Storing the building plans for a virus in its genome is much like how we store ideas in language. This may sound strange but, as an example, typos in spelling, grammar, or word usage, can lead to the meaning of a sentence either changing dramatically, remaining virtually unchanged, or becoming complete nonsense. The SARS-CoV-2 genome consists of RNA. Transcription of this RNA runs into a similar problem: errors can lead to the loss of function, a gain of function, or be completely inconsequential to the resulting protein (Figure 1). Large changes may break the virus, but smaller changes may provide an advantage and are essential for evolution.
In a previous article we spoke about the copy machinery of the virus, including the RNA-dependent RNA polymerase (RdRp), and drugs targeting it, such as Remdesivir. The goal of these drugs is to jam the enzyme and halt RNA production - or to cause more errors than are sustainable, with the end result being a less infectious virus. The reason the development of drugs targeting the copy machinery of RNA is worthwhile is that humans don’t have machinery to reproduce RNA from RNA. This means drugs targeting this machinery are less likely to interfere with normal processes in people. What if the virus could quickly repair these errors before the new genome is packed into a hull and kicked out the door? That would make finding a therapeutic much more difficult…
Unfortunately, SARS-CoV-2 has a way to repair the mistakes. When errors are introduced in transcription through environmental mutagenesis or even mutations caused by nucleotide analogs like Ribavarin1–3, the non-structural protein 14 (nsp14) has the ability to remove them. This multifunctional protein removes errors with the exoribonuclease (ExoN) activity of its N-terminal domain, while the C-terminal domain has the unrelated function of methylating the end cap of the viral RNA3,4.
However, this ExoN does not work alone. There is a replication complex made up of proteins performing many roles in the production of new RNA with high fidelity. Nsp12 is the main hub that makes a new RNA chain to complement the template. Nsp7 and nsp8 have a “processivity” role to enable nsp12 to function efficiently. In addition to these proteins there is a two-component proofreading system of Helicase (nsp13) and the ExoN domain of nsp14. Helicase can detect misshapen RNA helices caused by errors made by the copy machinery5. It then unwinds these double strands of RNA and feeds the strand containing the error into the ExoN domain of nsp14 where they are chopped out. This results in nsp12 continuing RNA replication where it left off.
The proofreading ability from Helicase and nsp14 ExoN allows SARS-CoV-2 to have a huge genome as compared to other viruses6(Figure 2). The large 29.9 kb genome of SARS-CoV-2 requires much more physical space to accommodate the necessary genetic information for reproduction when compared to other RNA viruses, such as Rhinovirus that has a genome between 7.2 kb and 8.5 kb in size (Figure 3). When no ExoN proofreading is present genomes cannot expand beyond 20 kb in size6(Figure 2). Maybe by removing the exoribonuclease activity, irreversible damage could be caused to the genome of SARS-CoV-2.
In order to understand how nsp14 can do this, we need to find out its atomic structure; this may also allow us to develop a drug which hinders its function. However, to this date, no structure of nsp14 from SARS-CoV-2 has been solved. However, structures have been solved of nsp14 in complex with another viral protein, nsp10, both from SARS-CoV (PDB entries 5nfy, 5c8s, 5c8t, 5c8u)2,7. As the protein sequences are very similar between SARS-CoV and SARS-CoV-2 (nsp14 is 95%, and nsp10 is 97% identical), it can be assumed that the SARS-CoV-2 structure as well as its functionality are very similar to SARS-CoV. The active site of the ExoN domain of nsp14 from SARS-CoV-2 has a DEEDh motif (named for the one-letter codes of the amino acids involved) containing a histidine as well as two aspartates and two glutamates2,3,7,8.
