Fast protein liquid chromatography-chromatofocusing of bovine and chicken alpha-crystallin subunits under denaturing conditions

Packing-induced conformational and functional changes in the subunits of alpha -crystallin

The heteroaggregate alpha-crystallin and homoaggregates of its subunits, alphaA- and alphaB-crystallins, function like molecular chaperones and prevent the aggregation of several proteins. Although modulation of the chaperone-like activity of alpha-crystallin by both temperature and chaotropic agents has been demonstrated in vitro, the mechanism(s) of its regulation in vivo have not been elucidated. The subunits of alpha-crystallin exchange freely, resulting in its dynamic and variable quaternary structure. Mixed aggregates of the alpha-crystallins and other mammalian small heat shock proteins (sHSPs) have also been observed in vivo. We have investigated the time-dependent structural and functional changes during the course of heteroaggregate formation by the exchange of subunits between homoaggregates of alphaA- and alphaB-crystallins. Native isoelectric focusing was used to follow the time course of subunit exchange. Circular dichroism revealed large tertiary structural alterations in the subunits upon subunit exchange and packing into heteroaggregates, indicating specific homologous and heterologous interactions between the subunits. Subunit exchange also resulted in quaternary structural changes as demonstrated by gel filtration chromatography. Interestingly, we found time-dependent changes in chaperone-like activity against the dithiothreitol-induced aggregation of insulin, which correlated with subunit exchange and the resulting tertiary and quaternary structural changes. Heteroaggregates of varying subunit composition, as observed during eye lens epithelial cell differentiation, generated by subunit exchange displayed differential chaperone-like activity. It was possible to alter chaperone-like activity of preexisting oligomeric sHSPs by alteration of subunit composition by subunit exchange. Our results demonstrate that subunit exchange and the resulting structural and functional changes observed could constitute a mechanism of regulation of chaperone-like activity of alpha-crystallin (and possibly other mammalian sHSPs) in vivo.

A dynamic quaternary structure of bovine alpha-crystallin as indicated from intermolecular exchange of subunits

The structural bovine eye lens protein alpha-crystallin was dissociated in 7 M urea and its four subunits, A1, A2, B1, and B2, were separated by means of ion-exchange chromatography. Homopolymeric reaggregates of these subunits were prepared by removal of the denaturant via dialysis. It was found that subunits were exchanged upon incubation of mixtures of two homopolymers under native conditions. New hybrid species were formed within 24 h as demonstrated by isoelectric focusing. Moreover, native alpha-crystallin molecules also exchanged subunits when incubated with homopolymeric aggregates of B2 subunits. Subunit exchange between native alpha-crystallin molecules is postulated, and a "dynamic quaternary structure" is presented that allows the polydisperse protein to adapt to changes in cytoplasmic conditions upon aging of the lens tissue.

The in vivo phosphorylation sites of bovine alpha B-crystallin

Phosphate content determinations established that in alpha B-crystallin two phosphate groups can be present in vivo in bovine lenses. Comparison of tryptic digests of phosphorylated and unphosphorylated alpha B chains, revealed the location of the two phosphorylation sites in tryptic peptides T2 and T3. Thermolytic digestion and gas-phase sequencing demonstrated that Ser-19 and Ser-45 are the in vivo phosphorylation sites of bovine alpha B-crystallin. This pattern of phosphorylation differs from the previously reported in vitro obtained results.

Protein folding and aggregation studied by isoelectric focusing across a urea gradient and isoelectric focusing in two dimensions

Isoelectric focusing across a concentration gradient of urea was used to study the folding-unfolding and association-dissociation processes of proteins. Myoglobulin, albumin, RNase, papain, beta L- and alpha-crystallin were analyzed with this technique, and examples are given of visualized dissociation steps and of equilibrium-unfolding intermediates. Furthermore, a two-dimensional isoelectric focusing technique is presented that is useful to deduce whether a transition of a protein aggregate observed upon urea-gradient isoelectric focusing must be attributed to a change in the protein's tertiary or quaternary structure.

On the structure of alpha-crystallin: construction of hybrid molecules and homopolymers

The alpha A2 and alpha B2 subunits of bovine alpha-crystallin were purified by chromatofocussing in urea and assembled into homopolymers. Light-scattering measurements indicated their molecular masses were 360 and 420 kDa. The alpha A2 and alpha B2 polypeptides were also used to construct a series of hybrid molecules with alpha A/alpha B ratios ranging from 7:1 to 1:7. Sedimentation velocity analyses, isoelectric focussing under non-deaggregating conditions, circular dichroism spectroscopy and immunochemical analysis indicated that all of the subunits had copolymerized to alpha-crystallin-like aggregates with complete regeneration of the native structure. The polymers could be distinguished on the basis of their differing affinities for the antiserum. This was directly related to the proportion of alpha A2 subunits in each polymer. It was concluded that the alpha A2 and alpha B2 subunits are structurally equivalent and occupy equivalent site in the alpha-crystallin aggregates. It was also concluded that a micellar-like quaternary structure was consistent with most previous observations on the protein.