Evaluation of assay technologies for the identification of protein-Peptide interaction antagonists.

An increasing number of assay detection technologies are routinely used in small molecule drug discovery and lead optimization. These assays range from solid-phase heterogeneous assays such as enzyme-linked immunosorbent assay and dissociation-enhanced lanthanide fluorescent immunoassay (DELFIA, PerkinElmer Life and Analytical Sciences, Boston, MA) to solution phase, bead-based assays such as electrochemiluminescence assay (BioVeris [Gaithersburg, MD] technology) and amplified luminescent proximity homogeneous assay (AlphaScreen, PerkinElmer Life and Analytical Sciences) to completely solution-based homogeneous assays such as time-resolved fluorescence resonance energy transfer and fluorescence polarization. The aim of this study is to compare these assay technologies and assess the advantages and disadvantages of each in the context of our efforts to develop small molecule antagonists to the melanoma inhibitor of apoptosis protein. In this study, seven peptides have been evaluated for their potencies in each assay format. Our results indicate that these assay technologies produce similar relative potencies; however, some methods may be more susceptible to interference than others. Consequently, the choice of the method used frequently depends on a number of factors in addition to assay reproducibility and performance, such as throughput of the assay, cost, compound interference, and ease of use.

Engineering ML-IAP to produce an extraordinarily potent caspase 9 inhibitor: implications for Smac-dependent anti-apoptotic activity of ML-IAP.

ML-IAP (melanoma inhibitor of apoptosis) is a potent anti-apoptotic protein that is strongly up-regulated in melanoma and confers protection against a variety of pro-apoptotic stimuli. The mechanism by which ML-IAP regulates apoptosis is unclear, although weak inhibition of caspases 3 and 9 has been reported. Here, the binding to and inhibition of caspase 9 by the single BIR (baculovirus IAP repeat) domain of ML-IAP has been investigated and found to be significantly less potent than the ubiquitously expressed XIAP (X-linked IAP). Engineering of the ML-IAP-BIR domain, based on comparisons with the third BIR domain of XIAP, resulted in a chimeric BIR domain that binds to and inhibits caspase 9 significantly better than either ML-IAP-BIR or XIAP-BIR3. Mutational analysis of the ML-IAP-BIR domain demonstrated that similar enhancements in caspase 9 affinity can be achieved with only three amino acid substitutions. However, none of these modifications affected binding of the ML-IAP-BIR domain to the IAP antagonist Smac (second mitochondrial activator of caspases). ML-IAP-BIR was found to bind mature Smac with low nanomolar affinity, similar to that of XIAP-BIR2-BIR3. Correspondingly, increased expression of ML-IAP results in formation of a ML-IAP-Smac complex and disruption of the endogenous interaction between XIAP and mature Smac. These results suggest that ML-IAP might regulate apoptosis by sequestering Smac and preventing it from antagonizing XIAP-mediated inhibition of caspases, rather than by direct inhibition of caspases.

Rapid identification of small binding motifs with high-throughput phage display: discovery of peptidic antagonists of IGF-1 function.

A panel of 22 naïve peptide libraries was constructed in a polyvalent phage display format and sorted against insulin-like growth factor-1 (IGF-1). The libraries were pooled to achieve a total diversity of 4.4 x 10(11). After three rounds of selection, the majority of the phage clones bound specifically to IGF-1, with a disulfide-constrained CX(9)C scaffold dominating the selection. Four monovalently displayed sub-libraries were designed on the basis of these conserved motifs. Sub-library maturation in a monovalent format yielded an antagonistic peptide that inhibited the interactions between IGF-1 and two cell-surface receptors and those between IGF-1 and two soluble IGF binding proteins with micromolar potency. NMR analysis revealed that the peptide is highly structured in the absence of IGF-1, and peptides that preorganize the binding elements were selected during the sorting.

A selective, slow binding inhibitor of factor VIIa binds to a nonstandard active site conformation and attenuates thrombus formation in vivo.