The N-terminus of nsp14 interacts with nsp10 (pink and blue, respectively, in Figure 4). The following domain (orange) has been shown to have exoribonuclease activity on double stranded RNA in a 3’ to 5’ direction9. When nsp10 is interacting with nsp14 there is a 35 fold increase in exoribonuclease activity, which is thought to occur due to conformational changes caused by formation of the complex2,9. The ExoN domain of nsp14 (orange) is connected to the methyltransferase domain (green) by a flexible hinge (black)7,10. This flexible region opens up the methyltransferase active site to allow methylation of the N7 of the 5’ Guanosine triphosphate of RNA10. There are three zinc finger motifs in nsp14 with two found in the ExoN domain and one in the methyltransferase domain2,7. In combination with the two further zinc sites in nsp10, these zinc fingers hold loops of the proteins together and are involved with nucleotide interaction2,7.
Nsp14 has also been demonstrated to form complexes with the copy machinery , nsp12, nsp7, and nsp8, although this interaction is independent of nsp102,11,12.
Scientists are searching for drugs that could be used to target nsp14 in order to find a cure for COVID-19. The active site of the ExoN domain of nsp14 has five residues that are essential for activity that form a negatively charged pocket (Figure 5A)7. Currently researchers are using the nsp14 structure from SARS-CoV to model a SARS-CoV-2 structure which can be used to identify compounds that could bind to the active site (Figure 5). These in silico screens start with nucleotide analog drugs like Remdesivir, Ribivarin or Ritonavir that are currently used as antiviral treatments for other viruses13–15. These nucleotide analogs are then changed to achieve a better binding to Nsp14’s active site in order to block it (Figure 5B).
As the ExoN is essential to support the huge 29.9kb genome of SARS-CoV-2, targeting nsp14 could lead to an effective treatment to COVID-19. Although drugs that target just nsp14 could be effective at increasing the error rate in RNA production by the virus, a more effective treatment will require inhibition of the RdRp of the copy machinery at the same time!
If you would like to look at the currently available structures for Nsp14(currently only available from SARS-CoV), they are available from our data base; we provide information on the quality of measurement data and models as well as improved structures. The highest resolution structure of nsp14 is PDB entry 5c8t at 3.2Å. This has a bound S-Adenosyl methionine ligand as well as zinc atoms present. Alongside this, another structure of Nsp14 bound to S-Adenosyl homocysteine and a guanosine-triphosphate-adenosine ligand as well as zinc at 3.33Å resolution has been published (PDB: 5c8s). Additionally, two structures with zinc atoms but no ligands are available (PDB 5c8u 3.4Å at and 5nfy at 3.34Å). Both PDB entry 5c8t and 5nfy have improved structures re-refined by our group.
Have you heard that the coronavirus “mutates”? Or that there are “several strains” of it around the world? Sounds scary, right? However, the reality is that everything “mutates”. All organisms, over time, acquire differences in their genes, from bacteria to humans. You might be aware that this can happen when your DNA (Deoxyribonucleic Acid) is exposed to UV light (like from the sun!), but this can also happen during DNA replication. This is when a cell uses the template of one of the two DNA strands to make a new complimentary copy of the other strand. Mutation is common to all living organisms (and viruses) and a driver of evolution. This is the first post in a series that will explore coronavirus replication with a focus on the proteins involved.
SARS-CoV-2 uses single-strand Ribonucleic acid (RNA) to encode its genome, not DNA, and hence belongs to a class of “single-strand RNA viruses”. For this reason, the virus needs a different way to copy its genome than “normal” cells have. The viral protein that copies the RNA is called an “RNA-dependent RNA polymerase” (RdRp). This protein uses the viral RNA as a template to make a new copy of viral RNA, by stringing single ribonucleotides together like beads on a string. This process is called polymerization.
A study by the Morse lab at Texas A&M University showed that SARS-CoV-2 RNA polymerase has a remarkable similarity to the RNA polymerase of SARS-CoV (>95%) as well as MERS-CoV , the virus which causes Middle-Eastern Respiratory Syndrome. This means that research performed in response to the SARS and MERS epidemics can inform our response to SARS-CoV-2. Unfortunately, a lack of consistent pandemic-preparedness funding means that we didn’t learn as much about RdRp in time as we could have. Still, RNA polymerase might be a viable drug target for halting the spread and reducing the fatality rate of COVID-19.
