P1' specificity of the S219V/R203G mutant tobacco etch virus protease.

Proteases that recognize linear amino acid sequences with high specificity became indispensable tools of recombinant protein technology for the removal of various fusion tags. Due to its stringent sequence specificity, the catalytic domain of the nuclear inclusion cysteine protease of tobacco etch virus (TEV PR) is also a widely applied reagent for enzymatic removal of fusion tags. For this reason, efforts have been made to improve its stability and modify its specificity. For example, P1' autoproteolytic cleavage-resistant mutant (S219V) TEV PR was found not only to be nearly impervious to self-inactivation, but also exhibited greater stability and catalytic efficiency than the wild-type enzyme. An R203G substitution has been reported to further relax the P1' specificity of the enzyme, however, these results were obtained from crude intracellular assays. Until now, there has been no rigorous comparison of the P1' specificity of the S219V and S219V/R203G mutants in vitro, under carefully controlled conditions. Here, we compare the P1' amino acid preferences of these single and double TEV PR mutants. The in vitro analysis was performed by using recombinant protein substrates representing 20 P1' variants of the consensus TENLYFQ*SGT cleavage site, and synthetic oligopeptide substrates were also applied to study a limited set of the most preferred variants. In addition, the enzyme-substrate interactions were analyzed in silico. The results indicate highly similar P1' preferences for both enzymes, many side-chains can be accommodated by the S1' binding sites, but the kinetic assays revealed lower catalytic efficiency for the S219V/R203G than for the S219V mutant.

Biochemical Characterization, Specificity and Inhibition Studies of HTLV-1, HTLV-2, and HTLV-3 Proteases.

The human T-lymphotropic viruses (HTLVs) are causative agents of severe diseases including adult T-cell leukemia. Similar to human immunodeficiency viruses (HIVs), the viral protease (PR) plays a crucial role in the viral life-cycle via the processing of the viral polyproteins. Thus, it is a potential target of anti-retroviral therapies. In this study, we performed in vitro comparative analysis of human T-cell leukemia virus type 1, 2, and 3 (HTLV-1, -2, and -3) proteases. Amino acid preferences of S4 to S1' subsites were studied by using a series of synthetic oligopeptide substrates representing the natural and modified cleavage site sequences of the proteases. Biochemical characteristics of the different PRs were also determined, including catalytic efficiencies and dependence of activity on pH, temperature, and ionic strength. We investigated the effects of different HIV-1 PR inhibitors (atazanavir, darunavir, DMP-323, indinavir, ritonavir, and saquinavir) on enzyme activities, and inhibitory potentials of IB-268 and IB-269 inhibitors that were previously designed against HTLV-1 PR. Comparative biochemical analysis of HTLV-1, -2, and -3 PRs may help understand the characteristic similarities and differences between these enzymes in order to estimate the potential of the appearance of drug-resistance against specific HTLV-1 PR inhibitors.

Identification of Host Cellular Protein Substrates of SARS-COV-2 Main Protease.

The novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of coronavirus disease-19 (COVID-19) being associated with severe pneumonia. Like with other viruses, the interaction of SARS-CoV-2 with host cell proteins is necessary for successful replication, and cleavage of cellular targets by the viral protease also may contribute to the pathogenesis, but knowledge about the human proteins that are processed by the main protease (3CLpro) of SARS-CoV-2 is still limited. We tested the prediction potentials of two different in silico methods for the identification of SARS-CoV-2 3CLpro cleavage sites in human proteins. Short stretches of homologous host-pathogen protein sequences (SSHHPS) that are present in SARS-CoV-2 polyprotein and human proteins were identified using BLAST analysis, and the NetCorona 1.0 webserver was used to successfully predict cleavage sites, although this method was primarily developed for SARS-CoV. Human C-terminal-binding protein 1 (CTBP1) was found to be cleaved in vitro by SARS-CoV-2 3CLpro, the existence of the cleavage site was proved experimentally by using a His6-MBP-mEYFP recombinant substrate containing the predicted target sequence. Our results highlight both potentials and limitations of the tested algorithms. The identification of candidate host substrates of 3CLpro may help better develop an understanding of the molecular mechanisms behind the replication and pathogenesis of SARS-CoV-2.

Analysis of the efficacy of HIV protease inhibitors against SARS-CoV-2's main protease.

