Scintillation proximity assay

Scintillation proximity assay technologies provide a rapid non-separation method to measure common biological interactions using radioactively tagged molecules. This unit identifies potential uses of the technology for the measurement of receptor-ligand binding, cAMP accumulation, GTP binding to heterotrimeric G proteins, protease activity and cellular uptake.

High-throughput screening of low molecular weight NS3-NS4A protease inhibitors using a fluorescence resonance energy transfer substrate

Hepatitis C virus (HCV) NS3-NS4A protease is an attractive target for anti-HCV agents because of its important role in replication. An optimized fluorescence resonance energy transfer (FRET) substrate for NS3-NS4A protease, based on the sequence of the NS5A-5B cleavage site, was designed and synthesized. High-throughput screening of in-house compound libraries was performed using a FRET substrate FS10 (MOCAcDKIVPC-SMSYK-Dnp) and MBP-NS3-NS4A fusion protein. Several hit compounds were found, including YZ-9577 (2-oxido-1,2,5-oxadiazole-3,4-diyl) bis (phenylmethanone) with potent inhibitory activity (IC50=1.6 microM) and good selectivity against other human serine proteases.

Cinanserin is an inhibitor of the 3C-like proteinase of severe acute respiratory syndrome coronavirus and strongly reduces virus replication in vitro

The 3C-like proteinase (3CLpro) of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is one of the most promising targets for anti-SARS-CoV drugs due to its crucial role in the viral life cycle. In this study, a database containing structural information of more than 8,000 existing drugs was virtually screened by a docking approach to identify potential binding molecules of SARS-CoV 3CLpro. As a target for screening, both a homology model and the crystallographic structure of the binding pocket of the enzyme were used. Cinanserin (SQ 10,643), a well-characterized serotonin antagonist that has undergone preliminary clinical testing in humans in the 1960s, showed a high score in the screening and was chosen for further experimental evaluation. Binding of both cinanserin and its hydrochloride to bacterially expressed 3CLpro of SARS-CoV and the related human coronavirus 229E (HCoV-229E) was demonstrated by surface plasmon resonance technology. The catalytic activity of both enzymes was inhibited with 50% inhibitory concentration (IC50) values of 5 microM, as tested with a fluorogenic substrate. The antiviral activity of cinanserin was further evaluated in tissue culture assays, namely, a replicon system based on HCoV-229E and quantitative test assays with infectious SARS-CoV and HCoV-229E. All assays revealed a strong inhibition of coronavirus replication at nontoxic drug concentrations. The level of virus RNA and infectious particles was reduced by up to 4 log units, with IC50 values ranging from 19 to 34 microM. These findings demonstrate that the old drug cinanserin is an inhibitor of SARS-CoV replication, acting most likely via inhibition of the 3CL proteinase.

Identification and characterization of a monofunctional glycosyltransferase from Staphylococcus aureus

A gene (mgt) encoding a monofunctional glycosyltransferase (MGT) from Staphylococcus aureus has been identified. This first reported gram-positive MGT shared significant homology with several MGTs from gram-negative bacteria and the N-terminal glycosyltransferase domain of class A high-molecular-mass penicillin-binding proteins from different species. S. aureus MGT contained an N-terminal hydrophobic domain perhaps involved with membrane association. It was expressed in Escherichia coli cells as a truncated protein lacking the hydrophobic domain and purified to homogeneity. Analysis by circular dichroism revealed that secondary structural elements of purified truncated S. aureus MGT were consistent with predicted structural elements, indicating that the protein might exhibit the expected folding. In addition, purified S. aureus MGT catalyzed incorporation of UDP-N-acetylglucosamine into peptidoglycan, proving that it was enzymatically active. MGT activity was inhibited by moenomycin A, and the reaction product was sensitive to lysozyme treatment. Moreover, a protein matching the calculated molecular weight of S. aureus MGT was identified from an S. aureus cell lysate using antibodies developed against purified MGT. Taken together, our results suggest that this enzyme is natively present in S. aureus cells and that it may play a role in bacterial cell wall biosynthesis.

Protease inhibitors as potential antiviral agents for the treatment of picornaviral infections

The picornavirus family contains several human pathogens including human rhinovirus (HRV) and hepatitis A virus (HAV). In the case of HRVs, these small single-stranded positive-sense RNA viruses translate their genetic information into a polyprotein precursor which is further processed mainly by two viral proteases designated 2A and 3C. The 2A protease (2Apro) makes the first cleavage between the structural and non-structural proteins, while 3C protease (3Cpro) catalyzes most of the remaining internal cleavages. It has been shown that both 2Apro and 3Cpro are cysteine proteases but their overall protein folding is more like trypsin-type serine proteases. Due to their unique protein structure and essential roles in viral replication, 2Apro and 3Cpro have been viewed as excellent targets for antiviral intervention. In recent years, considerable efforts have been made in the development of antiviral compounds targeting these proteases. This article summarizes the recent approaches in the design of novel 2A and 3C protease inhibitors as potential antiviral agents for the treatment of picornaviral infections.

