FIG. 7.

Triton X-100 insolubility of cell surface proteins. At 4 h p.i., influenza virus-infected (2.5 h p.i. for VSV-infected cells) MDCK cells were metabolically labeled for 60 min with 300 μCi of ³⁵S-protein labeling mix/ml and chased for 90 min. Cell surface proteins were biotinylated and extracted with 1% Triton X-100 on ice for 10 min. Triton X-100-soluble (S) and -insoluble (I) fractions of surface biotinylated proteins were immunoprecipitated with specific antibodies for NA, HA, or VSV G proteins, analyzed by SDS-PAGE, autoradiographed, and quantified. The percentage of Triton X-100-insoluble protein was calculated from the results from three to five independent experiments, with average variation of less than 10%. *, the Triton X-100 insolubility of HA was calculated with both HA0 and HA1 bands.