ABSTRACT
A high-affinity RNA aptamer to human activated protein C (APC) was selected from a pool of random sequences using in vitro selection. Activated protein C, a trypsin-like serine protease plays an important role along with thrombin as a regulator in blood clotting cascade. After seven rounds of selection and amplification, a single predominant nucleic acid sequence APC-167, a 167-base oligonucleotide with a random sequence core of 120 bases, was obtained. The selected aptamer did not bind to thrombin or factor Xa and thus demonstrated specificity to APC. Furthermore, this aptamer was a non-competitive inhibitor to the cleavage reaction of a fluorogenic substrate catalyzed by APC. The inhibition constant (Ki) of APC-167 was 83 nM. The 99-base oligonucleotide (APC-99) derived from APC-167 by deleting both primer binding sites, was also found to inhibit APC strongly (Ki = 137 nM). Two stem-loop structures and at least one G x U wobble base pair in the stem were elucidated as important structural motifs for binding.
Subject(s)
Nucleic Acid Conformation , Protein C/metabolism , RNA/chemistry , RNA/metabolism , Base Composition , Base Sequence , Binding Sites , DNA Primers , Ethylnitrosourea , Humans , Kinetics , Models, Molecular , Molecular Sequence Data , Oligoribonucleotides/chemistry , Oligoribonucleotides/metabolism , Polymerase Chain Reaction , Protein C/antagonists & inhibitorsABSTRACT
Hepatitis C virus (HCV) is a single-stranded RNA virus and its genome is translated into a single large polyprotein. The viral-encoded NS3 protein possesses protease, nucleoside triphosphatase, and helicase activities. Since these activities appear to be important for viral replication, efforts are being made to identify compounds that might inhibit the enzymatic activities of NS3 and serve as potential anti-HCV agents. We used a genetic selection strategy in vitro to isolate, from a pool of completely random RNA (120 random bases), those RNA aptamers that could bind to NS3. After six cycles of selection and amplification, 14% of the pooled RNAs could bind specifically to the NS3 protein. When the aptamers in the pool (cycle 6) were analyzed for binding and inhibition of the proteolytic activity of NS3 with the NS5A/NS5B peptide as substrate (S1), two aptamers, designated G6-16 and G6-19 RNA, were found to inhibit NS3 in vitro. Kinetic studies of the inhibition revealed that the aptamer G6-16 inhibited the NS3 protease with an inhibitory constant (Ki) of 3 microM. We also analyzed aptamers G6-16 and G6-19 for their action with a longer protein substrate (amino acid region 2203-2506) and found that these aptamers efficiently inhibited the proteolytic activity of NS3. In addition, both G6-16 and G6-19 aptamers were found to inhibit the helicase activity of NS3. Since these aptamers possesses dual inhibitory function for NS3, they could prove to be useful as anti-HCV drug leads.
Subject(s)
Hepacivirus/genetics , RNA, Viral/genetics , Viral Nonstructural Proteins/genetics , Base Sequence , Humans , Molecular Sequence Data , RNA, Viral/isolation & purification , Serine Endopeptidases/geneticsABSTRACT
The RNA genome of human hepatitis C virus (HCV) is translated into a large precursor polyprotein. The NS3 protease of HCV has a crucial role in the processing of the polyprotein into functional viral proteins. We have used an in vitro genetic-selection strategy to isolate high-affinity RNA aptamers that bind to the NS3 protein, especially to its protease domain. Starting from a RNA pool that had a random sequence core of 12-18 nucleotides, aptamers that bind specifically to the NS3 protein were selected after 10 rounds of selection and amplification. A single aptamer, 10G-1, was found predominantly (71%) in the selected pool. This aptamer could bind to the NS3 protein with a binding constant of 650 nM and inhibit the proteolytic activity in vitro. By phosphate-modification-interference analysis we showed that the phosphate residues that are critical for the binding of 10G-1 to NS3 lie within the selected regions of the aptamer and that binding involves electrostatic contacts with the phosphates of regions G28-U34 and A47-A55. The NS3-binding region in 10G-1 can serve as a basis for designing more potential inhibitors of the NS3 protein.
Subject(s)
Hepacivirus/enzymology , Hepacivirus/genetics , RNA, Viral/genetics , RNA, Viral/metabolism , Serine Endopeptidases/metabolism , Viral Nonstructural Proteins/metabolism , Base Sequence , Binding Sites/genetics , Carrier Proteins/genetics , Carrier Proteins/metabolism , DNA Primers/genetics , Drug Design , Hepacivirus/drug effects , Humans , Ligands , Maltose-Binding Proteins , Molecular Sequence Data , Nucleic Acid Conformation , Phosphates/chemistry , RNA, Viral/chemistry , Recombinant Fusion Proteins/genetics , Recombinant Fusion Proteins/metabolism , Sequence Homology, Nucleic Acid , Serine Endopeptidases/genetics , Serine Proteinase Inhibitors/pharmacology , Static Electricity , Viral Nonstructural Proteins/antagonists & inhibitors , Viral Nonstructural Proteins/geneticsABSTRACT
The serine protease domain of HCV comprising amino acids 1027-1218 (deltaNS3) was expressed in E. coli with a His tag at its N-terminal end. The protease was purified to apparent homogeneity by a single step affinity chromatography resulting in high yields (approximately 3 mg/l of cultured cells). The deltaNS3 efficiently cleaves a 17-mer peptide corresponding to the NS5A-NS5B junction with kcat/Km = 160 x 10(-3) min(-1) microM(-1) in the presence of NS4A peptide. Our deltaNS3 represents the minimal domain possessing highly active protease of NS3 constructed so far. The deltaNS3 protein also efficiently processed a longer substrate corresponding to NS5A/5B junction (2203-2506 amino acids) that was synthesized by in vitro transcription and translation system.
