Dimerization by colicin E3 in the absence of immunity protein

Using small angle x-ray scattering from solutions of colicin E3-immunity protein complex and the separated proteins, we have measured the radii of gyration of each species. We find that the complex is an elongated molecule with a radius of gyration of 35.5 +/- 0.7 A. Immunity protein is more compact with a radius of gyration of 13.4 +/- 0.1 A. Upon removal of immunity protein from the complex, colicin E3 (form minus immunity protein) undergoes further elongation and dimerizes, such that its radius of gyration increases to 75.6 +/- 3.1 A. Dimerization is not reversed by simple addition of a stoichiometric amount of immunity protein to the E3 solution, even though this is sufficient to block in vitro activity. We suggest that the elongation, and perhaps dimer formation, occurs in vivo, concomitant with membrane translocation.