Characterization of a phosphorylated form of the intermediate filament-aggregating protein filaggrin

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Abstract
[Rat] filaggrin and a phosphorylated form of filaggrin, shown by pulse-chase studies to be a precursor form of the protein (Dale et Ling, 1979), were compared for functional, biochemical, and physical properties. Filaggrin reacts with keratin filaments to form visible macrofibrils, unlike the precursor which does not. Biochemical and peptide-mapping studies suggest that the 2 proteins have similar, perhaps identical, amino acid sequences. The major differences between the 2 proteins are in MW (precursor, 44,200 g/mol; filaggrin, 38,400 g/mol), the existence of oligomeric forms of the precursor, and the presence of phosphate in the precursor (15-20 mol/mol of protein). Phosphoserine was identified in the precursor, but neither phosphothreoninine nor phosphotyrosine was observed. The results of proteolytic digests of [32P]phosphate-radiolabled precursor show that the phosphate is unevenly distributed throughout the molecule and may be localized in .apprx. 30% of the precursor. A discrete localization of the phosphate in the precursor may block a specific keratin filament combining site and so prevent premature aggregation of these filaments during epidermal differentiation. Probably, a specific phosphatase is involved in the dephosphorylation, because several phosphatases of general specificity, including rat epidermal lysosomal acid phosphatase, did not catalyze this conversion.