The human NPC is composed of approximately 34 nucleoporin proteins. The interior, or “symmetric,” core is composed of an inner ring, and two outer rings that sit on either side of the nuclear envelope. The NPC symmetric core helps to stabilize the extreme curvature of the nuclear membranes abutting the pore, forms the diffusion barrier, and provides a central transport channel. On the outside, the NPC symmetric core is decorated with proteins called cytoplasmic filaments that participate in protein transport and mRNA export; on the inside, the symmetric core is associated with nuclear basket nucleoporins, which interact with chromatin and the transcription machinery. In the central transport channel, a diffusion barrier is composed of extensive amino-acid regions found in about a third of the nucleoporins called phenylalanine-glycine (FG) repeats, which are intrinsically disordered and self-assemble into a mesh-like network that prevents passive diffusion of macromolecules.
© SCOTT LEIGHTON
