Sequence and expression of murine type I hair keratins mHa2 and mHa3

A cDNA library was constructed with poly(A)+ RNA from mouse tail epidermis which contained all hair follicles of tail skin. The library was subjected to sequential screening procedures aimed at selecting cDNA clones coding for acidic, type I hair keratins. Two clones, pktI-2 and pktI-3, encoded keratins that could be identified as murine type I hair keratins mHa2 and mHa3, respectively, by positive hybridization selection analysis. Sequence comparisons with the known murine type I hair keratins mHa1 (Bertolino et al., J. Invest. Dermatol. 91, 541-546, 1988) and mHa4 (Bertolino et al., J. Invest. Dermatol. 94, 297-303, 1990) revealed a structural heterogeneity within the type I hair keratin subfamily. Three keratins, mHa1, mHa3, and mHa4, are highly related, differing mainly in the penultimate part of their amino and carboxy termini. In contrast, mHa2 is structurally distinct from the three other keratins in both the alpha-helix and, in particular, the non-alpha-helical domains. These findings are confirmed by evolutionary investigations and flexibility calculations which indicate a more flexible nature of the mHa2 amino terminus when compared to the corresponding region of the three other keratins. In situ hybridization experiments with specific 3' fragments of mHa2 and mHa3 show that mHa3 is expressed in cortex cells, whereas mHa2 transcripts are strictly limited to the cuticle of the hair shaft. mHa3 mRNA expression can also be demonstrated in the central unit of the murine lingual filiform papillae, whereas the cuticular keratin mHa2 is not expressed in this body site. These data indicate that the structural heterogeneity within the type I hair keratin subfamily is functionally relevant in the morphogenesis of hard alpha-keratin-expressing tissues.