A magnetic bead-based fluorescent substrate for sensitive assay of SARS-CoV-2 3C-like protease activity

The 3C-like protease (3CLpro) of SARS-CoV-2 is a crucial target for antiviral drugs due to its essential role in viral polyprotein processing. In this study, we designed and produced a modular fluorescent recombinant substrate (6×His-ECFP-AVLQSGFRK-EYFP), which was then immobilized on Ni-NTA magnetic beads (Ni-NTA-6×His-ECFP-AVLQSGFRK-EYFP) for the assay of 3CLpro activity. Upon cleavage at the specific AVLQ↓SG motif, the EYFP fragment was released into the supernatant and quantified via fluorescence measurement (Ex/Em = 480/528 nm). A standard curve (y = 725.29x - 52.356; R² = 0.998) was obtained, enabling accurate quantification of the cleaved product and kinetic parameters. The assay using the designed substrate revealed a K_m of 22.01 ± 3.5 μM, k_cat of 0.021 s^-1, and catalytic efficiency (k_cat/K_m) of 946 M^-1.s^-1. The assay showed ∼50-fold greater sensitivity compared to SDS-PAGE and the inhibitory effect of GC376 for 3CLpro was also determined, with IC₅₀ of 0.88 μM. Since the modular substrate design allows for substitution of the N-terminal domain and cleavage motif, our development of the substrate and assay could be expanded to other high-specificity proteases.