Epidermal filaggrin is synthesized on a large messenger ribonucleic acid as a high-molecular-weight precursor

Filaggrin is a keratin filament associated protein found in the differentiated cells of the epidermis. The mouse protein has a molecular weight of 26.5 X 10(3), while the molecular weight of rat filaggrin is 49 X 10(3), when analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. In order to clarify our understanding of the precursor form of filaggrin, RNA was isolated from mouse and rat epidermis and the poly(A+) fraction was translated in a cell-free protein synthesis system. Immunoprecipitation of translated proteins with specific antibody against rat filaggrin revealed a diffuse radiolabeled band with a molecular weight of approximately 250 X 10(3) on SDS-polyacrylamide gels. The size of filaggrin mRNA was estimated by sedimenting poly(A+) RNA through isokinetic sucrose gradients. Mouse filaggrin mRNA activity was located in the 30S region of the gradient, while rat filaggrin mRNA was located in the 34S region. The molecular weight of rat filaggrin mRNA was estimated to be 5 X 10(6) by electrophoresis in denaturing agarose gels containing methylmercury(II) hydroxide. A messenger RNA of this size could code for a polypeptide as large as approximately 600 kilodaltons. We conclude that filaggrin is synthesized as a large molecular weight precursor that must undergo substantial processing prior to formation of the mature form of the protein.