Crystallization of Feline Coronavirus Mpro With GC376 Reveals Mechanism of Inhibition.

Coronaviruses infect a variety of hosts in the animal kingdom, and while each virus is taxonomically different, they all infect their host via the same mechanism. The coronavirus main protease (Mpro, also called 3CLpro), is an attractive target for drug development due to its essential role in mediating viral replication and transcription. An Mpro inhibitor, GC376, has been shown to treat feline infectious peritonitis (FIP), a fatal infection in cats caused by internal mutations in the feline enteric coronavirus (FECV). Recently, our lab demonstrated that the feline drug, GC373, and prodrug, GC376, are potent inhibitors of SARS-CoV-2 Mpro and solved the structures in complex with the drugs; however, no crystal structures of the FIP virus (FIPV) Mpro with the feline drugs have been published so far. Here, we present crystal structures of FIPV Mpro-GC373/GC376 complexes, revealing the inhibitors covalently bound to Cys144 in the active site, similar to SARS-CoV-2 Mpro. Additionally, GC376 has a higher affinity for FIPV Mpro with lower nanomolar Ki values compared to SARS-CoV and SARS-CoV-2 Mpro. We also show that improved derivatives of GC376 have higher potency for FIPV Mpro. Since GC373 and GC376 represent strong starting points for structure-guided drug design, determining the crystal structures of FIPV Mpro with these inhibitors are important steps in drug optimization and structure-based broad-spectrum antiviral drug discovery.

SARS-COV-2 recombinant Receptor-Binding-Domain (RBD) induces neutralizing antibodies against variant strains of SARS-CoV-2 and SARS-CoV-1.

SARS-CoV-2 is the etiological agent of COVID19. There are currently several licensed vaccines approved for human use and most of them target the spike protein in the virion envelope to induce protective immunity. Recently, variants that spread more quickly have emerged. There is evidence that some of these variants are less sensitive to neutralization in vitro, but it is not clear whether they can evade vaccine induced protection. In this study, we tested SARS-CoV-2 spike RBD as a vaccine antigen and explored the effect of formulation with Alum/MPLA or AddaS03 adjuvants. Our results show that RBD induces high titers of neutralizing antibodies and activates strong cellular immune responses. There is also significant cross-neutralization of variants B.1.1.7 and B.1.351 and to a lesser extent, SARS-CoV-1. These results indicate that recombinant RBD can be a viable candidate as a stand-alone vaccine or as a booster shot to diversify our strategy for COVID19 protection.

Insights into the catalytic properties of the mitochondrial rhomboid protease PARL.

The rhomboid protease PARL is a critical regulator of mitochondrial homeostasis through its cleavage of substrates such as PINK1, PGAM5, and Smac/Diablo, which have crucial roles in mitochondrial quality control and apoptosis. However, the catalytic properties of PARL, including the effect of lipids on the protease, have never been characterized in vitro. To address this, we isolated human PARL expressed in yeast and used FRET-based kinetic assays to measure proteolytic activity in vitro. We show that PARL activity in detergent is enhanced by cardiolipin, a lipid enriched in the mitochondrial inner membrane. Significantly higher turnover rates were observed for PARL reconstituted in proteoliposomes, with Smac/Diablo being cleaved most rapidly at a rate of 1 min-1. In contrast, PGAM5 is cleaved with the highest efficiency (kcat/KM) compared with PINK1 and Smac/Diablo. In proteoliposomes, a truncated ß-cleavage form of PARL, a physiological form known to affect mitochondrial fragmentation, is more active than the full-length enzyme for hydrolysis of PINK1, PGAM5, and Smac/Diablo. Multiplex profiling of 228 peptides reveals that PARL prefers substrates with a bulky side chain such as Phe in P1, which is distinct from the preference for small side chain residues typically found with bacterial rhomboid proteases. This study using recombinant PARL provides fundamental insights into its catalytic activity and substrate preferences that enhance our understanding of its role in mitochondrial function and has implications for specific inhibitor design.

PARL Protease: A Glimpse at Intramembrane Proteolysis in the Inner Mitochondrial Membrane.

Intramembrane proteolysis, although once a controversial concept, is a widely studied field. Four classes of intramembrane proteases have been identified and are classified by their catalytic mechanism of peptide bond hydrolysis: metallo, glutamyl, aspartyl, and serine proteases. One of the most studied of these classes is the rhomboid superfamily of serine intramembrane proteases. Rhomboids consist of six or seven transmembrane segments that form a helical bundle within the membrane and are involved in a multitude of cellular processes. These proteases are characterized by a catalytic dyad composed of a serine and a histidine residue, which distinguishes them from classical serine proteases wherein a catalytic triad is utilized. Of all currently identified rhomboid proteases, one that is of great interest is the mammalian mitochondrial rhomboid protease PARL. Most well known for its processing of the kinase PINK1 and potential link to Parkinson's disease, PARL has been shown to cleave a variety of substrates within the cell including PGAM5, Smac, TTC19, and others. While recent proteomic studies have provided insight on new potential substrates of PARL, its regulation, activity, and role in maintaining mitochondrial homeostasis remain largely unknown.