FG Nups, which consist of large natively unfolded domains, are the pore's gatekeepers—they keep out proteins that are not bound to transport receptors. Thus, says Lim, “the mechanics of transport lies in how FG domains behave at the nanoscale.”
To examine this behavior, the group used atomic force microscopy on clusters of one such FG domain, called cNup153, tethered at one end to gold nanodots. The forces exerted by the cluster just nanometers above the dot were reminiscent of the behavior of a physical phenomenon known as polymer brushes. Specifically, random flexible movements of polymers (or unfolded FG domains) create a large exclusion volume. Interactions between hydrophobic FG repeats were not seen, suggesting that FG Nups do not form a meshwork.
When packed tightly, says Lim, “being entropically dominated means that it is probably unfavorable for the FG domains to remain in confined spaces,” such as the center of the NPC. Most FG Nups therefore probably extend out of the pore, creating a corona-like barrier to incoming proteins.
The binding of a transport receptor to an FG domain locks it into an entropically reduced conformation that might thus collapse into the pore.