Special Biochemical Features of the Hair Follicle

Summary

Certain keratin proteins are discussed which are not present in the epidermis and appear to be unique to hair and wool They are the high-gfycine/tyrosine (HGT) proteins and ultra-high-sulphur (UHS) proteins found in the cells of the hair cortex and the trichohyalin protein of the inner root sheath cells surrounding the fibre in its follicle. The high-sulphur (HS) proteins also belong to this group but they are not discussed in this paper. A function common to all these proteins is that they act as matrices associated with intermediate filaments (IFs). The HGT and UHS proteins are of central interest because they vary in their occurrence between species and in the case of sheep, their abundance can alter with nutritional status. The availability of HGT gene probes has allowed studies to be made at the genomic level. For example, HGT sequences have been detected in human DNA and similarly, in the DNA of the sheep felting-lustre mutant. Since HGT proteins have not been found in human hair (or nail) and are found at very low levels in the felting-lustre mutant wool compared to normal wool, there must be transcriptional or translational modulation of expression. A long-recognised but virtually uncharacterised protein fraction from wool and hair is the UHS group with a half-cystine content of at least 30 moles %. The level of expression of these proteins in wool is known to increase with an increase in available cysteine. A UHS gene (human) has recently been isolated and sequenced and its use as a probe to gain an understanding of the molecular basis of the increased UHS protein expression is the prime aim of future experiments.

Trichohyalin protein which exists as granules in early cells of the inner root sheath and of the hair medulla has been partly characterised as a large protein (200 Kd) and is believed to play a matrix-type role. It has an unusual amino acid composition and undergoes major chemical and structural changes during cell differentiation which includes the appearance of citrulline residues and y-peptide cross-linking. Experiments at present are directed towards the complete sequencing of the protein via a cDNA clone prepared from size-selected cDNA using the)gtll expression system and a specific anti-trichohyalin polyclonal antibody.

Finally, attention is given to the presence of arginyl-tRNA transferase activity in hair follicle extracts. It is known from recent studies with other systems that a major role for this particular enzyme is to alter proteins that have acidic amino acid N-termini so that they can be targeted for ubiquitination prior to degradation. This enzyme activity could be important for the processing of proteins in the follicle.