Nonstructural protein 5A (NS5A) is a zinc-binding and proline-rich hydrophilic phosphoprotein that plays a key role in Hepatitis C virus RNA replication. [1] [2] It appears to be a dimeric form without trans-membrane helices. [3]
Nonstructural protein 5A (NS5A) is a zinc-binding and proline-rich hydrophilic phosphoprotein that plays a key role in Hepatitis C virus RNA replication. [1] [2] It appears to be a dimeric form without trans-membrane helices. [3]
NS5A is derived from a large polyprotein that is translated from the HCV genome, and undergoes post-translation processing by nonstructural protein 3 (NS3) viral protease. [4] Despite no inherent enzymatic activity being attributed to NS5A, its function is mediated through interaction with other nonstructural (NS) viral and cellular proteins. [2] [4] NS5A has two phosphorylated forms: p56 and p58, which differ in the electrophoretic mobility. [3] p56 is basally phosphorylated by host cellular protein kinase at the center and near the C terminus, whereas p58 is a form of hyper-phosphorylated NS5A at the center of the serine-rich region. [3] Protein mass spectrometry identified several phosphorylated serine residues in this region including serine 225, 229, 232, and 235 responsible for NS5A hyper-phosphorylation. [5] An array of phosphorylation-specific antibodies confirmed their phosphorylation in infected cells. [6] [7] [8] [9] It has been predicted that the N-terminal 30 aa of NS5A form an amphipathic α-helix with a highly preserved feature, which is essential to modulate the association between NS5A and ER membrane. [3] [4] The IFN-sensitivity determining region (ISDR) at the C-terminal of NS5A has been reported to perform strong trans-activating activities, suggesting that NS5A likely functions as a transcriptional activator. [3]
NS5A has three structurally different domains: Domain I was demonstrated to be an alternative dimeric structure by crystallography, while domain II and III remained unfolded. [1] Furthermore, the conformational flexibility of NS5A plays an important role in multiple HCV infection stages. [1] It is also possible that NS5A is a critical component during HCV replication and subcellular localization, which may shed light on the poorly understood HCV life cycle. [1] [4] Additionally, NS5A has been shown to modulate the polymerase activity of NS5B, an RNA-dependent RNA polymerase (RdRp). [3] Intriguingly, NS5A may be a RNA binding protein because it is able to bind to the 3’UTR of the plus and minus HCV RNA strands. [3] Moreover, NS5A is a key mediator in regulating host cell function and activity upon HCV infection. [4] Therefore, NS5A has been extensively studied in HCV research also due to its capability to regulate the interferon (IFN) response of the host cells. Because NS5A exerts functionally essential effects in regulation of viral replication, assembly and egress, it has been considered a potential drug target for antiviral therapeutic intervention. [1] [4] Indeed, small molecule drugs efficiently targeting NS5A displayed a much higher potency in controlling HCV infection than other drugs. [1] Therefore, NS5A related researches would have important implications in single molecule drug design and pegIFN-free direct-acting antiviral (DAA) combination therapies. [1]
Many antiviral drugs target NS5A, e.g. to treat hepatitis C; they are often described as NS5A inhibitors, and they are often used in combination with NS5B inhibitors:
Multiple copies of a polypeptide encoded by a gene often can form an aggregate referred to as a multimer. When a multimer is formed from polypeptides produced by two different mutant alleles of a particular gene, the mixed multimer may exhibit greater functional activity than the unmixed multimers formed by each of the mutants alone. When a mixed multimer displays increased functionality relative to the unmixed multimers, the phenomenon is referred to as intragenic complementation.[ citation needed ]
NS5A protein is a multimer, a dimer in this case, and intragenic complementation of replication-defective NS5A alleles has been demonstrated by Fridell et al. [10] On the bases of pairwise complementation tests between different NS5A mutant alleles, they identified three complementation groups that were considered to define three distinct and genetically separable functions of NS5A in RNA replication.
The hepatitis C virus (HCV) is a small, enveloped, positive-sense single-stranded RNA virus of the family Flaviviridae. The hepatitis C virus is the cause of hepatitis C and some cancers such as liver cancer and lymphomas in humans.
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Nonstructural protein 2 (NS2) is a viral protein found in the hepatitis C virus. It is also produced by influenza viruses, and is alternatively known as the nuclear export protein (NEP).
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Daclatasvir, sold under the brand name Daklinza, is an antiviral medication used in combination with other medications to treat hepatitis C (HCV). The other medications used in combination include sofosbuvir, ribavirin, and interferon, vary depending on the virus type and whether the person has cirrhosis. It is taken by mouth.
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Nonstructural protein 5B (NS5B) is a viral protein found in the hepatitis C virus (HCV). It is an RNA-dependent RNA polymerase, having the key function of replicating HCV's viral RNA by using the viral positive RNA strand as a template to catalyze the polymerization of ribonucleoside triphosphates (rNTP) during RNA replication. Several crystal structures of NS5B polymerase in several crystalline forms have been determined based on the same consensus sequence BK. The structure can be represented by a right hand shape with fingers, palm, and thumb. The encircled active site, unique to NS5B, is contained within the palm structure of the protein. Recent studies on NS5B protein genotype 1b strain J4's (HC-J4) structure indicate a presence of an active site where possible control of nucleotide binding occurs and initiation of de-novo RNA synthesis. De-novo adds necessary primers for initiation of RNA replication.
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Nonstructural protein 5A (NS5A) inhibitors are direct acting antiviral agents (DAAs) that target viral proteins, and their development was a culmination of increased understanding of the viral life cycle combined with advances in drug discovery technology. However, their mechanism of action is complex and not fully understood. NS5A inhibitors were the focus of much attention when they emerged as a part of the first curative treatment for hepatitis C virus (HCV) infections in 2014. Favorable characteristics have been introduced through varied structural changes, and structural similarities between NS5A inhibitors that are clinically approved are readily apparent. Despite the recent introduction of numerous new antiviral drugs, resistance is still a concern and these inhibitors are therefore always used in combination with other drugs.
Glecaprevir/pibrentasvir (G/P), sold under the brand names Mavyret and Maviret, is a fixed-dose combination medication used to treat hepatitis C. It contains glecaprevir and pibrentasvir. It works against all six types of hepatitis C. At twelve weeks following treatment between 81% and 100% of people have no evidence of hepatitis C. It is taken once a day by mouth with food.
Non-structural protein 5B (NS5B) inhibitors are a class of direct-acting antivirals widely used in the treatment of chronic hepatitis C. Depending on site of action and chemical composition, NS5B inhibitors may be categorized into three classes—nucleoside active site inhibitors (NIs), non-nucleoside allosteric inhibitors, and pyrophosphate analogues. Subsequently, all three classes are then subclassified. All inhibit RNA synthesis by NS5B but at different stages/sites resulting in inability of viral RNA replication. Expression of direct-acting NS5B inhibitors does not take place in cells that are not infected by hepatitis C virus, which seems to be beneficial for this class of drugs.