[ 35 S]methionine labeling and immunoprecipitation DBT cells were infected with MHV-A59 at an m.o.i. of 2. The cells were incubated with methionine-free medium for 30 min before labeling and were labeled in 100 mCi/ml [ 35 S]methionine starting at 1.5, 7 or 24 h p.i. After labeling for 2 h at each time point, the cells were harvested for protein extraction as described previously (Li et al., 1997) . The protein extracts were immunoprecipitated with anti-Flag antibody-conjugated beads (Sigma, St Louis, MO) in Tm 10 buffer (50 mM Tris±HCl pH 7.9, 0.1 M KCl, 12.5 mM MgCl 2 , 1 mM EDTA, 10% glycerol, 1 mM DTT, 0.1% NP-40, 1 mM phenylmethylsulfonyl¯uoride) at 4°C for 2 h. The immunoprecipitates were washed and separated on a 4±15% gradient SDS±polyacrylamide gel and visualized by autoradiography.