The intermediate filament cytoskeleton of the lens: An ever changing network through development and differentiation

In the eye lens, intermediate filament proteins form two morphologically distinct polymers, 10-nm intermediate-sized filaments and beaded filaments. Coincidently, the beaded filament polymer and the proteins filensin and CP49 are specific to lens fibre cells and are therefore excellent markers for fibre cell differentiation. In the adult lens, filensin and CP49 are maintained throughout all stages of lens fibre cell differentiation whilst vimentin is apparently lost at a specific stage from the deeper cortical fibres. The expression of CP49 and filensin is coincident with the presence of beaded filaments suggesting these proteins are filament components. In association with α-crystallin, CP49 and filensin form beaded filaments in vitro. During fibre cell differentiation, filensin and CP49 are post-transla-tionally modified. In the case of filensin, proteolysis results in two functionally distinct fragment sets, one derived from the α-helical rod domain and the other from the C-terminal tail domain of filensin. It is proposed that both filensin and CP49 are critically involved in organising the cyto-plasmic and plasma membrane domains of the fibre cell and therefore essential to the optical properties of the lens.