Cohesiveness tunes assembly and morphology of FG nucleoporin domain meshworks – Implications for nuclear pore permeability

Nico B Eisele,Aksana A Labokha,Steffen Frey,Dirk Görlich,Ralf P Richter

doi:10.1016/j.bpj.2013.09.006

Abstract

Nuclear pore complexes control the exchange of macromolecules between the cytoplasm and the nucleus. A selective permeability barrier that arises from a supramolecular assembly of intrinsically unfolded nucleoporin domains rich in phenylalanine-glycine dipeptides (FG domains) fills the nuclear pore. There is increasing evidence that selective transport requires cohesive FG domain interactions. To understand the functional roles of cohesive interactions, we studied monolayers of end-grafted FG domains as a bottom-up nanoscale model system of the permeability barrier. Based on detailed physicochemical analysis of the model films and comparison of the data with polymer theory, we propose that cohesiveness is tuned to promote rapid assembly of the permeability barrier and to generate a stable and compact pore-filling meshwork with a small mesh size. Our results highlight the functional importance of weak interactions, typically a few kBT per chain, and contribute important information to understand the mechanism of size-selective transport.

Highlights

Bulk macromolecular transport between the cytosol and the nucleus of eukaryotic cells is gated through nuclear pore complexes (NPCs) [1,2,3,4,5,6], large protein assemblies that perforate the nuclear envelope
Specialized nucleoporin domains that are natively unfolded and rich in phenylalanine-glycine dipeptides (FG domains) are grafted at high density to the channel walls [9] and constitute a selective permeability barrier: molecules smaller than 5 nm in diameter [10] can diffuse efficiently through the channel, whereas larger molecules are delayed or blocked, unless they are bound to nuclear transport receptors (NTRs) that bind to FG motifs as a prerequisite for facilitated NPC passage [1,2,3,4,11]
We argue that the self-organization phenomena that we observe on planar surfaces are relevant for the NPC topology, and we discuss the broad implications for the assembly and function of the permeability barrier

Summary

Introduction

Bulk macromolecular transport between the cytosol and the nucleus of eukaryotic cells is gated through nuclear pore complexes (NPCs) [1,2,3,4,5,6], large protein assemblies that perforate the nuclear envelope. To form an NPC, several types of nucleoporin proteins self-assemble in multiple copies into a ring-like structure with a central channel of 30 to 50 nm in diameter [7,8]. Specialized nucleoporin domains that are natively unfolded and rich in phenylalanine-glycine dipeptides (FG domains) are grafted at high density to the channel walls [9] and constitute a selective permeability barrier: molecules smaller than 5 nm in diameter [10] can diffuse efficiently through the channel, whereas larger molecules are delayed or blocked, unless they are bound to nuclear transport receptors (NTRs) that bind to FG motifs as a prerequisite for facilitated NPC passage [1,2,3,4,11]. Several models have been proposed [12,13,14,15,16] They share the idea that the permeability barrier of NPCs arises from the supramolecular assembly of FG domains. There is, increasing evidence that FG domains can interact attractively with each other [17,18,19,20], and that these interactions are essential for the formation of a functional permeability barrier [17,21]

Methods

Results

Discussion

Conclusion