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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.