By determining the structure of RdRp, and deeply understanding how it works, we can optimize a drug to specifically target it and hinder its function. To this end, in the last few months, several structures of SARS-CoV-2 RNA polymerase have been published.
One interesting structure shows RNA polymerase in action, in the process of elongating an RNA strand (see Figure 1). This structure clearly show the polymerase in complex with smaller proteins, non-structural protein 7 and 8 (nsp7 and nsp8). These proteins improve how well the RNA polymerase binds the template RNA and also how long it stays bound before dissociating – a feature called “processivity”.
In the center of the protein is the area where the main action happens, called the “active site”. The amino acids of the polymerase that form the active site have a particular shape and chemical properties, which enable the polymerization reaction to occur very rapidly. In fact, the polymerase can string together as many as 100 nucleotides per second!  New RNA molecules can enter the active site through a little window to be added to the growing RNA chain. It is here that the antiviral drugs make their move!
First, let’s talk about Gilead’s FDA-approved drug, Remdesivir, which has taken the spotlight in the search for COVID-19 cures. Remdesivir (which has a fancy chemistry ID, GS-5734, and is sold under the brand name Veklury), is a “nucleotide analog”, which means that it mimics the shape and chemistry of the nucleotides that make up RNA and DNA (see figure).
Remdesivir was developed originally as a general antiviral drug and was later shown to protect cells (in a test tube) and monkeys (not in a test tube) from the Ebola Virus . However, this was recent enough, and science is slow enough that, until the COVID-19 pandemic, large-scale clinical trials of Remdesivir hadn’t been done yet. So scientists and doctors have been rushing to test the drug in COVID-19 patients. In fact, the US and Japan both approved the drug for “Emergency Use Authorization'' for severe COVID-19 patients as early as May , . And, in July, the European Medicines Agency gave Remdesivir a “conditional marketing authorization” (used for drugs that meet an unmet medical need but have insufficient data for normal approval). This allows the use of Remdesivir in severe COVID-19 patients through the next year . So, how the heck does a drug for Ebola, Influenza, or some other viruses also work against COVID-19? I was concerned by this when the news about all the drug trials were coming out – and I’m sure I wasn’t the only one...
The simple answer to that is all these viruses need to do the same thing - copy their RNA genome from an RNA template. And in order to do that, they all end up using basically the same tool, an RNA-Dependent RNA polymerase. And all drugs that are nucleotide analogs use the very same trick: they dress up like ribonucleotides (the "beads on a string" from before) and fool the RNA polymerase into letting them into the active site. Once inside, they get “stuck” in the active site, jamming the polymerase machine. Since this trick should work for any viral RNA polymerase, we can use these drugs for any RNA virus, and call them ‘general antivirals’. Of course, in practice, this doesn't always work, because there are differences between the different RNA polymerases. However, it is a great place to start! In the future, if we have general antivirals for SARS-CoV-2 all ready-to-go, we may be better equipped to deal with another coronavirus outbreak!
Remdesivir resembles the nucleotide adenine in structure, although it has some fancy chemical add-ons which help make it a better drug (thank you, medicinal chemistry!). When Remdesivir is injected into a vein, it travels through the bloodstream and enters into our cells, which recognize it as a foreign substance and try to digest it. However, what ends up happening is that the cells remove just the fancy chemical add-ons, and then confuse it for a normal adenine nucleotide. In infected cells, the viral RNA-dependent RNA polymerase then starts grabbing these molecules and inserting them into the new viral RNA strand in place of adenine molecules. Remdesivir, now attached to the RNA, jams the polymerase, rendering the virus unable to make more copies of its genome. Ultimately, this halts viral replication and helps the patient fight off the virus.
