface 
FG repeat–binding sites cannot melt the 
mesh, and their diffusion through the NPC 
is comparatively slow.