FIG. 4.

Requirements for the μ1 conformational change in solution. Trypsin sensitivity assays were carried out to test whether free mutant μ1 had undergone a conformational change. Free wild-type and mutant μ1 trimers [μ1₃, (μ1-DS1)₃, (μ1-DS2)₃, and (μ1-N42A)₃], prepared as described in Materials and Methods, were incubated at 42°C for 30 min in different buffers containing either NaCl (Na) or CsCl (Cs). Each sample was treated with trypsin to detect the conformational change. (A) The μ1 conformational change requires separation of the μ1 head domains. (B) The μ1 conformational change does not require autocleavage at the μ1N/C junction. Note that the δ/φ cleavage is more extensive here than in the experiment whose results are shown in Fig. 2C, probably because of slightly different chymotrypsin to μ1₃σ3₃ heterohexamer ratios.