BACKGROUND: The pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has resulted in millions of infections worldwide. While the search for an effective antiviral is still ongoing, experimental therapies based on repurposing of available antivirals is being attempted, of which HIV protease inhibitors (PIs) have gained considerable interest. Inhibition profiling of the PIs directly against the viral protease has never been attempted in vitro, and while few studies reported an efficacy of lopinavir and ritonavir in SARS-CoV-2 context, the mechanism of action of the drugs remains to be validated. METHODS: We carried out an in-depth analysis of the efficacy of HIV PIs against the main protease of SARS-CoV-2 (Mpro) in cell culture and in vitro enzymatic assays, using a methodology that enabled us to focus solely on any potential inhibitory effects of the inhibitors against the viral protease. For cell culture experiments a dark-to-bright GFP reporter substrate system was designed. RESULTS: Lopinavir, ritonavir, darunavir, saquinavir, and atazanavir were able to inhibit the viral protease in cell culture, albeit in concentrations much higher than their achievable plasma levels, given their current drug formulations. While inhibition by lopinavir was attributed to its cytotoxicity, ritonavir was the most effective of the panel, with IC50 of 13.7 µM. None of the inhibitors showed significant inhibition of SARS-CoV-2 Mpro in our in vitro enzymatic assays up to 100 µM concentration. CONCLUSION: Targeting of SARS-CoV-2 Mpro by some of the HIV PIs might be of limited clinical potential, given the high concentration of the drugs required to achieve significant inhibition. Therefore, given their weak inhibition of the viral protease, any potential beneficial effect of the PIs in COVID-19 context might perhaps be attributed to acting on other molecular target(s), rather than SARS-CoV-2 Mpro.

Biochemical Characterization of Human Retroviral-Like Aspartic Protease 1 (ASPRV1).

The human retroviral-like aspartic protease 1 (ASPRV1) is a mammalian retroviral-like enzyme that catalyzes a critical proteolytic step during epidermal differentiation; therefore, it is also referred to as skin-specific aspartic protease (SASPase). Neutrophil granulocytes were also found recently to express ASPRV1 that is involved in the progression of acute chronic inflammation of the central nervous system, especially in autoimmune encephalomyelitis. Thus, investigation of ASPRV1 is important due to its therapeutic or diagnostic potential. We investigated the structural characteristics of ASPRV1 by homology modeling; analysis of the proposed structure was used for interpretation of in vitro specificity studies. For in-vitro characterization, activities of SASP28 and SASP14 enzyme forms were measured using synthetic oligopeptide substrates. We demonstrated that self-processing of SASP28 precursor causes autoactivation of the protease. The highest activity was measured for GST-SASP14 at neutral pH and at high ionic strength, and we proved that pepstatin A and acetyl-pepstatin can also inhibit the protease. In agreement with the structural characteristics, the relatively lower urea dissociation constant implied lower dimer stability of SASP14 compared to that of HIV-1 protease. The obtained structural and biochemical characteristics support better understanding of ASPRV1 function in the skin and central nervous system.

Functional Study of the Retrotransposon-Derived Human PEG10 Protease.

Paternally expressed gene 10 (PEG10) is a human retrotransposon-derived imprinted gene. The mRNA of PEG10 encodes two protein isoforms: the Gag-like protein (RF1PEG10) is coded by reading frame 1, while the Gag-Pol-like polyprotein (RF1/RF2PEG10) is coded by reading frames 1 and 2. The proteins are translated by a typical retroviral frameshift mechanism. The protease (PR) domain of RF2PEG10 contains an -Asp-Ser-Gly- sequence, which corresponds to the consensus -Asp-Ser/Thr-Gly- active-site motif of retroviral aspartic proteases. The function of the aspartic protease domain of RF2PEG10 remains unclear. To elucidate the function of PEG10 protease (PRPEG10), we designed a frameshift mutant (fsRF1/RF2PEG10) for comparison with the RF1/RF2PEG10 form. To study the effects of PRPEG10 on cellular proliferation and viability, mammalian HEK293T and HaCaT cells were transfected with plasmids coding for either RF1/RF2PEG10, the frameshift mutant (fsRF1/RF2PEG10), or a PR active-site (D370A) mutant fsRF1/RF2PEG10. Our results indicate that fsRF1/RF2PEG10 overexpression results in increased cellular proliferation. Remarkably, transfection with fsRF1/RF2PEG10 had a detrimental effect on cell viability. We hypothesize that PRPEG10 plays an important role in the function of this retroviral remnant, mediating the proliferation of cells and possibly implicating it in the inhibition of apoptosis.