Direct interactions between proteins are essential for the regulation of their functions in biological pathways. Targeting the complex network of protein-protein interactions (PPIs) has now been widely recognized as an attractive means to therapeutically intervene in disease states. Even though this is a challenging endeavor and PPIs have long been regarded as ‘undruggable’ targets, the last two decades have seen an increasing number of successful examples of PPI modulators resulting in a growing interest in this field. PPI modulation requires novel approaches and the integrated efforts of multiple disciplines to be a fruitful strategy. This Perspective focuses on the hub protein 14-3-3, which has several hundred identified protein interaction partners and is therefore involved in a wide range of cellular processes and diseases. Here, we aim to provide an integrated overview of the approaches explored for the modulation of 14-3-3 PPIs and review the examples resulting from these efforts in both inhibiting and stabilizing specific 14-3-3 protein complexes by small molecules, peptide-mimetics and natural products.

The specific recognition of protein surface elements is a fundamental challenge in the life sciences. New developments in this field will form the basis of new therapeutic approaches and will lead to applications such as sensors, affinity-tags, immobilization techniques and protein-based materials. Synthetic supramolecular molecules and materials are creating new opportunities for protein recognition, which are orthogonal to classical small molecule and protein-based approaches. As outlined here, their unique molecular features enable the recognition of amino acids, peptides and even whole protein surfaces, which can be applied to the modulation and assembly of proteins. We believe that structural insights into these pro-cesses are of great value for the further development of this field and have therefore focused the perspective on contributions that provide such structural data.

Introduction: PPIs are involved in every disease and specific modulation of these PPIs with small molecules would significantly improve our prospects of developing therapeutic agents. Both industry and academia have engaged in the identification and use of PPI inhibitors. However in comparison, the opposite strategy of employing small-molecule stabilizers of PPIs is underrepresented in drug discovery. Areas covered: PPI stabilization has not been exploited in a systematic manner. Rather, this concept validated by a number of therapeutically used natural products like rapamycin and paclitaxel has been shown retrospectively to be the basis of the activity of synthetic molecules originating from drug discovery projects among them lenalidomide and tafamidis. Here, the authors cover the growing number of synthetic small-molecule PPI stabilizers to advocate for a stronger consideration of this as a drug discovery approach. Expert opinion: Both the natural products and the growing number of synthetic molecules show that PPI stabilization is a viable strategy for drug discovery. There is certainly a significant challenge to adapt compound libraries, screening techniques and downstream methodologies to identify, characterize and optimize PPI stabilizers, but the examples of molecules reviewed here in our opinion justify these efforts.

Significance: It has been shown that 14-3-3 proteins increase trafficking of cystic fibrosis transmembrane conductance regulator (CFTR) to the plasma membrane by binding to its regulatory (R) domain. This paper contains a detailed characterization of the 14-3-3/CFTR interaction, showing that multiple phosphorylated binding sites in the CFTR R-domain are necessary for significant binding with 14-3-3. We find that one of these binding sites serves as an anchor, while surrounding weaker sites enhance the interaction. Furthermore, we show the druggability of this interaction using natural-product fusicoccin-A, which stabilizes the 14-3-3/CFTR interaction by selectively modifying a weaker binding site. This mechanism of action can serve as a model for the development of new trafficking corrector molecules to treat cystic fibrosis.