Truncation of βA3/A1-crystallin during aging of the bovine lens;: Possible implications for lens optical quality

During aging, extensive modifications of eye lens proteins take place, which may contribute to the development of cataract. Truncation of the accessible extensions of beta-crystallins has been suggested to be an important factor in this process. We therefore studied the truncations of bovine beta A3- and beta Al-crystallin in more detail. These proteins are identical except for the length of their N-terminal extension, 30 and 13 residues, respectively. The water-soluble and -insoluble proteins from cortex and nucleus of bovine lenses of different ages were separated by 2D-gel electrophoresis and immune-blotted with an antiserum against beta A3. Two major truncation products were detected, which by sequence analysis were found to correspond to beta A3 having lost 11 or 22 amino acids. beta A3(-11) was only observed in the nucleus, whereas beta A3(-22) was present both in cortex and nucleus. We argue, therefore, that each of these two products is produced by a separate proteolytic enzyme. beta A3(-22) can originate by cleavage of beta A3, beta A1 and beta A3(-11). Truncation of beta A3 occurs more readily than that of beta A1, while beta A3(-11) disappears at an intermediate rate, It appears that the longer the N-terminal extension, the easier proteolysis takes place. Truncated proteins are not necessarily prone to end up in the water-insoluble fractions; other modifications leading to charge changes are more likely to be responsible for insolubilization, Truncation of the extensions of beta-crystallins could be a functional rather than a harmful process during aging of the lens; by modulating protein repulsion, it may help to maintain the protein concentration gradient that is necessary for the optical quality of the lens. (C) 1999 Academic Press.