Design of Potent Inhibitors of Human RAD51 Recombinase Based on BRC Motifs of BRCA2 Protein: Modeling and Experimental Validation of a Chimera Peptide

We have previously shown that a 28-amino acid peptide derived from the BRC4 motif of BRCA2 tumor suppressor inhibits selectively human RAD51 recombinase (HsRad51). With the aim of designing better inhibitors for cancer treatment, we combined an in silico docking approach with in vitro biochemical testing to construct a highly efficient chimera peptide from eight existing human BRC motifs. We built a molecular model of all BRC motifs complexed with HsRad51 based on the crystal structure of the BRC4 motif-HsRad51 complex, computed the interaction energy of each residue in each BRC motif, and selected the best amino acid residue at each binding position. This analysis enabled us to propose four amino acid substitutions in the BRC4 motif. Three of these increased the inhibitory effect in vitro, and this effect was found to be additive. We thus obtained a peptide that is about 10 times more efficient in inhibiting HsRad51-ssDNA complex formation than the original peptide

The Src Homology 3 Domain Is Required for Junctional Adhesion Molecule Binding to the Third PDZ Domain of the Scaffolding Protein ZO-1

Tight junctions are cell-cell contacts that regulate the paracellular flux of solutes and prevent pathogen entry across cell layers. The assembly and permeability of this barrier are dependent on the zonula occludens (ZO) membrane-associated guanylate kinase (MAGUK) proteins ZO-1, -2, and -3. MAGUK proteins are characterized by a core motif of protein-binding domains that include a PDZ domain, a Src homology 3 (SH3) domain, and a region of homology to guanylate kinase (GUK); the structure of this core motif has never been determined for any MAGUK. To better understand how ZO proteins organize the assembly of protein complexes we have crystallized the entire PDZ3-SH3-GUK core motif of ZO-1. We have also crystallized this core motif in complex with the cytoplasmic tail of the ZO-1 PDZ3 ligand, junctional adhesion molecule A (JAM-A) to determine how the activity of different domains is coordinated. Our study shows a new feature for PDZ class II ligand binding that implicates the two highly conserved Phe−2 and Ser−3 residues of JAM. Our x-ray structures and NMR experiments also show for the first time a role for adjacent domains in the binding of ligands to PDZ domains in the MAGUK proteins family

Structural Basis of a Key Factor Regulating the Affinity between the Zonula Occludens First PDZ Domain and Claudins

The molecular seal between epithelial cells, called the tight junction (TJ), is built by several membrane proteins, with claudins playing the most prominent role. The scaffold proteins of the zonula occludens family are required for the correct localization of claudins and hence formation of the TJ. The intracellular C terminus of claudins binds to the N-terminal PDZ domain of zonula occludens proteins (PDZ1). Of the 23 identified human claudin proteins, nine possess a tyrosine at the −6 position. Here we show that the claudin affinity for PDZ1 is dependent on the presence or absence of this tyrosine and that the affinity is reduced if the tyrosine is modified by phosphorylation. The PDZ1 β2-β3 loop undergoes a significant conformational change to accommodate this tyrosine. Cell culture experiments support a regulatory role for this tyrosine. Plasticity has been recognized as a critical property of TJs that allow cell remodeling and migration. Our work provides a molecular framework for how TJ plasticity may be regulated

Co-targeting of convergent nucleotide biosynthetic pathways for leukemia eradication

Pharmacological targeting of metabolic processes in cancer must overcome redundancy in biosynthetic pathways. Deoxycytidine (dC) triphosphate (dCTP) can be produced both by the de novo pathway (DNP) and by the nucleoside salvage pathway (NSP). However, the role of the NSP in dCTP production and DNA synthesis in cancer cells is currently not well understood. We show that acute lymphoblastic leukemia (ALL) cells avoid lethal replication stress after thymidine (dT)-induced inhibition of DNP dCTP synthesis by switching to NSP-mediated dCTP production. The metabolic switch in dCTP production triggered by DNP inhibition is accompanied by NSP up-regulation and can be prevented using DI-39, a new high-affinity small-molecule inhibitor of the NSP rate-limiting enzyme dC kinase (dCK). Positron emission tomography (PET) imaging was useful for following both the duration and degree of dCK inhibition by DI-39 treatment in vivo, thus providing a companion pharmacodynamic biomarker. Pharmacological co-targeting of the DNP with dT and the NSP with DI-39 was efficacious against ALL models in mice, without detectable host toxicity. These findings advance our understanding of nucleotide metabolism in leukemic cells, and identify dCTP biosynthesis as a potential new therapeutic target for metabolic interventions in ALL and possibly other hematological malignancies

