The profilin--actin complex: further characterization of profilin and studies on the stability of the complex

Two forms of profilin can be isolated from calf spleen profilactin by chromatography on phosphocellulose. They can be distinguished by C-terminal analysis, which suggests that one of them lacks the C-terminal tyrosine and the penultimate glutamine residue. This is confirmed by treatment of profilin (+Tyr) with carboxypeptidase A, which removes the C-terminal tyrosine (rapidly) and the penultimate glutamine residue (slowly), and thereby converts it to the other form as judged by chromatography on phosphocellulose. The two forms of profilin differ also in solubility and in mobility during so-called 'charge shift' electrophoresis, indicating differences in their ability to bind detergents. Recombination studies using profilin with or without a modified C-terminus demonstrated that this part of profilin is relatively unimportant for the interaction with actin. On the other hand, experiments with native and modified actin revealed that the C-terminus of actin is of the utmost importance for the stability of the profilactin complex. Analysis of the u.v. absorbance and far-u.v. circular dichroism spectra of profilin and actin did not reveal any major changes in the conformation of the proteins accompanying the modifications at the C-terminal ends. Finally, it is reported that purified profilactin contains variable amounts of a protein factor which causes an apparent stabilization of profilactin in solution.