The Quaternary Structure of Bovine α‐Crystallin

Abstract

The stability of the native quaternary structure of bovine .alpha.-crystallin [from eye lens cells] was studied, by sedimentation analysis and EM, as a function of pH (7-11), ionic strength (0.01-0.5), temperature (6-60.degree. C) and Ca ion concentration (0 and 10 mM). Three successive transitions are distinguished at 20.degree. C: a slow transconformation step, which is independent of pH, ionic strength or Ca ions; an irreversible primary dissociation step.sbd.favored by increasing pH above 8 and/or a lower ionic strength.sbd.with formation of alkali-modified .alpha.-crystallin that is spherically shaped like the native protein, but has a smaller average diameter, sedimentation coefficient and MW; and, with further increase of pH above 9, a rapidly reversible dissociation of alkali-modified .alpha.-crystallin, characterized by a single reaction boundary in sedimentation velocity analysis. In the presence of Ca ions the quaternary structure is stabilized to the extent that no dissociation is observed up to, at least, pH 10.3. Upon increase of temperature, at pH 7.3, a slow irreversible dissociation and swelling run parallel, until a limit is reached around 37.degree. C with formation of temperature-modified .alpha.-crystallin; it is indistinguishable from the native protein by EM, but has a higher relative viscosity, and lower sedimentation coefficient and MW. Ca ions have little or no effect on this transition. Above 37.degree. C a reversal of this transition or aggregation is indicated. These findings, and previous structural data on microheterogeneity, reassociation from urea, and aging of .alpha.-crystallin in vivo, are incorporated into a hypothetical scheme of transitions based on a 3-layer model for the quaternary structure.