Unique amino acid signatures that are evolutionarily conserved distinguish simple-type, epidermal and hair keratins

Abstract

SummaryKeratins (Ks) consist of central a-helical rod domains that are flanked by non-a-helical head and tail domains. The cellular abundance of keratins, coupled with their selective cell expression patterns, suggests that they diversified to fulfill tissue-specific functions although the primary structure differences between them have not been comprehensively compared. We analyzed keratin sequences from many species: K1, K2, K5, K9, K10, K14 were studied as representatives of epidermal keratins, and compared with K7, K8, K18, K19, K20 and K31, K35, K81, K85, K86, which represent simple-type (single-layered or glandular) epithelial and hair keratins, respectively. We show that keratin domains have striking differences in their amino acids. There are many cysteines in hair keratins but only a small number in epidermal keratins and rare or none in simple-type keratins. The heads and/or tails of epidermal keratins are glycine and phenylalanine rich but alanine poor, whereas parallel domains of hair keratins are abundant in prolines, and those of simple-type epithelial keratins are enriched in acidic and/or basic residues. The observed differences between simple-type, epidermal and hair keratins are highly conserved throughout evolution. Cysteines and histidines, which are infrequent keratin amino acids, are involved in de novo mutations that are markedly overrepresented in keratins. Hence, keratins have evolutionarily conserved and domain-selectively enriched amino acids including glycine and phenylalanine (epidermal), cysteine and proline (hair), and basic and acidic (simple-type epithelial), which reflect unique functions related to structural flexibility, rigidity and solubility, respectively. Our findings also support the importance of human keratin 'mutation hotspot' residues and their wild-type counterparts.

Deciphering gene expression program of MAP3K1 in mouse eyelid morphogenesis

“…6D). MAP3K1 ablation decreased the expression of keratin 17, a hair follicle and epidermal appendage marker; it also down-regulated the expression of keratins 25 and 27, which are type I intermediate filaments, and keratins 71 and 73, which are type II intermediate filaments, and trichohyalin ( Tchh ) and hair keratin-associated proteins (KRTAP), which are intermediate filament-associated proteins specifically implicated in inner root sheath of hair follicles(Gritli-Linde et al, 2007;Strnad et al, 2011). These observations led us to ask whether MAP3K1 ablation affected the development and structure of hair follicle and epidermis.…”

Keratin mediates the recognition of apoptotic and necrotic cells through dendritic cell receptor DEC205/CD205

“…The DEC205 binding data show that it only recognizes the C-terminal tail domains of keratin 1 and keratin 10, and notably, the binding tests can be done by Western blot assays, suggesting that the target sequence of DEC205 on keratins adopts a linear conformation. Indeed, the tail domains of keratin 1 and keratin 10 contain a high percentage of glycine and serine in a X(Y) n format, where X represents an aromatic or a long-chain aliphatic residue and Y is either glycine or serine (32,37). The X(Y) n pattern is quasirepetitive and may form "glycine loops" where the Gly-rich peptides have flexible loop-like structures without rigid conformations (37), thus explaining the results of Western blot assays.…”

“…Recently, it has been shown that a bacterial protein ClfB could recognize the Gly-rich tail domain of keratin 8 and keratin 10 (38, 39), and the crystal structure of ClfB complexed with the keratin 10 tail shows that the Gly-rich sequence also adopts a linear conformation (29). In fact, the Gly-rich sequences are found in the C-terminal tail domains of many keratin members including both type I and type II keratins (32,37), suggesting that DEC205 might be a generic receptor for keratins targeting to these sequences.…”

“…The results showed that DEC205 only recognized the C-terminal tail domain of keratin 1 specifically at acidic pH (Fig. 4 C and D), which contains a high percentage of glycine and serine in sequence, a feature shared by many keratin family members (32). Similarly, two truncation mutants of keratin 10 were also expressed for binding assays (Fig.…”

Types I and II Keratin Intermediate Filaments

“…The designation “intermediate filaments” conveys that their diameter, ~10 nm, is intermediate between those of actin microfilaments (~6–8 nm) and microtubules (~25 nm) (Ishikawa et al 1968). Proteins belonging to this large superfamily, comprisingapproximately 70 conserved genes, show substantial amino acid sequence diversity, with some sharing as little as 20% identity (Szeverenyi et al 2008; Strnad et al 2011). Regardless of this heterogeneity, each memberof the IF familyshares a tripartite protein substructure, with a family-defining, centrally located α-helical rod domain, and readily self-assembles to form 10-nm filaments on its own or, more commonly, in partnership with other IF proteins (Fig.…”