Experimental and Computational Druggability Exploration of the 14-3-3ζ/SOS1pS1161 PPI Interface.

The exploration of the druggability of certain protein-protein interactions (PPIs) still remains a challenging task in drug discovery. Here, we present a case study using the 14-3-3-PPI, showing how small molecules can be located that are able to modulate this key oncogenic pathway. A workflow embracing biophysical techniques and MD simulations was developed to evaluate the potential of a 14-3-3ζ PPI system to bind new tool compounds. The significance of the use of computational approaches to compensate for the limitations of experimental techniques is demonstrated.

A new soaking procedure for X-ray crystallographic structural determination of protein-peptide complexes.

Interactions between a protein and a peptide motif of its protein partner are prevalent in nature. Often, a protein also has multiple interaction partners. X-ray protein crystallography is commonly used to examine these interactions in terms of bond distances and angles as well as to describe hotspots within protein complexes. However, the crystallization process presents a significant bottleneck in structure determination since it often requires notably time-consuming screening procedures, which involve testing a broad range of crystallization conditions via a trial-and-error approach. This difficulty is also increased as each protein-peptide complex does not necessarily crystallize under the same conditions. Here, a new co-crystallization/peptide-soaking method is presented which circumvents the need to return to the initial lengthy crystal screening and optimization processes for each consequent new complex. The 14-3-3σ protein, which has multiple interacting partners with specific peptidic motifs, was used as a case study. It was found that co-crystals of 14-3-3σ and a low-affinity peptide from one of its partners, c-Jun, could easily be soaked with another interacting peptide to quickly and easily generate new structures at high resolution. Not only does this significantly reduce the production time, but new 14-3-3-peptide structures that were previously not accessible with the 14-3-3σ isoform, despite screening hundreds of other different conditions, were now also able to be resolved. The findings achieved in this study may be considered as a supporting and practical guide to potentially enable the acceleration of the crystallization process of any protein-peptide system.

14-3-3 binding creates a memory of kinase action by stabilizing the modified state of phospholamban.

The cardiac membrane protein phospholamban (PLN) is targeted by protein kinase A (PKA) at Ser16 and by Ca2+/calmodulin-dependent protein kinase II (CaMKII) at Thr17 ß-Adrenergic stimulation and PKA-dependent phosphorylation of Ser16 acutely stimulate the sarcoplasmic reticulum calcium pump (SERCA) by relieving its inhibition by PLN. CaMKII-dependent phosphorylation may lead to longer-lasting SERCA stimulation and may sustain maladaptive Ca2+ handling. Here, we demonstrated that phosphorylation at either Ser16 or Thr17 converted PLN into a target for the phosphoadaptor protein 14-3-3 with different affinities. 14-3-3 proteins were localized within nanometers of PLN and endogenous 14-3-3 coimmunoprecipitated with pentameric PLN from cardiac membranes. Molecular dynamics simulations predicted different molecular contacts for peptides phosphorylated at Ser16 or Thr17 with the binding groove of 14-3-3, resulting in varied binding affinities. 14-3-3 binding protected either PLN phosphosite from dephosphorylation. ß-Adrenergic stimulation of isolated adult cardiomyocytes resulted in the membrane recruitment of endogenous 14-3-3. The exogenous addition of 14-3-3 to ß-adrenergic-stimulated cardiomyocytes led to prolonged SERCA activation, presumably because 14-3-3 protected PLN pentamers from dephosphorylation. Phosphorylation of Ser16 was disrupted by the cardiomyopathy-associated ∆Arg14 mutation, implying that phosphorylation of Thr17 by CaMKII may become crucial for 14-3-3 recruitment to ∆Arg14 PLN. Consistent with PLN acting as a dynamic hub in the control of Ca2+ handling, our results identify 14-3-3 binding to PLN as a contractility-augmenting mechanism.

Author Correction: The endocannabinoid hydrolase FAAH is an allosteric enzyme.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Interaction of an IκBα Peptide with 14-3-3.

Inflammatory responses mediated by the transcription factor nuclear factor kappa-light-chain enhancer of activated B cells (NF-κB) play key roles in immunity, autoimmune diseases, and cancer. NF-κB is directly regulated through protein-protein interactions, including those with IκB and 14-3-3 proteins. These two important regulatory proteins have been reported to interact with each other, although little is known about this interaction. We analyzed the inhibitor of nuclear factor kappa B α (IκBα)/14-3-3σ interaction via a peptide/protein-based approach. Structural data were acquired via X-ray crystallography, while binding affinities were measured with fluorescence polarization assays and time-resolved tryptophan fluorescence. A high-resolution crystal structure (1.13 Å) of the uncommon 14-3-3 interaction motif of IκBα (IκBαpS63) in a complex with 14-3-3σ was evaluated. This motif harbors a tryptophan that makes this crystal structure the first one with such a residue visible in the electron density at that position. We used this tryptophan to determine the binding affinity of the unlabeled IκBα peptide to 14-3-3 via tryptophan fluorescence decay measurements.