Another drug that inhibits the RNA polymerase activity is Favipiravir, sold under brand names Avigan, Abigan, and FabiFlu. Favipiravir has been discovered by Toyama Chemical Co., Ltd. in Japan and it has a similar mechanism to Remdesivir, except that it mimics a guanosine nucleoside instead of an adenine nucleotide . This drug was approved in Japan back in 2014 for use in resistant cases of Influenza A and B, but still remains unapproved in the US (still in Phase II and Phase III clinical trials) and the UK . This drug is also being tested for use against Ebola virus, Lassa virus, and currently SARS-CoV-2 in 43 countries. The approval of Favipiravir for COVID-19 has been much faster in China (Mar 15, 2020), Russia (Jun 3, 2020), and India (Jun 20, 2020), . Nonetheless, other countries, including Japan, are in various stages of clinical trials, and the results are anticipated to be out by the end of July .
Sadly, not yet. While the speed at which Remdesivir has gone through clinical trials is unprecedented, more work needs to be done to make sure it is safe and effective. Since (in the big scope of things) not a lot have people have taken Remdesivir, we aren’t really sure what all the side effects are, although there is emerging evidence for liver and kidney damage [12, 13]. The most common side effects are nausea (10% and 9% of patients), indigestion (7%) and increase of transaminases (6% and 8%). In one study, 3.6% of patients in a 10-day trial needed to stop taking therapy due to the latter. However, serious viral infections can also cause liver damage, so separating the two causes is a challenge! Remdesivir is not a cure-all, either. In one study it improved the recovery time from 15 days to 11 days, but it showed no effect for patients with mild to moderate disease, and no difference in median recovery time for patients who were already on a ventilator . Since the drug has to be given by infusion over several days, there is a pretty small window in which Remdesivir can actually help.
Likewise, Favipiravir has its own side effects such as liver damage, elevated uric acid levels, kidney damage, skin allergies, etc. . These effects restrict it for use by severe diabetes and heart patients. On top of that, it is not suitable for pregnant women because it can cause potential fetal deaths and deformities. It has been shown that Favipiravir works only during the earlier stages of SARS-CoV-2 infection when the body’s immune system isn’t totally drained, whereas it can result in a cytokine storm (when your immune system really freaks out) in severely ill patients. But, unfortunately, the virus doesn’t differentiate between humans while attacking, so a universal drug for COVID-19 has to be safe for use by all people.
However, these drugs are better than nothing, and by understanding the mechanisms involved, scientists can continue to improve upon the existing drugs for the benefit of all. While most of the ‘general antivirals’ that target RNA Polymerase have failed with SARS-CoV-2, Remdesivir has been relatively successful. Scientists think that this is actually because of a proofreading protein in SARS-CoV-2 called exonuclease. Immediately after the RNA-polymerase makes new RNA, exnuclease checks to make sure the new RNA is correct. In one study, another drug that mimics RNA called Ribivarin was shown to be removed from newly synthesized RNA by exonuclease . Thankfully, Remdesivir is not excised , which is likely why it has been more successful than the other options , . To read more about how nsp14 maintains the integrity and virulence of SARS-CoV-2, tune in to a future blog entry!
For those interested in reviewing the structures further, they are available in our GitHub repo, along with information about validation and, where relevant, improved structures. For a high-resolution comparison of the active site with and without Remdesivir, 7BV2 and 7BV1 (respectively) were published together at 2.5 and 2.8 Å. The elongating structure of the complex shown above (6YYT) has the polymerase as well as the cofactors and RNA very well resolved, with little "missing" density and a resolution of 2.9 Å. It is likely preferable to 6M71 and 7BTF, which were published with a similar resolution but with less of the complex resolved, and no RNA. For those interested, 7C2K and 7BZF (at 2.93 Å and 3.26 Å) show the complex bound to RNA in a pre- and post-translocation state.
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