Biophysical and structural characterization of the impacts of MET phosphorylation on tepotinib binding.

The receptor tyrosine kinase MET is activated by hepatocyte growth factor binding, followed by phosphorylation of the intracellular kinase domain (KD) mainly within the activation loop (A-loop) on Y1234 and Y1235. Dysregulation of MET can lead to both tumor growth and metastatic progression of cancer cells. Tepotinib is a highly selective, potent type Ib MET inhibitor and approved for treatment of non-small cell lung cancer harboring METex14 skipping alterations. Tepotinib binds to the ATP site of unphosphorylated MET with critical π-stacking contacts to Y1230 of the A-loop, resulting in a high residence time. In our study, we combined protein crystallography, biophysical methods (surface plasmon resonance, differential scanning fluorimetry), and mass spectrometry to clarify the impacts of A-loop conformation on tepotinib binding using different recombinant MET KD protein variants. We solved the first crystal structures of MET mutants Y1235D, Y1234E/1235E, and F1200I in complex with tepotinib. Our biophysical and structural data indicated a linkage between reduced residence times for tepotinib and modulation of A-loop conformation either by mutation (Y1235D), by affecting the overall Y1234/Y1235 phosphorylation status (L1195V and F1200I) or by disturbing critical π-stacking interactions with tepotinib (Y1230C). We corroborated these data with target engagement studies by fluorescence cross-correlation spectroscopy using KD constructs in cell lysates or full-length receptors from solubilized cellular membranes as WT or activated mutants (Y1235D and Y1234E/1235E). Collectively, our results provide further insight into the MET A-loop structural determinants that affect the binding of the selective inhibitor tepotinib.

Quantification and demonstration of the collective constriction-by-ratchet mechanism in the dynamin molecular motor.

Dynamin oligomerizes into helical filaments on tubular membrane templates and, through constriction, cleaves them in a GTPase-driven way. Structural observations of GTP-dependent cross-bridges between neighboring filament turns have led to the suggestion that dynamin operates as a molecular ratchet motor. However, the proof of such mechanism remains absent. Particularly, it is not known whether a powerful enough stroke is produced and how the motor modules would cooperate in the constriction process. Here, we characterized the dynamin motor modules by single-molecule Förster resonance energy transfer (smFRET) and found strong nucleotide-dependent conformational preferences. Integrating smFRET with molecular dynamics simulations allowed us to estimate the forces generated in a power stroke. Subsequently, the quantitative force data and the measured kinetics of the GTPase cycle were incorporated into a model including both a dynamin filament, with explicit motor cross-bridges, and a realistic deformable membrane template. In our simulations, collective constriction of the membrane by dynamin motor modules, based on the ratchet mechanism, is directly reproduced and analyzed. Functional parallels between the dynamin system and actomyosin in the muscle are seen. Through concerted action of the motors, tight membrane constriction to the hemifission radius can be reached. Our experimental and computational study provides an example of how collective motor action in megadalton molecular assemblies can be approached and explicitly resolved.

A Snu114-GTP-Prp8 module forms a relay station for efficient splicing in yeast.

The single G protein of the spliceosome, Snu114, has been proposed to facilitate splicing as a molecular motor or as a regulatory G protein. However, available structures of spliceosomal complexes show Snu114 in the same GTP-bound state, and presently no Snu114 GTPase-regulatory protein is known. We determined a crystal structure of Snu114 with a Snu114-binding region of the Prp8 protein, in which Snu114 again adopts the same GTP-bound conformation seen in spliceosomes. Snu114 and the Snu114-Prp8 complex co-purified with endogenous GTP. Snu114 exhibited weak, intrinsic GTPase activity that was abolished by the Prp8 Snu114-binding region. Exchange of GTP-contacting residues in Snu114, or of Prp8 residues lining the Snu114 GTP-binding pocket, led to temperature-sensitive yeast growth and affected the same set of splicing events in vivo. Consistent with dynamic Snu114-mediated protein interactions during splicing, our results suggest that the Snu114-GTP-Prp8 module serves as a relay station during spliceosome activation and disassembly, but that GTPase activity may be dispensable for splicing.

