A Fluorogenic Peptide Cleavage Assay to Screen the Proteolytic Activity of Proteases

Enveloped viruses with transmembrane spike proteins contain a proteolytic cleavage site.

Membrane-bound proteases on host cells cleave the spike proteins, exposing fusion peptides that facilitate virus entry.

To begin, take wild-type peptides with the canonical cleavage site and variant peptides, each bearing a unique mutation at the site.

The fluorogenic peptides contain a fluorophore and a quencher. Due to proximity, the quencher absorbs the fluorescence of the fluorophore.

Take a multi-well plate on ice, containing assay buffer and host proteases.

Add the wild-type and variant peptides into separate wells. Incubate at an appropriate temperature for optimum protease activity.

The protease cleaves the peptides, separating the quencher from the fluorophore and enabling its fluorescence emission.

Measure the change in the fluorescence signal to determine the rate of proteolytic cleavage.

A higher rate for the wild-type sequence indicates higher proteolytic activity, while a lower rate for the variants indicates altered cleavage efficiency.  

Prepare the appropriate assay buffers for the proteases as described in the text protocol, and chill the buffers on ice. Place a solid black polystyrene non-treated flat-bottom 96-well plate on ice with a thin metal plate underneath to support cooling and stability.

It is essential to use a Black plate as the assay plate to prevent fluorescent leakage from adjacent wells.

Per peptide, prepare three technical replicates per assay in a total volume of 100 microliters per sample. Pipette the appropriate amount of assay buffer into each well of the assay plate. Add 0.5 microliters of protease to each well. To six wells, add 0.5 microliters of buffer instead of the respective protease. Three of these will be blank controls, and the other three will be peptide controls.

Add 5 microliters of the peptide to a final concentration of 50 micromolar to each well, except the blank controls. To each of the three blank control wells, add 5 microliters of buffer instead of peptide. Insert the plate into the fluorescence plate reader and click Start.