This is the human Nup54 subunit of the nucleoporin complex, equivalent to Nup57 of yeast. Nup54, Nup58 and Nup62 all have similar affinities for importin-beta. It seems likely that they are the only FG-repeat nucleoporins of the central channel, and as such they would form a zone of equal affinity spanning the central channel. The diffusion of importin-beta import complexes through the central channel may be a stochastic process as the affinities are similar, whereas movement from cytoplasmic fibrils to the central channel and from the central channel to the nuclear basket would be facilitated by the subtle differences in affinity between them [1,2].

1: Peering through the pore: nuclear pore complex structure, assembly, and function. Suntharalingam M, Wente SR; Dev Cell 2003;4:775-789. PMID:12791264

2: Nucleocytoplasmic transport: navigating the channel. Bednenko J, Cingolani G, Gerace L; Traffic. 2003;4:127-135. PMID:12656985