Every living organism has been equipped with some form of barrier, separating the body from the environment. Its complexity depends on the degree of protection required from physical, mechanical and chemical insults. In humans this function is performed by the skin, which covers the outside of the body, protecting inner cells and organs. The outermost layer of the skin, the epidermis, is a continuously regenerating tissue as its cells are lost to normal "wear and tear". This process, called terminal or epidermal differentiation, is highly organised and involves the tightly controlled expression of many specific proteins. The most predominant cells of the epidermis, the keratinocytes, arise from the stem cells that line the basement membrane and pass outwards through a series of differentiation stages, towards the surface of the skin where they become a part of the cornified cell envelope, from which they are eventually sloughed into
the environment. Barrier function and the renewal of the epidermis are highly dependent on the individual genes, their protein products and interaction.
One of the most predominant protein markers of epidermal differentiation is filaggrin. It is a product of proteolytic processing of the large precursor protein, profilaggrin, and has been shown to interact with keratin filaments to form highly ordered structures known as macrofibrils. In later stages of the epidermal differentiation individual filaggrin repeats undergo further post-translational modifications and are proteolysed into free amino acids that may be involved both in protection against ultraviolet light and in the water-holding capacity of the skin. This thesis primarily examines a novel repetitive protein identified in mouse epidermis that has high homology to the human and mouse profilaggrin genes. The chromosomal localisation, repetitive structure, amino acid composition and expression studies
suggest that this protein is involved in the late stages of the epidermal differentiation and cornified cell envelope formation. In a mouse individual repeats are of 74-80 residues with each repeat having a pi of between 8.0 and 11.6 and are rich in proline, glycine, serine, glutamine and arginine residues. The amino terminus, as in profilaggrin, contains two calcium-binding motifs of the EF-hand type with a high homology to the SI00 proteins. Northern analysis revealed that the gene is expressed predominantly in the epidermis, with RT PCR detecting very low abundance in the tongue. The gene has been mapped to the epidermal differentiation complex (EDC) region on chromosome 3, 20 kb upstream of the profilaggrin gene. Blast searches of the available human and mouse genomes suggest that this is the last filaggrin-like protein containing EF-hand calcium binding domain. A polyclonal antibody produced, detects high molecular weight products as well as putative processed products in
western blots of epidermal extracts. Double-label immunofluorescence localised the novel protein to the more differentiated cells of the granular layer and stratum corneum.
Functionally the results show that this protein is similar to filaggrin; this protein assembles keratin filaments into macrofibrils in vitro and binds calcium. The processed repeats rich in arginine, glutamine, glutamic acid and histidine are potential sources of these amino acids for the integral processes involved in proteolytic breakdown of macrofibrils, hydration of the stratum comeum and UV protection. Additional mapping studies have identified the human homolog that has been located on chromosome lq21, the syntenic region of mouse chromosome 3. Based on homology data, the repetitive structure and amino acid composition, it has been proposed to name this protein filaggrin related protein (Flg-2).