High-molecular-weight precursor of epidermal filaggrin and hypothesis for its tandem repeating structure

Abstract

Filaggrin is a histidine-rich protein that is intimately involved in mammalian epidermal keratinization. Using a combination of immunologic and in vivo pulse-chase studies with radiolabeled histidine and phosphate, the phosphorylated precursor of both rat and mouse filaggrin has an apparent MW much higher than previously realized (6 .times. 105 and 3.9 .times. 105, respectively). These high-MW filaggrin precursors can be rapidly labeled with histidine and extracted from the epidermis under denaturing conditions. More than half of the label incorporated in the precursor at 2 h is found in filaggrin at 24 h after injection, even though filaggrin is < 10% of the size of the precursor. Limited proteolytic digestion of the precursor in vitro results in the formation of an oligomeric series of peptides based on a phosphorylated fragment slightly larger than filaggrin itself. More extensive digestion of this fragment shows that it is composed of filaggrin with few or no additional unrelated peptides, suggesting that the major part of the high MW filaggrin precursor must be composed of repeated domains of filaggrin. Because the primary translation product of filaggrin mRNA is large, these domains are repeated in tandem. From MW computations and peptide map analyses, the filaggrins are themselves composed of multiple repeating units of an unidentified peptide of approximately MW 8600. This value is derived from the MW of filaggrin from several mammalian species that differ by integral multiples of 8600. A model for the structure of the high MW precursor of filaggrin is presented. It has 2 types of repeating units: those that make up the filaggrin molecule itself and the tandem repeated copies of filaggrin.