Protease inhibitors as potential antiviral agents for the treatment of picornaviral infections

The picornavirus family contains several human pathogens including human rhinovirus (HRV) and hepatitis A virus (HAV). In the case of HRVs, these small single-stranded positive-sense RNA viruses translate their genetic information into a polyprotein precursor which is further processed mainly by two viral proteases designated 2A and 3C. The 2A protease (2Apro) makes the first cleavage between the structural and non-structural proteins, while 3C protease (3Cpro) catalyzes most of the remaining internal cleavages. It has been shown that both 2Apro and 3Cpro are cysteine proteases but their overall protein folding is more like trypsin-type serine proteases. Due to their unique protein structure and essential roles in viral replication, 2Apro and 3Cpro have been viewed as excellent targets for antiviral intervention. In recent years, considerable efforts have been made in the development of antiviral compounds targeting these proteases. This article summarizes the recent approaches in the design of novel 2A and 3C protease inhibitors as potential antiviral agents for the treatment of picornaviral infections.

Development of in vitro peptide substrates for human rhinovirus-14 2A protease

Purified 2A protease from human rhinovirus serotype-14 (HRV14) was unable to efficiently cleave a 16-mer peptide representing its authentic cis-cleavage site on the viral polyprotein, implying that in vivo cis cleavage by this enzyme might be very different from its in vitro trans activity. Presence of a serine at position P2 and a leucine at P2' in the 16-mer peptide was found to be responsible for the low peptide cleavage efficiency. To search for an efficient peptide substrate for HRV14 2A, small peptides derived from other rhinovirus 2A protease cleavage sites were synthesized and tested. These results suggested that the N-terminal 8 amino acids were sufficient for HRV14 2A cleavage to occur, although the P1' and P2' residue identities were important to the cleavage of peptides with amino acids occupying both sides of the scissile bond. On the basis of the 2A substrate requirements, a sensitive fluorometric assay for the viral 2A proteases was developed using peptides with anthranilide and 3-nitrotyrosine as the resonance energy transfer donor/quencher pair. Our data indicated that these fluorescent peptide substrates were suitable for 2A protease characterization and inhibitor evaluation.

Enzymatic characterization of refolded human rhinovirus type 14 2A protease expressed in Escherichia coli

Reported here is the production of recombinant human rhinovirus 14 (HRV14) 2A protease from bacterial cells transformed with a heat-inducible plasmid containing the HRV14 2A cDNA sequence. Overexpressed 2A protein partitioned into the inclusion bodies was solubilized in urea and then refolded in the presence of Zn2+ Transition metals were required for the restoration of 2A protease activity as a structural component, but appeared to be inhibitory if added exogenously once the enzyme was refolded. Based on the cleavage specificity studies, a colorimetric assay was developed for the highly purified HRV14 2A protease. A peptide with the sequence RKGDIKSY-p-nitroanilide was found to be cleaved by the 2A protease with a k(cat)/Km ratio of approximately 335 M(-1)s(-1), which allows its activity to be measured continuously with a spectrophotometer or a microplate reader.

Expression and purification of recombinant rhinovirus 14 3CD proteinase and its comparison to the 3C proteinase

Human rhinovirus (HRV) is a positive-stranded RNA virus with an open reading frame that encodes for a single polyprotein of about 3000 amino acids. The HRV polyprotein is proteolytically processed; eight of nine cleavages are catalyzed by the 3C and/or the 3CD proteinases. We have expressed and purified recombinant HRV14 3C and 3CD proteinases and investigated their substrate selectivity and inhibitor sensitivity. Expressed 3CD proteinase had the P1/P1' residues of the 3C/3D cleavage site mutated from Gln/Gly to Ala/Ala in order to prevent autocleavage. The 3CD proteinase activities were measured by utilization of native, chromogenic, and fluorogenic peptide substrates. The 3CD proteinase exhibited < or =15% activity, compared to 3C, toward peptidyl p-nitroanilide substrates which contain only the p-nitroaniline moiety in the prime side. The 3C and 3CD proteinases exhibited similar activities for both internally quenched fluorogenic and native peptides. These results suggest that the two enzymes have similar but slightly different substrate specificity, especially on their preference for prime side residues. Inhibitor sensitivities toward classical proteinase inhibitors were generally similar for both enzymes. Small peptidyl inhibitors, specifically designed and synthesized for HRV14 3C, also inhibited the 3CD proteinase. Taken together, our data indicate that the 3D domain of 3CD proteinase had some influence on substrate recognition, but did not have dramatic impact on its interaction with inhibitors.