The serine protease factor VIIa (FVIIa) in complex with its cellular cofactor tissue factor (TF) initiates the blood coagulation reactions. TF.FVIIa is also implicated in thrombosis-related disorders and constitutes an appealing therapeutic target for treatment of cardiovascular diseases. To this end, we generated the FVIIa active site inhibitor G17905, which displayed great potency toward TF.FVIIa (Ki = 0.35 +/- 0.11 nM). G17905 did not appreciably inhibit 12 of the 14 examined trypsin-like serine proteases, consistent with its TF.FVIIa-specific activity in clotting assays. The crystal structure of the FVIIa.G17905 complex provides insight into the molecular basis of the high selectivity. It shows that, compared with other serine proteases, FVIIa is uniquely equipped to accommodate conformational disturbances in the Gln217-Gly219 region caused by the ortho-hydroxy group of the inhibitor's aminobenzamidine moiety located in the S1 recognition pocket. Moreover, the structure revealed a novel, nonstandard conformation of FVIIa active site in the region of the oxyanion hole, a "flipped" Lys192-Gly193 peptide bond. Macromolecular substrate activation assays demonstrated that G17905 is a noncompetitive, slow-binding inhibitor. Nevertheless, G17905 effectively inhibited thrombus formation in a baboon arterio-venous shunt model, reducing platelet and fibrin deposition by approximately 70% at 0.4 mg/kg + 0.1 mg/kg/min infusion. Therefore, the in vitro potency of G17905, characterized by slow binding kinetics, correlated with efficacious antithrombotic activity in vivo.

Crystal structure of Clostridium botulinum neurotoxin protease in a product-bound state: Evidence for noncanonical zinc protease activity.

Clostridium botulinum neurotoxins (BoNTs), the most potent toxins known, disrupt neurotransmission through proteolysis of proteins involved in neuroexocytosis. The light chains of BoNTs are unique zinc proteases that have stringent substrate specificity and require exceptionally long substrates. We have determined the crystal structure of the protease domain from BoNT serotype A (BoNT/A). The structure reveals a homodimer in a product-bound state, with loop F242-V257 from each monomer deeply buried in its partner's catalytic site. The loop, which acts as a substrate, is oriented in reverse of the canonical direction for other zinc proteases. The Y249-Y250 peptide bond of the substrate loop is hydrolyzed, leaving the Y249 product carboxylate coordinated to the catalytic zinc. From the crystal structure of the BoNT/A protease, detailed models of noncanonical binding and proteolysis can be derived which we propose are also consistent with BoNT/A binding and proteolysis of natural substrate synaptosome-associated protein of 25 kDa (SNAP-25). The proposed BoNT/A substrate-binding mode and catalytic mechanism are markedly different from those previously proposed for the BoNT serotype B.

SMAC negatively regulates the anti-apoptotic activity of melanoma inhibitor of apoptosis (ML-IAP).

Inhibitors of apoptosis (IAPs) physically interact with a variety of pro-apoptotic proteins and inhibit apoptosis induced by diverse stimuli. X-linked IAP (X-IAP) is a prototype IAP family member that inhibits several caspases, the effector proteases of apoptosis. The inhibitory activity of X-IAP is regulated by SMAC, a protein that is processed to its active form upon receipt of a death stimulus. Cleaved SMAC binds X-IAP and antagonizes its anti-apoptotic activity. Here we show that melanoma IAP (ML-IAP), a potent anti-cell death protein and caspase inhibitor, physically interacts with SMAC through its BIR (baculovirus IAP repeat) domain. In addition to binding full-length SMAC, ML-IAP BIR associates with SMAC peptides that are derived from the amino terminus of active, processed SMAC. This high affinity interaction is very specific and can be completely abolished by single amino acid mutations either in the amino terminus of active SMAC or in the BIR domain of ML-IAP. In cells expressing ML-IAP and X-IAP, SMAC coexpression or addition of SMAC peptides abrogates the ability of the IAPs to inhibit cell death. These results demonstrate the feasibility of using SMAC peptides as a way to sensitize IAP-expressing cells to pro-apoptotic stimuli such as chemotherapeutic agents.

