Structure and Function of the Hypertension Variant A486V of G Protein-coupled Receptor Kinase 4.

G-protein-coupled receptor (GPCR) kinases (GRKs) bind to and phosphorylate GPCRs, initiating the process of GPCR desensitization and internalization. GRK4 is implicated in the regulation of blood pressure, and three GRK4 polymorphisms (R65L, A142V, and A486V) are associated with hypertension. Here, we describe the 2.6 Å structure of human GRK4α A486V crystallized in the presence of 5'-adenylyl ß,γ-imidodiphosphate. The structure of GRK4α is similar to other GRKs, although slight differences exist within the RGS homology (RH) bundle subdomain, substrate-binding site, and kinase C-tail. The RH bundle subdomain and kinase C-terminal lobe form a strikingly acidic surface, whereas the kinase N-terminal lobe and RH terminal subdomain surfaces are much more basic. In this respect, GRK4α is more similar to GRK2 than GRK6. A fully ordered kinase C-tail reveals interactions linking the C-tail with important determinants of kinase activity, including the αB helix, αD helix, and the P-loop. Autophosphorylation of wild-type GRK4α is required for full kinase activity, as indicated by a lag in phosphorylation of a peptide from the dopamine D1 receptor without ATP preincubation. In contrast, this lag is not observed in GRK4α A486V. Phosphopeptide mapping by mass spectrometry indicates an increased rate of autophosphorylation of a number of residues in GRK4α A486V relative to wild-type GRK4α, including Ser-485 in the kinase C-tail.

Structural basis for selective small molecule kinase inhibition of activated c-Met.

The receptor tyrosine kinase c-Met is implicated in oncogenesis and is the target for several small molecule and biologic agents in clinical trials for the treatment of cancer. Binding of the hepatocyte growth factor to the cell surface receptor of c-Met induces activation via autophosphorylation of the kinase domain. Here we describe the structural basis of c-Met activation upon autophosphorylation and the selective small molecule inhibiton of autophosphorylated c-Met. MK-2461 is a potent c-Met inhibitor that is selective for the phosphorylated state of the enzyme. Compound 1 is an MK-2461 analog with a 20-fold enthalpy-driven preference for the autophosphorylated over unphosphorylated c-Met kinase domain. The crystal structure of the unbound kinase domain phosphorylated at Tyr-1234 and Tyr-1235 shows that activation loop phosphorylation leads to the ejection and disorder of the activation loop and rearrangement of helix αC and the G loop to generate a viable active site. Helix αC adopts a orientation different from that seen in activation loop mutants. The crystal structure of the complex formed by the autophosphorylated c-Met kinase domain and compound 1 reveals a significant induced fit conformational change of the G loop and ordering of the activation loop, explaining the selectivity of compound 1 for the autophosphorylated state. The results highlight the role of structural plasticity within the kinase domain in imparting the specificity of ligand binding and provide the framework for structure-guided design of activated c-Met inhibitors.

Structural basis for the inhibition of RNase H activity of HIV-1 reverse transcriptase by RNase H active site-directed inhibitors.

HIV/AIDS continues to be a menace to public health. Several drugs currently on the market have successfully improved the ability to manage the viral burden in infected patients. However, new drugs are needed to combat the rapid emergence of mutated forms of the virus that are resistant to existing therapies. Currently, approved drugs target three of the four major enzyme activities encoded by the virus that are critical to the HIV life cycle. Although a number of inhibitors of HIV RNase H activity have been reported, few inhibit by directly engaging the RNase H active site. Here, we describe structures of naphthyridinone-containing inhibitors bound to the RNase H active site. This class of compounds binds to the active site via two metal ions that are coordinated by catalytic site residues, D443, E478, D498, and D549. The directionality of the naphthyridinone pharmacophore is restricted by the ordering of D549 and H539 in the RNase H domain. In addition, one of the naphthyridinone-based compounds was found to bind at a second site close to the polymerase active site and non-nucleoside/nucleotide inhibitor sites in a metal-independent manner. Further characterization, using fluorescence-based thermal denaturation and a crystal structure of the isolated RNase H domain reveals that this compound can also bind the RNase H site and retains the metal-dependent binding mode of this class of molecules. These structures provide a means for structurally guided design of novel RNase H inhibitors.

Structure of human prostasin, a target for the regulation of hypertension.

