Combining ultracentrifugation with fluorescence to follow the unfolding of modules 16-17 of complement receptor type 1

Complement receptor type 1 (CR1) is a member of a family of regulators of complement activation with therapeutic potential. A fragment of CR1 comprising modules 16 and 17 was overexpressed as a recombinant nonglycosylated protein in Pichia pastoris. Intrinsic fluorescence studies of the unfolding of recombinant CR1-16-17 caused by increasing concentrations of guanidinium chloride revealed that the intermodular junction unfolds first, followed by module 17, with module 16 the last to unfold. Sedimentation velocity studies in the analytical ultracentrifuge revealed a corresponding clear change in conformation from the native macromolecules with an axial ratio of about 5:1 to a much more extended conformation.