Tissue expression, protease specificity, and Kunitz domain functions of hepatocyte growth factor activator inhibitor-1B (HAI-1B), a new splice variant of HAI-1.

Hepatocyte growth factor activator inhibitor-1 (HAI-1) is an integral membrane protein expressed on epithelial cells and contains two extracellular Kunitz domains (N-terminal KD1 and C-terminal KD2) known to inhibit trypsin-like serine proteases. In tumorigenesis and tissue regeneration, HAI-1 regulates the hepatocyte growth factor (HGF)/c-Met pathway by inhibiting the activity of HGF activator (HGFA) and matriptase, two serine proteases that convert pro-HGF into its biologically active form. By screening a placental cDNA library, we discovered a new splice variant of HAI-1 designated HAI-1B that contains an extra 16 amino acids adjacent to the C terminus of KD1. To investigate possible consequences on Kunitz domain function, a soluble form of HAI-1B (sHAI-1B) comprising the entire extracellular domain was produced. First, we found that sHAI-1B displayed remarkable enzyme specificity by potently inhibiting only HGFA (IC50 = 30.5 nm), matriptase (IC50 = 16.5 nm), and trypsin (IC50 = 2.4 nm) among 16 serine proteases examined, including plasminogen activators (urokinase- and tissue-type plasminogen activators), coagulation enzymes thrombin, factors VIIa, Xa, XIa, and XIIa, and activated protein C. Relatively weak inhibition was found for plasmin (IC50 = 399 nm) and plasma kallikrein (IC50 = 686 nm). Second, the functions of the KD1 and KD2 domains in sHAI-1B were investigated using P1 residue-directed mutagenesis to show that inhibition of HGFA, matriptase, trypsin, and plasmin was due to KD1 and not KD2. Furthermore, analysis by reverse transcription-PCR demonstrated that HAI-1B and HAI-1 were co-expressed in normal tissues and various epithelial-derived cancer cell lines. Both isoforms were up-regulated in eight examined ovarian carcinoma specimens, three of which had higher levels of HAI-1B RNA than of HAI-1 RNA. Therefore, previously demonstrated roles of HAI-1 in various physiological and pathological processes likely involve both HAI-1B and HAI-1.

Structure and function analysis of peptide antagonists of melanoma inhibitor of apoptosis (ML-IAP).

Melanoma inhibitor of apoptosis (ML-IAP) is a potent anti-apoptotic protein that is upregulated in a number of melanoma cell lines but not expressed in most normal adult tissues. Overexpression of IAP proteins, such as ML-IAP or the ubiquitously expressed X-chromosome-linked IAP (XIAP), in human cancers has been shown to suppress apoptosis induced by a variety of stimuli. Peptides based on the processed N-terminus of Smac/DIABLO can negate the ability of overexpressed ML-IAP or XIAP to suppress drug-induced apoptosis. Such peptides have been demonstrated to bind to the single baculovirus IAP repeat (BIR) of ML-IAP and the third BIR of XIAP with similar high affinities (approximately 0.5 microM). Herein, we use phage-display of naïve peptide libraries and synthetic peptides to investigate the peptide-binding properties of ML-IAP-BIR and XIAP-BIR3. X-ray crystal structures of ML-IAP-BIR in complex with Smac- and phage-derived peptides, together with peptide structure-activity-relationship data, indicate that the peptides can be modified to provide increased binding affinity and selectivity for ML-IAP-BIR relative to XIAP-BIR3. For instance, substitution of Pro3' in the Smac-based peptide (AVPIAQKSE) with (2S,3S)-3-methylpyrrolidine-2-carboxylic acid [(3S)-methyl-proline] results in a peptide with 7-fold greater affinity for ML-IAP-BIR and about 100-fold specificity for ML-IAP-BIR relative to XIAP-BIR3.