The Supramolecular Assembly of Intrinsically Disordered Nucleoporin Domains is Tuned by Inter-Chain Interactions

Abstract

Nuclear pore complexes (NPCs) are the gates that mediate the exchange of all macromolecules between the cytoplasm and the nucleus. They are highly selective: inert proteins bigger than 30 kDa cannot diffuse through NPCs unless they are bound to nuclear transport receptors (NTRs). This selectivity arises from a supramolecular assembly of intrinsically unfolded nucleoporin domains which contain phenylalanine-glycine (FG)-rich repeats, so called FG-repeat domains (FGRDs). Different models for the size and species selectivity of the permeability barrier have been suggested that build on different putative supramolecular assemblies of FGRDs. A controversially discussed question today is whether and how inter-FGRD interactions contribute to the function of the permeability barrier. We have studied the inter-FGRD interactions in monolayers of end-grafted FGRDs which constitute a biomimetic nano-scale model system of the NPC's permeability barrier. A toolbox of biophysical characterization techniques enabled us to relate the strength of inter-FGRD interactions of different FGRD types to monolayer formation and morphology. Our main findings are that inter-FGRD interactions strongly affect the kinetics of formation, the morphology (thickness and lateral homogeneity), and the mechanical properties of FGRD monolayers. Based on these results, we propose that the strength of inter-FGRD interactions in the NPC has evolved to promote the formation of a homogeneous meshwork with a small mesh size. Our results highlight the importance of inter-FGRD interactions for the functionality, and hence contribute important information to refine the model of transport across the nuclear pore permeability barrier.