EVIDENCE THAT ALPHA-CRYSTALLIN PREVENTS NONSPECIFIC PROTEIN AGGREGATION IN THE INTACT EYE LENS

The ocular lens is a transparent organ comprised of a highly concentra ted and highly ordered matrix of structural proteins, called crystalli ns, which are probably the longest lived proteins of the body. Lens tr ansparency is dependent upon maintenance of the short range order of t he crystallin matrix. This transparency must be maintained for decades in the absence of normal protein synthesis or repair capacity. We pre sent evidence here that alpha-crystallin, one of the major lens protei ns, plays a central role in vivo in stabilizing the other crystallins and preventing uncontrolled aggregation of these progressively modifie d and aging molecules. alpha-Crystallin has previously been shown to s uppress non-specific aggregation of denaturing proteins in simple bina ry systems through a chaperone-like activity. Our studies using solubl e homogenates of monkey lenses demonstrate a strong resistance to heat induced non-specific aggregation when the complete complement of crys tallins is present; in contrast, if alpha-crystallin is selectively re moved prior to heating, the remaining crystallins undergo extensive no n-specific aggregation as indicated by light scattering. When alpha-cr ystallin is present it complexes with denaturing proteins forming a so luble heavy molecular weight (HMW) fraction but no insolubilization is observed, while when alpha-crystallin is absent there is heavy insolu bilization and no HMW formed. When intact monkey lenses were heated it could be demonstrated that soluble HMW was generated. Similar HMW pro tein appears in vivo in the human lens as a function of age. These fin dings suggest that the soluble HMW protein present in the human lens i s the product of the chaperone-like function of alpha-crystallin and t hat under physiological conditions alpha-crystallin inhibits the uncon trolled aggregation of damaged proteins, thereby preventing the format ion of light scattering centers and opacification of the lens.