Allostery is an inherent feature of proteins, but it remains challenging to reveal the mechanisms by which allosteric signals propagate. A clearer understanding of this intrinsic circuitry would afford new opportunities to modulate protein function. Here, we have identified allosteric sites in protein tyrosine phosphatase 1B (PTP1B) by combining multiple-temperature X-ray crystallography experiments and structure determination from hundreds of individual small-molecule fragment soaks. New modeling approaches reveal 'hidden' low-occupancy conformational states for protein and ligands. Our results converge on allosteric sites that are conformationally coupled to the active-site WPD loop and are hotspots for fragment binding. Targeting one of these sites with covalently tethered molecules or mutations allosterically inhibits enzyme activity. Overall, this work demonstrates how the ensemble nature of macromolecular structure, revealed here by multitemperature crystallography, can elucidate allosteric mechanisms and open new doors for long-range control of protein function.

Many proteins have small molecule-binding pockets that are not easily detectable in the ligand-free structures. These cryptic sites require a conformational change to become apparent; a cryptic site can therefore be defined as a site that forms a pocket in a holo structure, but not in the apo structure. Because many proteins appear to lack druggable pockets, understanding and accurately identifying cryptic sites could expand the set of drug targets. Previously, cryptic sites were identified experimentally by fragment-based ligand discovery, and computationally by long molecular dynamics simulations and fragment docking. Here, we begin by constructing a set of structurally defined apo-holo pairs with cryptic sites. Next, we comprehensively characterize the cryptic sites in terms of their sequence, structure, and dynamics attributes. We find that cryptic sites tend to be as conserved in evolution as traditional binding pockets, but are less hydrophobic and more flexible. Relying on this characterization, we use machine learning to predict cryptic sites with relatively high accuracy (for our benchmark, the true positive and false positive rates are 73% and 29%, respectively). We then predict cryptic sites in the entire structurally characterized human proteome (11,201 structures, covering 23% of all residues in the proteome). CryptoSite increases the size of the potentially “druggable” human proteome from ~40% to ~78% of disease-associated proteins. Finally, to demonstrate the utility of our approach in practice, we experimentally validate a cryptic site in protein tyrosine phosphatase 1B using a covalent ligand and NMR spectroscopy. The CryptoSite web server is available at http://salilab.org/cryptosite.

There is great interest in developing selective protein kinase inhibitors by targeting allosteric sites, but these sites often involve protein-protein or protein-peptide interfaces that are very challenging to target with small molecules. Here we present a systematic approach to targeting a functionally conserved allosteric site on the protein kinase PDK1 called the PDK1-interacting fragment (PIF)tide-binding site, or PIF pocket. More than two dozen prosurvival and progrowth kinases dock a conserved peptide tail into this binding site, which recruits them to PDK1 to become activated. Using a site-directed chemical screen, we identified and chemically optimized ligand-efficient, selective, and cell-penetrant small molecules (molecular weight ∼380 Da) that compete with the peptide docking motif for binding to PDK1. We solved the first high-resolution structure of a peptide docking motif (PIFtide) bound to PDK1 and mapped binding energy hot spots using mutational analysis. We then solved structures of PDK1 bound to the allosteric small molecules, which revealed a binding mode that remarkably mimics three of five hot-spot residues in PIFtide. These allosteric small molecules are substrate-selective PDK1 inhibitors when used as single agents, but when combined with an ATP-competitive inhibitor, they completely suppress the activation of the downstream kinases. This work provides a promising new scaffold for the development of high-affinity PIF pocket ligands, which may be used to enhance the anticancer activity of existing PDK1 inhibitors. Moreover, our results provide further impetus for exploring the helix αC patches of other protein kinases as potential therapeutic targets even though they involve protein-protein interfaces.allostery | peptide mimicry | drug discovery | protein-protein interaction P rotein kinases are a rich source of targets for the development of chemical probes and therapeutics; however, the remarkable similarity of their ATP-binding pockets presents a formidable challenge for the development of selective ATPcompetitive inhibitors. Previous efforts to address these limitations have focused on targeting allosteric sites in kinases. Exquisitely selective allosteric inhibitors of the protein kinases AKT, MEK, and ABL are now in clinical trials for cancer, and various other allosteric kinase inhibitors and activators are in preclinical development (1). Despite these recent successes, finding allosteric modulators remains challenging, because most allosteric opportunities are the sites of protein-protein or protein-peptide interactions, which are very difficult to mimic with small molecules. Moreover, traditional chemical screening approaches most often identify ligands for the more druggable ATP-binding pocket.The helix αC patch is an ancient allosteric site present on various serine/threonine and tyrosine kinases (2). The binding of effector proteins to the helix αC patch activates some kinases and inhibits others. The helix αC patch is seen most frequently in the AGC family of serine/threonine ki...

Caspases are a family of proteases found in all metazoans, including a dozen in humans, that drive the terminal stages of apoptosis as well as other cellular remodeling and inflammatory events. Caspases are named because they are cysteine class enzymes shown to cleave after aspartate residues. In the past decade, we and others have developed unbiased proteomic methods that collectively identified~2000 native proteins cleaved during apoptosis after the signature aspartate residues. Here, we explore non-aspartate cleavage events and identify 100s of substrates cleaved after glutamate in both human and murine apoptotic samples. The extended consensus sequence patterns are virtually identical for the aspartate and glutamate cleavage sites suggesting they are cleaved by the same caspases. Detailed kinetic analyses of the dominant apoptotic executioner caspases-3 and -7 show that synthetic substrates containing DEVD↓ are cleaved only twofold faster than DEVE↓, which is well within the 500-fold range of rates that natural proteins are cut. X-ray crystallography studies confirm that the two acidic substrates bind in virtually the same way to either caspases-3 or -7 with minimal adjustments to accommodate the larger glutamate. Lastly, during apoptosis we found 121 proteins cleaved after serine residues that have been previously annotated to be phosphorylation sites. We found that caspase-3, but not caspase-7, can cleave peptides containing DEVpS↓ at only threefold slower rate than DEVD↓, but does not cleave the unphosphorylated serine peptide. There are only a handful of previously reported examples of proteins cleaved after glutamate and none after phosphorserine. Our studies reveal a much greater promiscuity for cleaving after acidic residues and the name 'cacidase' could aptly reflect this broader specificity. Human caspases are a family of 12 homologous intracellular proteases known for driving cellular state changes such as apoptosis and differentiation, as well as inflammatory responses. Caspases are cysteine-class proteases named for their signature ability to cleave after aspartate residues, or P1 is aspartate using the Schetchter and Berger notation. 1,2Synthetic peptide profiling for purified caspases show distinctive subsite preferences extending from P1 to P4 or P5. 3-5Sequence conservation and signal pathway analyses have further grouped the proteases into apoptotic initiators (−2, − 8, − 9 and − 10), apoptotic executioners (−3, − 6 and − 7), regulators of inflammation (−1, − 4, − 5 and − 12) and keratinocyte differentiation − 14).The past decade has seen a significant advancement in the use of LC-MS to define the spectrum of natural proteins cleaved by caspases in cells. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] In addition to providing unbiased information about which proteins are cleaved, in some cases, these experiments locate the precise sites and quantify the rates of cleavage.13-17 Using the subtiligasebased N-terminomics approach, we identified more than 1700 aspartate cleavage eve...