Conformational study and determination of the molecular weight of highly charged basic proteins by sedimentation equilibrium and gel electrophoresis.

We study several highly charged protamines and some related proteins from the sperm of molluscs. Circular dichroism and hydrodynamic parameters obtained from the sedimentation constant and intrinsic viscosity show that these proteins behave as random coils. However, it appears that a small amount of structure is present at basic pH. The molecular weight of these proteins is determined by several methods. When sedimentation equilibrium is used, we have found that the influence of concentration is much smaller than expected. We have also found that the highly charged nature of these proteins can be properly taken into account by using the methodology presently available [Williams, J. W., Van Holde, K. E., Baldwin, R.L., & Fujita, H. (1958) Chem. Rev. 58, 715; Eisenberg, H. (1976) Biological Macromolecules and Polyelectrolytes in Solution, Oxford University Press, London]. The calculations have been carried out in most cases by the method of Chernyak & Margretova [Chernyak, V. Ya., & Margretova, N.N. (1975) Biochem. Biophys. Res. Commun. 65, 990], which does not require the knowledge of the protein concentration. The overall adequacy of this approach has been ascertained by using as standards histone H1 and the protamine thynnine, both of known molecular weight and different charge densities. An electrophoretic method for the rapid estimation of the molecular weights of this type of proteins is also given. The values obtained by this method, as well as those found either with the Scheraga-Mandelkern equation or from the sedimentation and diffusion constants, agree within experimental error with the values obtained from sedimentation equilibrium.