EFFECT OF CROSS-LINKING ON THE CHAPERONE-LIKE FUNCTION OF ALPHA-CRYSTALLIN

Alpha crystallin can function as a molecular chaperone in suppressing the heat-induced aggregation of other crystallins and proteins. During cataractogenesis, alpha-crystallin becomes a water-insoluble, high-mo lecular-weight, cross-linked aggregate. To determine whether the chape rone activity of alpha crystallin is lost during this age-related modi fication, extracts were prepared by sonication of water-insoluble prot eins isolated from aged bovine lenses and human cataract lenses. All t he preparations were tested for chaperone-like activity using beta L-c rystallin as the target protein and the percentage of alpha-crystallin in water-insoluble sonicated supernatant (WISS) was determined by slo t blot immunoassay. The WISS from bovine as well as human lenses were still effective in protecting beta L-crystallin aggregation at 56 degr ees C. The bovine cortical WISS with 50% immunoreactive alpha-crystall in showed 62% of the chaperone-like activity displayed by native alpha -crystallin. The WISS from bovine lens nucleus and human lenses with 1 7% and 5% immunoreactive alpha-crystallin showed 19% and 4% chaperone- like activity compared to native alpha-crystallin. Prior treatment of the WISS of both bovine and human lenses with dithiothreitol resulted in nearly 50% increase in chaperone-like activity suggesting possible loss of chaperone-like activity due to disulfide cross-links. To see i f the chaperone-like activity of alpha-crystallin can be altered by no n-disulfide cross-linking, native alpha-crystallin isolated from bovin e lenses was cross-linked with dimethylsuberimidate (DMS) and dimethyl 3,3'-dithiobispropionimidate (DTBP) and tested for chaperone-like act ivity. The DMS cross-linked alpha-crystallin was effective in inhibiti ng the aggregation of beta L-crystallins at 56 degrees C, but required a two- to five-fold higher concentration than the native alpha-crysta llin, alpha-Crystallin with higher degree of cross-linking showed lowe r chaperone-like activity. alpha-Crystallin cross-linked with DTBP, a cleavable cross-linking agent, also showed a 80% loss in chaperone-lik e activity. However, when the DTBP cross-linked alpha-crystallin was t reated with dithiothreitol to cleave the cross-links there was a 50% r ecovery in the chaperone-like activity. These data suggest that the ag e-related cross-linking, which restricts the molecular flexibility of alpha-crystallin decreases its chaperone-like function. (C) 1995 Acade mic Press Limited