Syncytium formation in cells expressing wild-type or mutant proteins. (A) pH dependence of polykaryon formation. BHK-E2E1 (WT) and BHK cells were induced with ZnSO4 for 16 h, exposed to fusion medium at pH 5.0, 6.0, or 7.0 for 20 min, incubated in regular growth medium for 4 h, and photographed. (B) Polykaryon formation by mutant proteins. Induced BHK cells were exposed to fusion medium at pH 5.0 for 20 min, incubated in regular growth medium for 4 h, fixed, and photographed. WT, BHK-E2E1; Gly, BHK-E2E1(G93D); Pro, BHK-E2E1(P104G).