FC0099
Nsp1  -  Kap95

Biological function
Nsp1 is an FG-Nup, which is part of the yeast nuclear pore complex (NPC) that forms a selective filter to allow the rapid passage of transport factors (TFs) and their cargoes across the nuclear envelope, while blocks the passage of other macromolecules.

Domain organization/sequence features
The N-terminal Nsp1 (48-172) is asparagine-rich, which together with irregularly spaced FG repeats forms amyloid hydrogels under certain conditions. The central, highly charged segment (274-397) is lysine-rich and has regular FSFG repeats, which remain highly soluble (FSFG-K).

Structural evidence
Based on NMR data, FG Nups were fully disordered and highly dynamic in all cellular milieu, which were tested and ¹³C chemical shift values indicate negligible secondary structures. Upon interacting with Kap95, the majority of peaks in the 2D HSQC spectrum of FSFG-K are minimally affected, showing that the corresponding residues remain disordered and dynamic. The degree of signal attenuation falls off relatively uniformly at both sides of each FG repeat, consistent with the transient binding of individual FG residues, with the remaining residues remaining fully disordered and highly dynamic. No major state change of spacers resulting from TF addition, such as caused by the formation or breakage of secondary structure, or gel/sol transitions has been observed.

Biochemical evidence
The affinity of Kap95 for the FSFG-K was estimated from a titration observing ¹⁵N R₂ and resulted in K_d > 36 μM, which is fully consistent with rapid, reversible transport.

Structure/Mechanism
Interactions of FG repeats with the cellular milieu stabilize the unfolded state by engaging the hydrophobic phenylalanines in transient interactions that decrease their contact with the water, reducing the driving force for hydrophobic collapse and amyloid formation. Extremely weak, dynamic interactions may occur between FG repeats, and such interactions can modulate the long distance distribution of FG repeat regions within the nuclear pore complex. The binding interface with Kap95 consists of 2–4 residues surrounding the FG repeat itself—and the residues that do not interact directly with the TF sites remain highly mobile and dynamic. TF is able to rapidly diffuse, engaging transiently with multiple FG motifs across different FG Nups.

Mechanism category
tethering

Significance
Fuzziness enables FG repeat regions to form a highly dynamic phase and interact via a variety of TFs, consistent with the rapid rates of nuclear transport.