Prostasin (also called channel activating protease-1 (CAP1)) is an extracellular serine protease implicated in the modulation of fluid and electrolyte regulation via proteolysis of the epithelial sodium channel. Several disease states, particularly hypertension, can be affected by modulation of epithelial sodium channel activity. Thus, understanding the biochemical function of prostasin and developing specific agents to inhibit its activity could have a significant impact on a widespread disease. We report the expression of the prostasin proenzyme in Escherichia coli as insoluble inclusion bodies, refolding and activating via proteolytic removal of the N-terminal propeptide. The refolded and activated enzyme was shown to be pure and monomeric, with kinetic characteristics very similar to prostasin expressed from eukaryotic systems. Active prostasin was crystallized, and the structure was determined to 1.45 A resolution. These apoprotein crystals were soaked with nafamostat, allowing the structure of the inhibited acyl-enzyme intermediate structure to be determined to 2.0 A resolution. Comparison of the inhibited and apoprotein forms of prostasin suggest a mechanism of regulation through stabilization of a loop which interferes with substrate recognition.

Biochemical and structural characterization of a novel class of inhibitors of the type 1 insulin-like growth factor and insulin receptor kinases.

The type 1 insulin-like growth factor receptor (IGF-1R) is often overexpressed on tumor cells and is believed to play an important role in anchorage-independent proliferation. Additionally, cell culture studies have indicated that IGF-1R confers increased resistance to apoptosis caused by radiation or chemotherapeutic agents. Thus, inhibitors of the intracellular kinase domain of this receptor may have utility for the clinical treatment of cancer. As part of an effort to develop clinically useful inhibitors of IGF-1R kinase, a novel class of pyrrole-5-carboxaldehyde compounds was investigated. The compounds exhibited selectivity against the closely related insulin receptor kinase intrinsically and in cell-based assays. The inhibitors formed a reversible, covalent adduct at the kinase active site, and treatment of such adducts with sodium borohydride irreversibly inactivated the enzyme. Analysis of a tryptic digest of a covalently modified IGF-1R kinase fragment revealed that the active site Lys1003 had been reductively alkylated with the aldehyde inhibitor. Reductive alkylation of the insulin receptor kinase with one of these inhibitors led to a similarly inactivated enzyme which was examined by X-ray crystallography. The crystal structure confirmed the modification of the active site lysine side chain and revealed details of the key interactions between the inhibitor and enzyme.

Protein expression plasmids produced rapidly: streamlining cloning protocols and robotic handling.

As many processes in the preclinical drug discovery process become highly parallel, the need to also produce a large number of different proteins in parallel has become acute, such as for protein crystallization and activity screening. In turn, the requisite DNA constructions to produce these proteins must now be done at a rate that requires automated cloning procedures, each with an intrinsic low failure probability per sample. The high-throughput cloning solutions presented here achieve production of 192 different expression plasmids at a success rate of greater than 95% of the targeted open reading frames. Time for completion of the set by one person is reduced to approximately 11 working days, starting with polymerase chain reactions for a number of source clones and ending with purified expression plasmids. Achievement of this throughput utilizes the following: (1) the Beckman Coulter (Fullerton, CA) Biomek FX liquid handler for most manipulations, (2) Gateway cloning technology (Invitrogen Corp., Carlsbad, CA), and (3) computer programs designed for parallel processing of all sample information, including primer design and the resulting DNA and protein sequence assembly. Exemplary data are presented for discovery of a form of the Rho-kinase that crystallizes (ROCK2).

A general procedure for the purification of human beta-secretase expressed in Escherichia coli.

Expression and purification of human beta-secretase (BACE1) in bacteria have been plagued with issues concerning solubility, inhomogeneous N-terminus, and lack of enzymic activity. Several forms of the mature human BACE1 have been expressed in Escherichia coli with different N-terminal extensions and without the C-terminus transmembrane domain. Although each of the proteins expresses in inclusion bodies, a generalized protocol has been developed to solubilize, refold, and purify these BACE1 variants. The resultant proteins are homogeneous and monodispersed in solution. Each possesses a unique N-terminus. Activity assays using the peptide substrate 7-methoxycoumarin-4-yl-SEVNLDAEFK-2,4-dinitrophenyl-RR, corresponding to the beta-secretase cleavage sequence in the amyloid precursor protein with the Swedish mutations of N(670)L(671) substituting for the residues K(670)M(671), reveal a kcat and KM of 9.3 min(-1) and 55 microM, respectively.

Structure of apo, unactivated insulin-like growth factor-1 receptor kinase at 1.5 A resolution.