ProteoPlex: stability optimization of macromolecular complexes by sparse-matrix screening of chemical space.

Molecular machines or macromolecular complexes are supramolecular assemblies of biomolecules with a variety of functions. Structure determination of these complexes in a purified state is often tedious owing to their compositional complexity and the associated relative structural instability. To improve the stability of macromolecular complexes in vitro, we present a generic method that optimizes the stability, homogeneity and solubility of macromolecular complexes by sparse-matrix screening of their thermal unfolding behavior in the presence of various buffers and small molecules. The method includes the automated analysis of thermal unfolding curves based on a biophysical unfolding model for complexes. We found that under stabilizing conditions, even large multicomponent complexes reveal an almost ideal two-state unfolding behavior. We envisage an improved biochemical understanding of purified macromolecules as well as a substantial boost in successful macromolecular complex structure determination by both X-ray crystallography and cryo-electron microscopy.

CD200R1 immune checkpoint blockade by the first-in-human anti-CD200R1 antibody 23ME-00610: molecular mechanism and engineering of a surrogate antibody.

Human CD200R1 (hCD200R1), an immune inhibitory receptor expressed predominantly on T cells and myeloid cells, was identified as a promising immuno-oncology target by the 23andMe database. Blockade of CD200R1-dependent signaling enhances T cell-mediated antitumor activity in vitro and in vivo. 23ME-00610 is a potential first-in-class, humanized IgG1 investigational antibody that binds hCD200R1 with high affinity. We have previously shown that 23ME-00610 inhibits the hCD200R1 immune checkpoint function. Herein, we dissect the molecular mechanism of 23ME-00610 blockade of hCD200R1 by solving the crystal structure of 23ME-00610 Fab in complex with hCD200R1 and performing mutational studies, which show 23ME-00610 blocks the interaction between hCD200 and hCD200R1 through steric hindrance. However, 23ME-00610 does not bind CD200R1 of preclinical species such as cynomolgus monkey MfCD200R1. To enable preclinical toxicology studies of CD200R1 blockade in a pharmacologically relevant non-clinical species, we engineered a surrogate antibody with high affinity toward MfCD200R1. We used phage display libraries of 23ME-00610 variants with individual CDR residues randomized to all 20 amino acids, from which we identified mutations that switched on MfCD200R1 binding. Structural analysis suggests how the surrogate, named 23ME-00611, acquires the ortholog binding ability at the equivalent epitope of 23ME-00610. This engineering approach does not require a priori knowledge of structural and functional mapping of antibody-antigen interaction and thus is generally applicable for therapeutic antibody development when desired ortholog binding is lacking. These findings provide foundational insights as 23ME-00610 advances in clinical studies to gain understanding of the hCD200R1 immune checkpoint as a target in immuno-oncology.

Chemical space docking enables large-scale structure-based virtual screening to discover ROCK1 kinase inhibitors.

With the ever-increasing number of synthesis-on-demand compounds for drug lead discovery, there is a great need for efficient search technologies. We present the successful application of a virtual screening method that combines two advances: (1) it avoids full library enumeration (2) products are evaluated by molecular docking, leveraging protein structural information. Crucially, these advances enable a structure-based technique that can efficiently explore libraries with billions of molecules and beyond. We apply this method to identify inhibitors of ROCK1 from almost one billion commercially available compounds. Out of 69 purchased compounds, 27 (39%) have Ki values < 10 µM. X-ray structures of two leads confirm their docked poses. This approach to docking scales roughly with the number of reagents that span a chemical space and is therefore multiple orders of magnitude faster than traditional docking.

Crystal structure analysis reveals functional flexibility in the selenocysteine-specific tRNA from mouse.

