On the structure of alpha m-crystallin. The reversibility of urea dissociation.

Fetal calf alpha-crystallins were denatured using various concentrations of urea and analyzed by gel filtration and ultracentrifugation. alpha c-Crystallin (18.4 S) irreversibly dissociates to alpha m-crystallin (11.8 S) at low urea concentrations. These findings substantiate our previous proposal that alpha c-crystallin is an artefactual aggregate produced by low isolation temperatures (Thomson, J.A., and Augusteyn, R.C. (1983) Exp. Eye Res. 37, 367-377). The sedimentation coefficient of alpha m-crystallin decreases steadily with increasing urea, due to a parallel change in the diffusion coefficient and not to a decrease in molecular weight. The aggregate dissociates directly to subunits, with no evidence for stable intermediate species. Complete dissociation, in 8 M urea, is accompanied by gross disruption of the secondary and tertiary structures. Following removal of the urea, the reassociated alpha r-crystallin is indistinguishable from native alpha m-crystallin in secondary, tertiary, and quaternary structure, as judged from the sedimentation and diffusion coefficients, subunit contents, near- and far-UV CD spectra, the microenvironments of cyteine and aromatic amino acids, and from their immunochemical properties. We have concluded that the native structure of alpha-crystallin can be completely recovered after dissociation and denaturation in urea.