Further characterization of structural and electric properties of non-spherical alpha-crystallin

Recently we have shown that the population of native alpha-crystallin, isolated using size-exclusion chromatography from eye lenses of calves, is multimodal. Most of the protein probably possesses an almost spherical appearance, but at least one of the other modes represents more extended, ellipsoidally and/or cylindrically shaped molecules [Van Haeringen, B., Eden, D., Van den Bogaerde, M.R., Van Grondelle, R. & Bloemendal, M. (1992) Eur. J. Biochem. 210, 211-216]. In the present study, we characterize various subpools of a single alpha-crystallin size-exclusion chromatography elution peak by means of transient-electric-birefringence measurements, ultraviolet linear-dichroism spectroscopy and analytical fast protein liquid chromatography. It is concluded that the fractions have a well-defined stable mass and are not in reversible equilibrium with each other. All pools appear to be composed of at least two types of differently shaped molecules. The hydrodynamic dimensions and electric properties of the different alpha-crystallin species are characterized. The non-spherical alpha-crystallin is found to be optically and electrically more anisotropic, and to contain a larger permanent electric dipole moment than the spherical form. A model for the composition of the alpha-crystallin pool is presented.