Truncation of βA3/A1-crystallin during Aging of the Bovine Lens; Possible Implications for Lens Optical Quality

Abstract

During aging, extensive modifications of eye lens proteins take place, which may contribute to the development of cataract. Truncation of the accessible extensions of β-crystallins has been suggested to be an important factor in this process. We therefore studied the truncations of bovine βA3- and βA1-crystallin in more detail. These proteins are identical except for the length of theirN-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 immuno-blotted with an antiserum against βA3. Two major truncation products were detected, which by sequence analysis were found to correspond to βA3 having lost 11 or 22 amino acids. βA3(-11) was only observed in the nucleus, whereas βA3(-22) was present both in cortex and nucleus. We argue, therefore, that each of these two products is produced by a separate proteolytic enzyme. βA3(-22) can originate by cleavage of βA3, βA1 and βA3(-11). Truncation of βA3 occurs more readily than that of βA1, while βA3(-11) disappears at an intermediate rate. It appears that the longer theN-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 β-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.