Homology and Diversity Between Intermediate Filament Proteins of Neuronal and Nonneuronal Origin in Goldfish Optic Nerve

The predominant intermediate filament proteins of the goldfish optic nerve have molecular weights of 58K. They can be separated into a series of four major isoelectric variants of neuronal (ON1 and ON2) and nonneuronal (ON3 and ON4) origin. The extent of homology between the goldfish 58K intermediate filament proteins themselves and to rat optic nerve vimentin and glial fibrillary acidic protein (GFAP) was investigated. Unlabeled and [32P]orthophosphate-labeled proteins were subjected to partial hydrolysis by V8 protease, chymotrypsin, and CNBr. The results show that the goldfish intermediate filament proteins share with vimentin and GFAP a 40K chymotrypsin-resistant core fragment. Phosphorylated moieties appear to be located outside the core region since they are preferentially cleaved off by chymotrypsin and not found associated with the 40K core. In addition, the goldfish ON proteins contain the antigenic site within the core that is common to most intermediate filaments. V8 or CNBr digestion indicates that many fragments that are common to ON1 and ON2 are clearly distinct from fragments that are common to ON3 and ON4. In addition, structural variability is observed between the goldfish intermediate filament proteins and vimentin and GFAP. The results are discussed in terms of intermediate filament structure and their possible role in nerve growth.