FIG 2.

Proteolytic activation of the precleaved S protein. (A) Conformational changes in precleaved S proteins. To prepare the virus harboring a cleaved S protein, MHV-2 was treated with 1 μg/ml trypsin or PBS (uncleaved control) for 1 h, treated with trypsin inhibitors, and then purified on a Sephadex G-75 column. Conformational changes in PBS-treated MHV-2, trypsin-treated MHV-2, and MHV-2f harboring a precleaved S protein were examined as described for Fig. 1. Samples were boiled and subjected to Western blot analysis with MAb-10G antibody. (B) Cell entry by viruses. Viruses treated with PBS or trypsin were inoculated onto DBT cells, and cellular RNA was isolated after 0 or 6 h. Viral mRNA was quantified by real-time PCR (n = 4).