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Science  07 Sep 2012:
Vol. 337, Issue 6099, pp. 1151
DOI: 10.1126/science.337.6099.1151-a
CREDIT: HÜLSMANN ET AL., CELL 150, 738 (2012)

Nuclear pore complexes are gateways between the cytoplasm and the nucleus, and they contain multiple copies of nucleoporins (Nups). Inert objects larger than 5 nm cannot pass, yet nuclear transport receptors (NTRs) facilitate the translocation of cargo up to 40 nm in diameter. Phenylalanine-glycine repeat–containing Nups (FG-Nups) bind NTRs during facilitated translocation, but how binding increases permeability and how FG-Nups keep out the nonbinders are unclear.

CREDIT: HÜLSMANN ET AL., CELL 150, 738 (2012)

In a “reduction of dimensionality” model, the FG domains (black dots) form a surface that NTRs slide over to traverse the NPC. In two other models, the FG domains do bar passive transport: The virtual gate model relies on a brush-like behavior of noninteracting FG domains that repel inert material, whereas the selective phase model claims that the barrier is formed by a sieve-like hydrogel of interacting FG domains. Hülsmann et al. reconstituted nuclear pores and found that the FG-Nup Nup98 is essential both for active transport and maintenance of a passive barrier. The FG domain of Nup98 formed a hydrogel in vitro; this domain was required for the formation of a permeability barrier in reconstituted NPCs and could not be complemented by a noncohesive FG domain. These data support the selective phase model in which the meshwork formed by interacting domains excludes inert molecules, but NTRs bind to the FG domains and disrupt the mesh in order to transport their cargoes.

Cell 150, 738 (2012).

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