The endocannabinoid hydrolase FAAH is an allosteric enzyme.

Fatty acid amide hydrolase (FAAH) is a membrane-bound homodimeric enzyme that in vivo controls content and biological activity of N-arachidonoylethanolamine (AEA) and other relevant bioactive lipids termed endocannabinoids. Parallel orientation of FAAH monomers likely allows both subunits to simultaneously recruit and cleave substrates. Here, we show full inhibition of human and rat FAAH by means of enzyme inhibitors used at a homodimer:inhibitor stoichiometric ratio of 1:1, implying that occupation of only one of the two active sites of FAAH is enough to fully block catalysis. Single W445Y substitution in rat FAAH displayed the same activity as the wild-type, but failed to show full inhibition at the homodimer:inhibitor 1:1 ratio. Instead, F432A mutant exhibited reduced specific activity but was fully inhibited at the homodimer:inhibitor 1:1 ratio. Kinetic analysis of AEA hydrolysis by rat FAAH and its F432A mutant demonstrated a Hill coefficient of ~1.6, that instead was ~1.0 in the W445Y mutant. Of note, also human FAAH catalysed an allosteric hydrolysis of AEA, showing a Hill coefficient of ~1.9. Taken together, this study demonstrates an unprecedented allosterism of FAAH, and represents a case of communication between two enzyme subunits seemingly controlled by a single amino acid (W445) at the dimer interface. In the light of extensive attempts and subsequent failures over the last decade to develop effective drugs for human therapy, these findings pave the way to the rationale design of new molecules that, by acting as positive or negative heterotropic effectors of FAAH, may control more efficiently its activity.

Protein X-ray crystallography of the 14-3-3ζ/SOS1 complex.

Activation of Ras-MAPK signaling regulates essential cellular functions; its aberration leads to irregular cell proliferation and differentiation (i.e. pancreatic cancer). Previously, it was revealed that the formation of the complex of the 14-3-3 protein and the Son of sevenless homolog 1 (SOS1) - one of the main actors of the Ras-MAPK cascade -, would represent a key-process to downstream the deviant Ra-MAPK signaling. In this data article we attempt to shed some light on the 3D structure, providing useful details about the crystallization process of the 14-3-3ζ dimer in complex with the 13-mer SOS1pS1161. The crystal structure is deposited at the Protein Data Bank with identifier 6F08. This Data in Brief article refers to "Structural characterization of 14-3-3ζ in complex with the human Son of sevenless homolog 1 (SOS1) (2018)."

14-3-3: A Case Study in PPI Modulation.

In recent years, targeting the complex network of protein⁻protein interactions (PPIs) has been identified as a promising drug-discovery approach to develop new therapeutic strategies. 14-3-3 is a family of eukaryotic conserved regulatory proteins which are of high interest as potential targets for pharmacological intervention in human diseases, such as cancer and neurodegenerative and metabolic disorders. This viewpoint is built on the "hub" nature of the 14-3-3 proteins, binding to several hundred identified partners, consequently implicating them in a multitude of different cellular mechanisms. In this review, we provide an overview of the structural and biological features of 14-3-3 and the modulation of 14-3-3 PPIs for discovering small molecular inhibitors and stabilizers of 14-3-3 PPIs.

Biophysical and structural insight into the USP8/14-3-3 interaction.

The ubiquitin-specific protease 8 (USP8)/14-3-3 protein-protein interaction has recently been shown to exert a significant role in the pathogenesis of Cushing's disease (CD). USP8 is a deubiquitinase that prevents epidermal growth factor receptor (EGFR) degradation. Impairment of 14-3-3 binding leads to a higher deubiquitination of EGFR and results in a higher EGFR signaling and an increased production of adrenocorticotropic hormone. Here we report the high-resolution crystal structure of the 14-3-3 binding motif of USP8 surrounding Ser718 in complex with 14-3-3ζ and characterize the interaction with fluorescence polarization and isothermal titration calorimetry. Furthermore, we analyze the effect of USP8 mutations identified in CD on binding to 14-3-3.

Structural characterization of 14-3-3ζ in complex with the human Son of sevenless homolog 1 (SOS1).

The deviant Ras activation machinery is found in approximately 30% of all human cancers. SOS1 is an important protagonist of this pathway that plays a key-role in aberrant cell proliferation and differentiation. Interaction of SOS1 with 14-3-3 proteins modulates SOS1 activity in Ras-MAPK signaling. In the present study, we analyze the 14-3-3/SOS1 protein-protein interaction (PPI) by different biochemical assays and report the high resolution crystal structure of a 13-mer motif of SOS1 bound to 14-3-3ζ. These structural and functional insights are important for the evaluation of this PPI interface for small-molecule stabilization as a new starting point for modulating the Ras-Raf-MAPK pathway.