FASEB J. 2022 May;36 Suppl 1
Nuclear pore complexes (NPCs) mediate nucleocytoplasmic exchange controlling the flow of molecules into and out of the nucleus. The selective filter properties of NPCs enable translocation of specific molecules known as nuclear transport factors (NTRs) and their cargo. Central to this selectivity barrier is a group of largely intrinsically disordered nucleoporins (Nups) that contain multiple phenylalanine-glycine repeats, termed FG Nups. The interactions between FG Nups and NTRs enable NTRs to translocate rapidly yet selectively through the NPC in what is referred to as the "transport paradox". FG Nups in the NPC have a small folded anchor region tethering them to the NPC outer ring and otherwise are fully disordered, random coil polymers that remain predominantly disordered while engaged to NTRs. FG Nups interact with NTRs using mainly their FG motifs and minimally their intervening spacer residues. The overall enthalpy of the interaction increases as multivalency increases the frequency of individually weak FG-NTR contacts. Tight binding is limited by an entropy penalty that disfavors simultaneous engagements of FG motifs. Small angle neutron scattering (SANS) shows that the entropy loss is partly due to the local rigidity of an FG motif in the interacting state(Fig 1). All-atom molecular dynamics (MD) simulation indicates that spacers between the FG motifs behave as "entropic springs", disfavoring any static binding of the FG repeats. The dynamics of FG Nups enables the FG motifs to slide along the hydrophobic patches of NTRs enabling FG motifs to be easily displaced by other competing FG motifs (Fig. 2). This explanation provides a simple hypothesis for the rapid exchange of FG motif contacts during transport, focusing on the entropic exclusion of non-NTRs, and 'solubilization' of NTR complexes, in contrast to possible condensate formation which would provide a compartmentalization of components. These results reveal fundamental aspects of the functioning mechanisms underlying NPC transport at high structural resolution, something lacking in current models of nuclear transport. 1. Sparks, S., et al., Analysis of Multivalent IDP Interactions: Stoichiometry, Affinity, and Local Concentration Effect Measurements. Methods Mol Biol, 2020. 2141: p. 463-475. 2. Sparks, S., et al., Deciphering the "Fuzzy" Interaction of FG Nucleoporins and Transport Factors Using Small-Angle Neutron Scattering. Structure, 2018. 26(3): p. 477-484 e4. 3. Hayama, R., et al., Thermodynamic characterization of the multivalent interactions underlying rapid and selective translocation through the nuclear pore complex. J Biol Chem, 2018. 293(12): p. 4555-4563. 4. Hough, L.E., et al., The molecular mechanism of nuclear transport revealed by atomic-scale measurements. Elife, 2015. 4. 5. Raveh, B., et al., Slide-and-exchange mechanism for rapid and selective transport through the nuclear pore complex. Proc Natl Acad Sci U S A, 2016. 113(18): p. E2489-97.