Human hair keratins.

Human hair keratins were among the first to be studied but it is only recently that sufficient information has been obtained to gain a basic biologic perspective of these proteins. Hair keratins are members of the intermediate filament family of proteins, yet are sufficiently divergent from epidermal keratins to warrant separate classification: type Ia and IIa ("hard"/hair keratins) and type Ib and IIb (epidermal and other "soft" keratins). As with hair keratins from other species, the human proteins may be distinguished from their epidermal counterparts by a relatively higher cysteine content, 7.6% versus 2.9%, respectively. This feature reflects utilization of disulfide bonding in producing a tougher, more durable structure in the tissues in which the hair keratins are distributed. Although prominent in hair, their distribution is not strictly limited to this tissue. A number of molecular characteristics have been elucidated from human hair keratin gene studies including amino acid sequence data for a type Ia hair keratin. Studies of various pedigrees has revealed a fairly wide latitude of variation in human hair keratin expression that is tolerated without associated obvious hair phenotypic change. Thus, a foundation of knowledge regarding these proteins has emerged and continues to evolve.

Transient expression of mouse hair keratins in transfected HeLa cells: interactions between "hard" and "soft" keratins.

Although it has been shown previously that an acidic (type I) "soft" keratin can interact with many basic (type II) "soft" keratins to form 10-nm intermediate filaments, it has been unclear whether "soft" keratins are compatible with the "hard" keratins typically found in hair and nail. To address this issue and to generate more structural information about hard keratins, we have isolated and sequenced a cDNA clone that encodes a mouse hair basic keratin (b4). Our sequence data revealed new information regarding the structural conservation of hard keratins as a group, being significantly different from soft keratins. Using expression vectors containing appropriate cDNA inserts, we studied the expression of this basic (b4) as well as an acidic (a1) mouse hair keratin in HeLa cells. The expression of these alien hair keratins in the transfected cells was surveyed using a panel of monoclonal and polyclonal antibodies. Our results indicated that the basic and acidic hair keratin readily incorporated into the existing endogenous soft keratin network of HeLa cells. Overproduction of hair keratin, however, occasionally led to the formation of cytoplasmic aggregates containing both hard and soft keratins. These data suggest that although small amounts of newly synthesized hair keratins can incorporate into the "scaffolding" of the preformed soft keratin filament network, possibly through dynamic subunit exchange, overproduction of hard keratins can lead to the partial collapse of the soft keratin network. These observations, along with the deduced amino acid sequence data, support and extend the concept that hard and soft keratins, although closely related, are divergent enough to justify their being divided into two separate subgroups.

Differential expression of type I hair keratins.

The hair follicle provides an excellent system in which to study growth and differentiation. Hair keratins are useful tissue-specific molecular markers for these events. By comparing a second mouse Type I hair keratin cDNA clone, MHKA-2, with our previously described MHKA-1, we have been able to contrast the nucleotide sequences and corresponding deduced amino acid sequences of the smallest (mHa4) and the largest (mHa1) major Type I hair keratins. Both nucleotide sequences and both deduced amino acid sequences share high identity but have distinct segments suitable for generation of specific molecular probes. Comparison of amino acid sequences adjacent to the central helical domains has demonstrated homologous subdomains, designated H1 and H2, in the Type I hair keratin nonhelical termini. Although there is only 56% amino acid identity in the carboxy-terminal nonhelical domains, a common sequence, T-------CGPC----R, has been identified in this domain, suggesting a possible common functional role for this portion of the molecule. In addition, it appears that mHa4 may differ in part from mHa1 by deletion of a segment between the H2 subdomain and the conserved sequence. Staining of mouse and human hair follicles with AmHa1, a monospecific polyclonal antibody for mHa1, and AE13, an antibody specific for all Type I hair keratins, suggests differential expression of individual Type I hair keratins in both species. This supports our hypothesis that distinct functional requirements are satisfied by the multiplicity of hair keratins.

Cloning and characterization of a mouse type I hair keratin cDNA.

A cDNA library was prepared from poly(A)-containing C57BL/6J mouse hair-root-enriched mRNA. The library was screened using a radiolabeled nucleic acid probe prepared from a sheep wool, Type I keratin cDNA clone (SWK2). Clone MHKA-1 was shown to contain a mouse hair, Type I keratin cDNA insert by positive hybridization-selection translation assay, and by the corresponding deduced amino acid sequence. The size of the cDNA insert is 1585 bp (excluding homopolymer tails) and on the basis of Northern blot analysis it corresponds to a mRNA of approximately 1.6 Kb. The cloned cDNA sequence includes the entire coding region for a protein of 416 amino acids (including the initiation methionine). Comparison of the deduced amino acid sequence with that of sheep wool, Type I keratin (8c1) reveals an overall corresponding sequence identity of 87%. In contrast, the MHKA-1 protein is significantly less similar to non-hair Type I keratins. An additional 3 amino acids (adjacent proline residues) not present in the wool protein have been identified near the middle of the carboxy-terminal end of the mouse protein. MHKA-1 is the first of a series of mouse hair follicle cDNA clones to be identified and characterized that will enable us to study follicular regulatory mechanisms and the interrelationships among the proteins in the mammalian hair follicle.