DIFFERENTIAL EXPRESSION OF GENES ENCODING A CYSTEINE-RICH KERATIN FAMILY IN THE HAIR CUTICLE

In the hair follicle the cuticle develops as a thin layer of cells bet ween the hair shaft cortex and the inner root sheath. Once the cuticle cells begin to differentiate they accumulate cysteine-rich granules i n their cytoplasm but the identity of their constituent proteins has r emained largely an enigma. In this report we show differential express ion of a family of genes encoding cysteine-rich, glycine-rich keratins in the cuticle. Two clones of the sheep KAP5 gene family were isolate d: the KAP5.4 cDNA encodes a protein of 190 amino acids (M(r) = 16,936 ) containing 32 mol% cysteine, 26 mol% glycine and the partial KAP5.5 cDNA encodes a protein of at least 197 amino acids (M(r) greater than or equal to 17,474) containing 29 mol% cysteine, 28 mol% glycine. The predicted amino acid sequences of the KAP5 family show extensive seque nce conservation and all the proteins are composed almost entirely of cysteine-rich and glycine-rich repeats. Each KAP5 gene produces an sim ilar to 1.5-kb mRNA species but the KAP5.4 and KAP5.5 mRNA levels appe ar to be severalfold greater than the KAP5.1 mRNA. Comparative tissue in situ hybridizations reveal a positive correlation between the onset of expression and follicle depth. For a given KAP5 gene two widely di fferent cuticle expression patterns were noted amongst the follicle po pulations, and on the basis of follicle bulb cell kinetics they are co nsistent with expression in either sheep primary or secondary follicle types.