Thermodynamic characterization of the multivalent interactions underlying rapid and selective translocation through the nuclear pore complex

Abstract

Intrinsically disordered proteins (IDPs) play important roles in many biological systems. Given the vast conformational space that IDPs can explore, the thermodynamics of the interactions with their partners is closely linked to their biological functions. Intrinsically disordered regions of Phe-Gly nucleoporins (FG Nups) that contain multiple phenylalanineglycine repeats are of particular interest, as their interactions with transport factors (TFs) underlie the paradoxically rapid yet also highly selective transport of macromolecules mediated by the nuclear pore complex (NPC). Here, we used NMR and isothermal titration calorimetry (ITC) to thermodynamically characterize these multivalent interactions. These analyses revealed that a combination of low per-FG motif affinity and the enthalpy-entropy balance prevents highavidity interaction between FG Nups and TFs, while the large number of FG motifs promotes frequent FG-TF contacts, resulting in enhanced selectivity.Our thermodynamic model underlines the importance of functional disorder of FG Nups. It helps explain the rapid and selective translocation of TFs through the NPC and further expands our understanding of the mechanisms of "fuzzy" interactions involving IDPs.Intrinsically disordered proteins (IDPs) and proteins with intrinsically disordered regions (IDRs), constitute ~30-40% of the human proteome and are involved in many protein signaling and regulation processes (1). IDPs/IDRs can interact with their targets with high specificity, and yet often with low affinity and high reversibility. There is a broad interest in quantifying the thermodynamic driving forces governing IDP interactions. Many IDPs undergo a disorder-to-order transition upon binding to their targets (2), while others form 'fuzzy complexes' (3) where significant residual disorder is maintained in the interacting state. Due to their essential role in many biological processes, a better understanding of the energetics of IDP interactions is needed (4).Many IDP interactions are mediated by short linear motifs (SLiMs) that engage with receptor molecules. Because SLiMs do not have extensive interaction interfaces to induce high enthalpy, SLiM-containing IDPs often utilize multiple motifs to participate in multivalent interactions Thermodynamics of FG-Transport Factor Interaction2 enhances individually weak monovalent interactions, resulting in higher overall affinity (avidity) and specificity (6,7). One example of an IDR that utilizes multiple short linear motifs are disordered domains of Phe-Gly nucleoporins (FG Nups) which line the central channel of the nuclear pore complex (NPC) ( Figure 1A). FG Nups typically contain 5-50 FG motifs separated by spacer residues (8). These FG repeat regions collectively form a selectively permeable barrier for macromolecular transport through the NPC. Specific cargoes can translocate rapidly and efficiently through the NPC by binding to cognate transport factors (TFs). TFs make contacts with multiple FG repeat motifs, allowing them to diffuse rapidl...

Fuzzy and fast nuclear transport

“…The term "kiss-and-run" has previously been used in this context (1), and the data presented in Ref. 2 attest to the high promiscuity on the part of the IDP, where multiple similar interaction motifs may continuously exchange to pass on the partner (Fig. 1).…”

“…Nuclear pore complexes (NPCs) 2 mediate this molecular exchange and thereby play a pivotal role in the organization of cellular molecular traffic. Structurally, the NPC is an amazing macromolecular assembly of 60 -120 MDa, consisting of at least 30 different nucleoporin proteins (Nups) that unite into an hourglassshaped architecture (see, for example, Ref.…”

“…Nearly quantitative enthalpy-entropy compensation was observed, explaining why the multivalent complexes showed affinities that were not more than additive. The "transport paradox" is thus resolved by Hayama et al (2) in the following way: A high local concentration of FG repeats with short interaction half-lives results in a high frequency of contacts, thereby increasing the effective lifetime of two interaction partners of appropriate overall complementarity. The entropy loss associated with the binding of additional FG elements helps to prevent overly strong association and thereby provides a principle for highly dynamic interactions to maintain transport speed through the nuclear pore complex.…”

“…: 45-51447344; E-mail: fmulder@chem.au.dk. 2 The abbreviations used are: NPC, nuclear pore complex; IDP, intrinsically disordered protein; TF, transport factor; FG Nup, Phe-Gly nucleoporin.…”

“…To understand intrinsic disorder, we must embrace concepts like polyvalent interactions (6), along with thermodynamic experiments and models (7). The work of Hayama et al (2) on FSFG-NTF2 interactions shows how critical thinking and experiments pave the way to a comprehensive understanding of key biological functions. .…”