Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has had a profound impact on society and the economy worldwide. Since its inception in late 2019, extensive research on SARS-CoV-2 has been conducted in an effort to identify effective therapies that can be used to treat coronavirus disease 2019 (COVID-19). These efforts have led to the development of various antibodies and antiviral agents that are now approved for use in patients with COVID-19.
Study: The functional landscape of the SARS-CoV-2 3CL protease. Image credit: Natalya Rozhkova / Shutterstock.com
Aimed at SARS-CoV-2 3CLpro
Current COVID-19 therapies target primarily three viral targets, including peak glycoprotein, ribonucleic acid-dependent RNA polymerase (RNA) (RdRp), and 3-chymotrypsin-like protease (3CLpro). ). While monoclonal antibodies often target the SARS-CoV-2 ear protein, remdesivir and molnupiravir target the viral RdRp, and nirmatrelvir targets the SARS-CoV-2 3CLpro.
SARS-CoV-2 3CLpro is required for viral replication, making this enzyme an ideal target for new therapies. Several inhibitors of SARS-CoV-2 3CLpro have already been developed, some of which have even advanced in clinical trials. In fact, the SARS-CoV-2 inhibitor 3CLpro niramtrelvir has already been approved to treat patients with COVID-19 in combination with ritonavir to prevent serious illness.
Compared to the tip protein, which has already undergone many mutations as SARS-CoV-2 has evolved throughout the pandemic, 3CLpro is largely conserved in various coronaviruses. This suggests that the development of broad-spectrum antivirals targeting this enzyme may be advantageous for the current pandemic, as well as for future coronavirus outbreaks.
About the study
In a recent study published on the bioRxiv * server, researchers at Columbia University’s Irving Medical Center, Tehran University, and the Fred Hutchinson Cancer Research Center are evaluating the activity of all potential mutations in SARS-CoV-2 3CLpro using deep mutational scanning (DMS). ). This approach has been used previously to evaluate the impact of mutations on SARS-CoV-2 spike protein expression, as well as their binding capabilities to the angiotensin 2 converting enzyme receptor (ACE2). ) of the host cell and neutralizing antibodies.
The current study used Saccharomyces cerevisiae (except in buds) for its ease of use, its usefulness in previous studies of SARS-CoV-2 DMS, and the folding of shared proteins, post-translation modification, and billing with human proteins. A yeast strain carrying the cloned SARS-CoV-2 3CLpro gene was created using an inducible yeast expression vector.
3CLpro restricts in vitro growth
Cells expressing SARS-CoV-2 3CLpro grew at a slower rate compared to cells carrying the yellow fluorescent protein enhanced with non-toxic control (EYFP). A catalytically inactive C145A mutant was subsequently created to determine whether this observed growth reduction was caused by the enzymatic activity of 3CLpro or that of an exogenously produced protease.
To this end, the C145A mutant grew at a rate similar to that of non-toxic EYFP control when produced in yeast, thus indicating that the toxicity detected was a direct result of 3CLpro activity.
Mutant activity 3CLpro
The activity of each enrichment of the SARS-CoV-2 3CLpro variant in the induced and non-induced circumstances was then evaluated to subsequently provide an activity score for each enzyme. These scores were then normalized to wild-type and stop-type encoding, which was set to 0 and -1, respectively. These studies showed that protein tolerance to each mutation varied widely between each variant, with certain regions appearing to be highly restricted and resistant to mutation.
More specifically, mutations on the catalytic day, His41 and Cys145, both essential for 3CLpro activity, inevitably resulted in a loss of protease activity. Other important residues within the active site that were conserved included Asp187 and Arg40, both important for stabilizing various structures within 3CLpro.
Phe140, Glu143, His163, His164 and Gln192 were also the most conserved residues that appeared to be resistant to mutations. In comparison, Glu166 and Gln189 were found to be unconserved residues that may also be resistant to mutations.
The researchers also observed that several regions within the 3CLpro mutants, such as those that form the catalytic pocket within the enzyme or reside at the dimerization interface, appear to be strongly intolerant to the mutants. DMS observations on the activity of the 3CLpro mutant were subsequently confirmed by in vitro assays that evaluated the protease activity of these mutants.
The evolution of conservation 3CLpro
Despite the high degree of conservation observed in 3CLpro, other 3CL coronavirus proteases remain quite diverse. To better understand the activity of the less conserved 3CLpro waste that arises due to evolution, the researchers used the ConSurf server.
The ConSurf server provided a platform to assess the rate of evolution of each waste, in which poorly conserved waste was classified with a high rate of evolution. Comparatively, a higher degree of conservation within the waste led to the classification of a medium or low rate of evolution.
Tolerance to mutations was observed in residues with high mutation rates, while intolerance to mutations was observed in residues with low rates of evolution. A potential factor that may contribute to the evolutionary conservation of 3CLpro may include compensatory or enabling mutations that were beyond the scope of the current study.
Implications
Despite the highly conserved nature of 3CLpro in different coronaviruses, the current study found that this enzyme can still acquire several mutations; however, these mutations do not appear to significantly alter 3CLpro activity. However, the potential development of resistance to 3CLpro inhibitors justifies future studies on combination treatments that may help reduce the rate of viral escape.
Several highly conserved regions within 3CLpro that were identified in the current study appear to play a critical role in the overall structural integrity and catalytic function of this enzyme. Thus, the development of 3CLpro inhibitors in the future could use this waste as anchor points.
* Important news
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guided by clinical practice / health-related behavior, or treated as established information.