Profilaggrin is a major epidermal calcium-binding protein

Abstract

Profilaggrin is a major highly phosphorylated protein component of the keratohyalin granules of mammalian epidermis. It contains 10 to 12 tandemly repeated filaggrin units and is processed into the intermediate filament-associated protein filaggrin by specific dephosphorylation and proteolysis during terminal differentiation of the epidermal cells. Later, filaggrin itself is degraded to free amino acids that participate in maintenance of epidermal flexibility. The present paper describes the structural organization of the 5' region of the human profilaggrin gene as well as the amino terminus of the profilaggrin protein. The primary profilaggrin transcript consists of three exons and two introns. The first exon (exon I) is only 54 bp and is untranslated. The coding sequences are distributed between exon II (159 bp) and exon III, which contains the information for 10 to 12 filaggrin repeats (972 bp each) and the 3' noncoding sequences. A very large intron separates exons I and II. The combination of a very short exon I with an unusually long intron 1 makes the structure of the profilaggrin gene unique among the epidermally expressed genes investigated so far. Comparison of the expression patterns revealed by primer extension and RNase protection analysis of foreskin epidermal and cultured keratinocyte RNAs suggests that alternately spliced messages, which are different from profilaggrin mRNA, are transcribed from the profilaggrin gene system at earlier stages of epidermal differentiation. The amino terminus of profilaggrin exhibits a significant homology to the small calcium-binding S100-like proteins. It contains two alpha-helical regions, termed EF-hands, that bind calcium in vitro. This is the first example of functional calcium-binding domains fused to a structural protein. We suggest that in addition to its role in filament aggregation and the maintenance of epidermal flexibility, profilaggrin may play an important role in the differentiation of the epidermis by autoregulating its own processing in a calcium-dependent manner or by participating in the transduction of calcium signal in epidermal cells.