Crowding-induced phase separation of nuclear transport receptors in FG nucleoporin assemblies

The rapid (< 1 ms) transport of biological material to and from the cell nucleus is regulated by the nuclear pore complex (NPC). At the core of the NPC is a permeability barrier consisting of intrinsically disordered Phe-Gly (FG) nucleoporins (FG Nups). Various types of nuclear transport receptors (NTRs) facilitate transport by partitioning in the FG Nup assembly, overcoming the barrier by their affinity to the FG Nups, and comprise a significant fraction of proteins in the NPC barrier. In previous work, we revealed that the experimental binding of the NTRs NTF2 and – the larger – Impβ to different planar assemblies of FG Nups follows a universal physical law defined by negative cooperativity, which was further validated by a minimal physical model that treated the FG Nups as flexible homopolymers and the NTRs as uniformly cohesive spheres Zahn et al. (2016). Here, we build upon our original study by first parametrizing our model to experimental data, and next to predict the effects of crowding by different types of NTRs. We show how varying the amounts of one type of NTR modulates how the other NTR penetrates the FG Nup assembly. Notably, at similar and physiologically relevant NTR concentrations, our model predicts demixed phases of NTF2 and Impβ within the FG Nup assembly. The functional implication of NTR phase separation is that NPCs may sustain separate transport pathways that are determined by inter-NTR competition.