Calcium decreases transparency of homogenate from lens cortex and has no effect on nucleus

Susceptibility of the bovine lens 115kDa beaded filament protein to degradation by calcium and calpain

The 115kDa cytoskeletal beaded filament protein of bovine lens fibres is degraded during opacification induced by increased internal calcium. The monoclonal antibody R2D2 to this protein has been used in whole lenses and native homogenates to follow the process of degradation and the production of break-down products. In the opaque outer cortex of whole bovine lenses with an internal Ca2+ of 2.0mM, both the 115kDa parent protein and the main degradation product (57kDa) were reduced in amount by almost 60%. No additional products were detected by the antibody. When native homogenates were incubated overnight with 10mM Ca2+ the protein could no longer be detected in SDS gels, but faintly reactive bands were detected by the antibody. Since these changes were dependent on the presence of increased calcium they were compared with changes induced by incubating freshly isolated cytoskeletal proteins with Ca2+ and the Ca(2+)-activated protease, calpain. The 115kDa protein was shown to be susceptible to degradation by calpain, with the formation of a number of breakdown products. These results indicate that degradation of the beaded filament protein can be brought about by the activation of calpain. Since the enzyme is present in lens cortex it is likely to have a role in the protein degradation observed during Ca(2+)-induced opacification, and may also be involved in the changes occurring as the lens fibres mature.