The Cul4-Rbx1-DDB1-Cereblon E3 ubiquitin ligase complex is the target of thalidomide, lenalidomide and pomalidomide, therapeutically important drugs for multiple myeloma and other B-cell malignancies. These drugs directly bind Cereblon (CRBN) and promote the recruitment of substrates Ikaros (IKZF1) and Aiolos (IKZF3) to the E3 complex, thus leading to substrate ubiquitination and degradation. Here we present the crystal structure of human CRBN bound to DDB1 and the drug lenalidomide. A hydrophobic pocket in the thalidomide-binding domain (TBD) of CRBN accommodates the glutarimide moiety of lenalidomide, whereas the isoindolinone ring is exposed to solvent. We also solved the structures of the mouse TBD in the apo state and with thalidomide or pomalidomide. Site-directed mutagenesis in lentiviral-expression myeloma models showed that key drug-binding residues are critical for antiproliferative effects.

Selective inhibition of the neuronal isoform of nitric oxide synthase NOS (nNOS) has been shown to prevent brain injury and is important for the treatment of various neurodegenerative disorders. However, given the high active site conservation among all three NOS isoforms, the design of selective inhibitors is an extremely challenging problem. Here we present the structural basis for why novel and potent nNOS inhibitors exhibit the highest level of selectivity over eNOS reported so far (≈ 3,800-fold). By using a combination of crystallography, computational methods, and site-directed mutagenesis, we found that inhibitor chirality and an unanticipated structural change of the target enzyme control both the orientation and selectivity of these novel nNOS inhibitors. A new hot spot generated owing to enzyme elasticity provides important information for the future fragment-based design of selective NOS inhibitors.

Neuronal nitric oxide synthase (nNOS) represents an important therapeutic target for the prevention of brain injury and the treatment of various neurodegenerative disorders. A series of trans substituted amino pyrrolidinomethyl 2-aminopyridine derivatives (8–34) was designed and synthesized. A structure-activity relationship analysis led to the discovery of low nanomolar nNOS inhibitors [(±)-32 and (±)-34] with more than 1000-fold selectivity for nNOS over eNOS. Four enantiomerically pure isomers of 3′-[2″-(3‴-fluorophenethylamino)ethoxy]pyrrolidin-4′-yl}methyl}-4-methylpyridin-2-amine (4) also were synthesized. It was found that (3′R, 4′R)-4 can induce enzyme elasticity to generate a new “hot spot” for ligand binding. The inhibitor adopts a unique binding mode, the same as that observed for (3′R, 4′R)-3′-[2″-(3‴-fluorophenethylamino)ethylamino]pyrrolidin-4′-yl}methyl}-4-methylpyridin-2-amine ((3′R, 4′R)-3) (J. Am. Chem. Soc. 2010, 132(15), 5437–5442). On the basis of structure-activity relationships of 8–34 and different binding conformations of the cis and trans isomers of 3 and 4, critical structural requirements of the NOS active site for ligand binding are revealed.

G protein-coupled receptors (GPCRs) mediate signaling from extracellular ligands to intracellular signal transduction proteins 1 . Methuselah (Mth) is a class B (secretin-like) GPCR, a family typified by their large, ligand-binding, N-terminal extracellular domains2. Down-regulation of mth increases the lifespan of Drosophila melanogaster 3 -inhibitors of Mth signaling would thus be expected to enhance longevity. We used mRNA display selection 4,5 to identify high affinity (K D = 15 to 30 nM) peptide ligands that bind to the N-terminal ectodomain of Mth. The selected peptides are potent antagonists of Mth signaling, and structural studies suggest that they perturb the interface between the Mth ecto-and transmembrane (TM) domains. Flies constitutively expressing a Mth antagonist peptide exhibit a robust lifespan extension, suggesting that the peptides inhibit Mth signaling in vivo. Our work thus provides novel lifespan-extending ligands for a metazoan and a general approach for the design of modulators of this important class of GPCRs.Because of their participation in numerous cell processes, GPCRs are the targets of approximately half of marketed drugs, and new GPCR ligands continue to be pursued and developed 6 . Naïve approaches toward GPCR ligand identification typically involve highthroughput screening of a molecular library (10 2 to 10 5 unique members) in functional, cellbased assays [6][7][8] . A powerful, alternative approach for the rapid isolation of novel ligands is in vitro peptide selection using mRNA display, which allows access to very high library complexities (>10 13 ) in a robust format 4 . High affinity peptide ligands for RNA, small molecule, and protein targets have been identified by mRNA display selection 5 .The crystal structure of the hexahistidine-tagged Mth ectodomain was previously determined, demonstrating that the mature, N-terminal extracellular domain of Mth is a stably folded, glycosylated protein of 195 residues 9 . Since the ectodomains of other class B GPCRs maintain recognition of their cognate ligands independently of their TM cores 10,11 , we targeted the Mth ectodomain for in vitro selection to isolate putative modulators of Mth signaling. We expressed and purified a specifically biotinylated construct of the Mth ectodomain 12 to avoid using the weak hexahistidine epitope as an immobilization tag for selection 13 . Hence, the Mth ectodomain was homogeneously presented, perhaps mimicking the juxtaposition of the ectodomain and TM domain in the context of the full-length receptor.We constructed a random, 27-mer peptide mRNA display library. After eight rounds of selection, with the final four rounds including preclearing steps on matrix without target and specific elution with free, non-biotinylated Mth, we obtained a final 8 th round pool that exhibited high activity for Mth and negligible non-specific binding (Fig. 1). DNA sequencing of individual clones from the final selection round revealed a highly conserved consensus, [R/ P]xxWxxR, which we term the RWR motif ...