BACKGROUND: Selenocysteine tRNAs (tRNA(Sec)) exhibit a number of unique identity elements that are recognized specifically by proteins of the selenocysteine biosynthetic pathways and decoding machineries. Presently, these identity elements and the mechanisms by which they are interpreted by tRNA(Sec)-interacting factors are incompletely understood. METHODOLOGY/PRINCIPAL FINDINGS: We applied rational mutagenesis to obtain well diffracting crystals of murine tRNA(Sec). tRNA(Sec) lacking the single-stranded 3'-acceptor end ((ΔGCCA)RNA(Sec)) yielded a crystal structure at 2.0 Å resolution. The global structure of (ΔGCCA)RNA(Sec) resembles the structure of human tRNA(Sec) determined at 3.1 Å resolution. Structural comparisons revealed flexible regions in tRNA(Sec) used for induced fit binding to selenophosphate synthetase. Water molecules located in the present structure were involved in the stabilization of two alternative conformations of the anticodon stem-loop. Modeling of a 2'-O-methylated ribose at position U34 of the anticodon loop as found in a sub-population of tRNA(Sec)in vivo showed how this modification favors an anticodon loop conformation that is functional during decoding on the ribosome. Soaking of crystals in Mn(2+)-containing buffer revealed eight potential divalent metal ion binding sites but the located metal ions did not significantly stabilize specific structural features of tRNA(Sec). CONCLUSIONS/SIGNIFICANCE: We provide the most highly resolved structure of a tRNA(Sec) molecule to date and assessed the influence of water molecules and metal ions on the molecule's conformation and dynamics. Our results suggest how conformational changes of tRNA(Sec) support its interaction with proteins.

Structure and catalytic mechanism of eukaryotic selenocysteine synthase.

In eukaryotes and Archaea, selenocysteine synthase (SecS) converts O-phospho-L-seryl-tRNA [Ser]Sec into selenocysteyl-tRNA [Ser]Sec using selenophosphate as the selenium donor compound. The molecular mechanisms underlying SecS activity are presently unknown. We have delineated a 450-residue core of mouse SecS, which retained full selenocysteyl-tRNA [Ser]Sec synthesis activity, and determined its crystal structure at 1.65 A resolution. SecS exhibits three domains that place it in the fold type I family of pyridoxal phosphate (PLP)-dependent enzymes. Two SecS monomers interact intimately and together build up two identical active sites around PLP in a Schiff-base linkage with lysine 284. Two SecS dimers further associate to form a homotetramer. The N terminus, which mediates tetramer formation, and a large insertion that remodels the active site set SecS aside from other members of the family. The active site insertion contributes to PLP binding and positions a glutamate next to the PLP, where it could repel substrates with a free alpha-carboxyl group, suggesting why SecS does not act on free O-phospho-l-serine. Upon soaking crystals in phosphate buffer, a previously disordered loop within the active site insertion contracted to form a phosphate binding site. Residues that are strictly conserved in SecS orthologs but variant in related enzymes coordinate the phosphate and upon mutation corrupt SecS activity. Modeling suggested that the phosphate loop accommodates the gamma-phosphate moiety of O-phospho-l-seryl-tRNA [Ser]Sec and, after phosphate elimination, binds selenophosphate to initiate attack on the proposed aminoacrylyl-tRNA [Ser]Sec intermediate. Based on these results and on the activity profiles of mechanism-based inhibitors, we offer a detailed reaction mechanism for the enzyme.

Conformational switches in winged-helix domains 1 and 2 of bacterial translation elongation factor SelB.

The crystal structure of the first two winged-helix motifs of translation elongation factor SelB from Moorella thermoacetica has been determined at 1.1 A resolution. Compared with the previous structure of the two domains in conjunction with winged-helix modules 3 and 4, the first winged-helix domain underwent a substantial conformational change during which the alpha-helical and beta-sheet portions of the element opened up like a shell. This conformational rearrangement was elicited by a change in the orientation of Trp396, leading to the disclosure of a bona fide ligand-binding site in the direct vicinity of Trp396. Additionally, the C-terminal tail of the second domain followed a different path compared with the previous structure. It is conceivable that these conformational switches constitute part of the molecular mechanism that underlies the communication between the N-terminal part of SelB, which binds Sec-tRNA(Sec) and GTP, and the C-terminal part of the protein, which binds selenocysteine-insertion sequences.