The stability of the native quaternary structure of bovine cc-crystallin was studied. by sedimentation analysis and electron microscopy, as a function of pH (7 -1 I), ionic strength (0.01 -0.5), temperature (6 -60 'C) and calcium ion concentration (0 and 10 mM).Three successive transitions are distinguished at 20 ' C. Firstly, a slow transconformation step, which is independent of pH, ionic strength or calcium ions. Secondly, an irreversible primary dissociation step, favoured by increasing pH above 8 and/or a lower ionic strength, with formation of 'alkali-modified x-crystallin', which is spherically shaped like the native protein but has a smaller average diameter, sedimentation coefficient and molecular weight. Thirdly, with further increase of pH above 9, a rapidly reversible dissociation of alkali-modified a-crystallin characterized by a single reaction boundary in sedimentation velocity analysis. In the presence of calcium 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 '-C with formation of 'temperature-modified r-crystallin', which is indistinguishable from the native protein by electron microscopy, but has a higher relative viscosity and lower sedimentation coefficient and molecular weight. Calcium ions have little or no effect on this transition. Above 3 7 ' C a reversal of this transition or aggregation is indicated.These findings, together with previous structural data on microheterogeneity, reassociation from urea, and aging of a-crystallin in viva, are incorporated into a hypothetical scheme of transitions, based on a three-layer model for the quaternary structure.The crystallins are a group of structural proteins which occur in extremely high concentrations in the eye lens cells, and, as such, play a role in the refraction and focussing of light rays (reviewed in [l -31). The largest of these, a-crystallin, has a weight-average molecular weight ( M , ) of about 850000 in the bovine lens cortex, and it is a spherical assembly of A and B-type subunits, both of about 20 000 molecular weight, in 3 : 1 ratio [4,5]. Primarily in the lens nucleus agerelated structural changes affect the state of aggregation, the solubility and the interaction of x-crystallin with other crystallins and lens membranes, which in turn may affect light refraction and lens transparency, and in extreme cases may lead to visual impairment, Previous papers in this scries appeared earlier in this journal [4.5,7,8]. A preliminary report of part of this work was presented at the Workshop on Aging of the Lens, Bonn-Meckenheim, July 1977 [20]._ _ _ at least in humans [3,6]. The elucidation of the quaternary structure of a-crystallin will undoubtedly contribute to (and it may prove to be essential for) an understanding of these processes on the molecular level. In earlier papers of this series information on the qtructure has been acquir...