Restricting motion by introducing double Cys mutants that form intersubunit disulfide bridges. (A) Locations of the five double Cys mutants created in this study: I, P89C/F291C; II, G93C/V95C; III, P90C/E91C; IV, S65C/D315C; V, E59C/Q321C. For clarity, bridges are only shown for one of the three subunit interfaces. (B) Western blot in the absence of reducing agent indicates that all five mutants form either trimers or dimers.