The crystal structure of the wild-type unactivated kinase domain (IGFRK-0P) of insulin-like growth factor-1 receptor has been reported previously at 2.7 A resolution [Munshi et al. (2002), J. Biol. Chem. 277, 38797-38802]. In order to obtain a high-resolution structure, a number of variants of IGFRK-0P were prepared and screened for crystallization. A double mutant with E1067A and E1069A substitutions within the kinase-insert region resulted in crystals that diffracted to 1.5 A resolution. Overall, the structure of the mutant IGFRK-0P is similar to that of the wild-type IGFRK-0P structure, with the exception of the previously disordered kinase-insert region in the wild type having become fixed. In addition, amino-acid residues 947-952 at the N-terminus are well defined in the mutant structure. The monomeric protein structure is folded into two lobes connected by a hinge region, with the catalytic center situated at the interface of the two lobes. Two molecules of IGFRK-0P in the asymmetric unit are associated as a dimer and two different types of dimers with their ATP-binding clefts either facing towards or away from each other are observed. The current refined model consists of a dimer and 635 water molecules.

Crystal structure of the Apo, unactivated insulin-like growth factor-1 receptor kinase. Implication for inhibitor specificity.

The x-ray structure of the unactivated kinase domain of insulin-like growth factor-1 receptor (IGFRK-0P) is reported here at 2.7 A resolution. IGFRK-0P is composed of two lobes connected by a hinge region. The N-terminal lobe of the kinase is a twisted beta-sheet flanked by a single helix, and the C-terminal lobe comprises eight alpha-helices and four short beta-strands. The ATP binding pocket and the catalytic center reside at the interface of the two lobes. Despite the overall similarity to other receptor tyrosine kinases, three notable conformational modifications are observed: 1) this kinase adopts a more closed structure, with its two lobes rotated further toward each other; 2) the conformation of the proximal end of the activation loop (residues 1121-1129) is different; 3) the orientation of the nucleotide-binding loop is altered. Collectively, these alterations lead to a different ATP-binding pocket that might impact on inhibitor designs for IGFRK-0P. Two molecules of IGFRK-0P are seen in the asymmetric unit; they are associated as a dimer with their ATP binding clefts facing each other. The ordered N terminus of one monomer approaches the active site of the other, suggesting that the juxtamembrane region of one molecule could come into close proximity to the active site of the other.

A collaborative screening program for the discovery of inhibitors of HCV NS2/3 cis-cleaving protease activity.

This report describes the development of a cell-based assay for high-throughput screening and detection of small-molecule inhibitors for hepatitis C virus (HCV) NS2/3 protease. The HCV NS2/3 protease is essential for the normal infectious cycle of HCV. Generation of a cell-based assay for this cis-acting viral protease involved reporter constructs in which the NS2/3 protease sequence was inserted between the ,B-lactamase (BLA) reporter and a ubiquitin-based destabilization domain. In stable cell lines, NS2/3 cis cleavage of the NS2/3-BLA fusion protein resulted in differential stability of the cleaved versus uncleaved BLA reporter, providing a robust readout for protease activity. BLA reporter activity was shown to be a function of NS2/3-specific protease activity, by using genetic mutants of the NS2/3 sequence. In addition, the cell-based assay was validated and screened in a 384-well format on a fully automated robotic platform where small-molecule inhibitors of NS2/3 protease activity were identified.

Potent HIV-1 Protease Inhibitors: Stereoselective Synthesis of a Dipeptide Mimic.

The synthesis of a differentially protected dipeptide mimic 10 in enantiomerically pure form is described. The key step involves the epimerization of the C-2 center of the lactone 4, hydrolysis and protection of the resulting hydroxy acid, followed by Curtius rearrangement to introduce the urethane functionality. The scope and versatility of this isostere has been demonstrated by its conversion to potent HIV-1 protease inhibitors with nanomolar potencies. Also, established through the synthesis of compound 13 and 14, the 3S hydroxyl configuration of the dipeptide isostere 1 is the preferred configuration for its potency. The present synthesis is efficient and provides an access to other dipeptide mimics with a great deal of structural diversity.

3'-Tetrahydrofuranylglycine as a Novel, Unnatural Amino Acid Surrogate for Asparagine in the Design of Inhibitors of the HIV Protease.

The blockade of the HIV protease has become a major target in the search for an effective therapy for AIDS.1 While many reports of potent HIV-1 inhibitors have appeared recently, the compound Ro 31-8959 remains the least selective for the HIV-1 and HIV-2 proteases.2 This property may result in reduced susceptibility to resistance since these represent the genetically most divergent strains of HIV presently known to exist.