Subcellular redistribution of components of the ubiquitin-proteasome pathway during lens differentiation and maturation

PURPOSE. To determine the subcellular distribution of components of the ubiquitin - proteasome pathway (UPP) in lens epithelium and differentiating fibers and to evaluate potential roles of the UPP in eliminating nuclei and other organelles during maturation of lens fibers. METHODS. Adult bovine lens cryosections were stained for immunofluorescence and analyzed by confocal microscopy. The specificities of the antibodies used in this study were determined by Western blot. RESULTS Cryosections of bovine lenses show that E1 and Ubc1 were present in both the cytoplasm and the nucleus in epithelial cells, whereas Ubc3 and ubiquitin conjugates were mostly confined to the nucleus, and Ubc4/5 was preferentially localized in clusters in the vicinity of the nuclear membrane. The 19S and 20S proteasome complexes were preferentially localized in the cytoplasm. When the epithelial cells differentiated into fiber cells at the transition zone, all components of the UPP were primarily present in the nucleus, with the exception of Ubc4/5, which was associated with the nuclear membrane. CONCLUSIONS The results show that during lens fiber differentiation and maturation, components of the UPP are redistributed at subcellular levels. Subcellular localization of an enzyme indicates where the reaction takes place. The primary nuclear localization of the UPP components in the differentiating fibers supports the hypothesis that the UPP may play a role in elimination of nuclei and other organelles during differentiation and maturation of lens fibers.