Development of New Deoxycytidine Kinase Inhibitors and Noninvasive in Vivo Evaluation Using Positron Emission Tomography

Combined inhibition of ribonucleotide reductase and deoxycytidine kinase (dCK) in multiple cancer cell lines depletes deoxycytidine triphosphate pools leading to DNA replication stress, cell cycle arrest and apoptosis. Evidence implicating dCK in cancer cell proliferation and survival stimulated our interest in developing small molecule dCK inhibitors. Following a high throughput screen of a diverse chemical library, a structure-activity relationship study was carried out. Positron Emission Tomography (PET) using (18)F-L-1-(2′-deoxy-2′-FluoroArabinofuranosyl) Cytosine ((18)F-L-FAC), a dCK-specific substrate, was used to rapidly rank lead compounds based on their ability to inhibit dCK activity in vivo. Evaluation of a subset of the most potent compounds in cell culture (IC(50) = ∼1 – 12 nM) using the (18)F-L-FAC PET pharmacodynamic assay identified compounds demonstrating superior in vivo efficacy

Structures of Apo and Product-Bound Human l‑Asparaginase: Insights into the Mechanism of Autoproteolysis and Substrate Hydrolysis

Asparaginases catalyze the hydrolysis of the amino acid asparagine to aspartate and ammonia. Bacterial asparaginases are used in cancer chemotherapy to deplete asparagine from the blood, because several hematological malignancies depend on extracellular asparagine for growth. To avoid the immune response against the bacterial enzymes, it would be beneficial to replace them with human asparaginases. However, unlike the bacterial asparaginases, the human enzymes have a millimolar <i>K</i>_m value for asparagine, making them inefficient in depleting the amino acid from blood. To facilitate the development of human variants suitable for therapeutic use, we determined the structure of human l-asparaginase (hASNase3). This asparaginase is an N-terminal nucleophile (Ntn) family member that requires autocleavage between Gly167 and Thr168 to become catalytically competent. For most Ntn hydrolases, this autoproteolytic activation occurs efficiently. In contrast, hASNas3 is relatively stable in its uncleaved state, and this allowed us to observe the structure of the enzyme prior to cleavage. To determine the structure of the cleaved state, we exploited our discovery that the free amino acid glycine promotes complete cleavage of hASNase3. Both enzyme states were elucidated in the absence and presence of the product aspartate. Together, these structures provide insight into the conformational changes required for cleavage and the precise enzyme–substrate interactions. The new understanding of hASNase3 will serve to guide the design of variants that possess a decreased <i>K</i>_m value for asparagine, making the human enzyme a suitable replacement for the bacterial asparaginases in cancer therapy

Structural characterization of new deoxycytidine kinase inhibitors rationalizes the affinity‐determining moieties of the molecules

Deoxycytidine kinase (dCK) is a key enzyme in the nucleoside salvage pathway that is also required for the activation of several anticancer and antiviral nucleoside analog prodrugs. Additionally, dCK has been implicated in immune disorders and has been found to be overexpressed in several cancers. To allow the probing and modulation of dCK activity, a new class of small-molecule inhibitors of the enzyme were developed. Here, the structural characterization of four of these inhibitors in complex with human dCK is presented. The structures reveal that the compounds occupy the nucleoside-binding site and bind to the open form of dCK. Surprisingly, a slight variation in the nature of the substituent at the 5-position of the thiazole ring governs whether the active site of the enzyme is occupied by one or two inhibitor molecules. Moreover, this substituent plays a critical role in determining the affinity, improving it from &gt;700 to 1.5 nM in the best binder. These structures lay the groundwork for future modifications that would result in even tighter binding and the correct placement of moieties that confer favorable pharmacodynamics and pharmacokinetic properties