Fig. 7.

The fusion-block H230A mutation does not affect core trimer formation or proximal stem packing. (A) Formation of the target of SFV DIII binding in WT and H230A SFV. BHK cells were incubated with radiolabeled WT SFV or H230A mutant on ice and treated at low pH in the presence of exogenous DIII, and E1 was retrieved by the use of DIII-specific antibody, all as described for Fig. 5A. The data shown are a representative example of the results of two independent experiments. (B) Packing of the proximal stem region in WT and H230A SFV. WT and H230A mutant viruses were prebound to BHK cells on ice as described for panel A, treated for 1 min at pH 5.5 at 37°C, and adjusted to neutral pH. The cells were lysed and digested with trypsin to remove monomeric E1 as described in Materials and Methods. The resultant E1 homotrimers were denatured with SDS where indicated. Aliquots of the samples were precipitated with polyclonal serum to E2/E1, antibody to the proximal stem region (29), and control preimmune serum. The amount of retrieved E1 was expressed as a percentage of the total amount of E1 retrieved using the polyclonal antiserum. Data shown represent the average and range of the results of 2 independent experiments.