Fig. 6. KLK13 cleaves the HCoV-HKU1 S protein between the S1 and S2 domains.

(A) Fifteen nanograms of CleavEx proteins harboring the S1/S2 or S2′ sites was incubated at 37°C for 3 hours with the indicated concentrations of purified KLK13 protein. The samples were then denatured at 95°C, and the proteins were resolved by SDS-PAGE and analyzed by Western blotting with antibodies specific for His-tagged proteins. Blots are representative of three experiments. (B) The full-length HKU1-S protein or control sample was incubated at 37°C for 3 hours with the indicated concentrations of purified KLK13 protein. The samples were then denatured at 95°C, and the proteins were resolved by SDS-PAGE and analyzed by Western blotting with HRP-conjugated antibody against the His tag to detect the full-length protein with the N-terminal His tag (~200 kD) and the S1 after KLK13 cleavage (~80 kD). Blots are representative of three experiments.