Truncation, cross-linking and interaction of crystallins and intermediate filament proteins in the aging human lens

The optical properties of the lens are dependent upon the integrity of proteins within the fiber cells. During aging, crystallins, the major intra-cellular structural proteins of the lens, aggregate and become water-insoluble. Modifications to crystallins and the lens intermediate filaments have been implicated in this phenomenon. In this study, we examined changes to, and interactions between, human lens crystallins and intermediate filament proteins in lenses from a variety of age groups (0-86 years). Among the lens-specific intermediate filament proteins, filensin was extensively cleaved in all postnatal lenses, with truncated products of various sizes being found in both the lens cortical and nuclear extracts. Phakinin was also truncated and was not detected in the lens nucleus. The third major intermediate filament protein, vimentin, remained intact in lens cortical fiber cells across the age range except for an 86 year lens, where a single similar to 49 kDa breakdown product was observed. An alpha beta-crystallin fusion protein (maltose-binding protein-alpha beta-crystallin) was found to readily exchange subunits with endogenous alpha-crystallin, and following mild heat stress, to bind to filensin, phakinin and vimentin and to several of their truncated products. Tryptic digestion of a truncated form of filensin suggested that the binding site for alpha-crystallin may be in the N-terminal region. The presence of significant amounts of small peptides derived from gamma S- and beta B1-crystallins in the water-insoluble fraction of the lens indicates that these interact tightly with cytoskeletal or membrane components. Interestingly, water-soluble complexes (similar to 40 kDa) contained predominantly gamma S- and beta B1-crystallins, suggesting that cross-linking is an alternative pathway for modified beta- and gamma-crystallins in the lens. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.