Microscopy of keratin filaments inside cells

Keratin (/ˈkɛrətɪn/[1][2]) is one of a family of structural fibrous proteins also known as scleroproteins. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, horns, claws, hooves, and the outer layer of skin among vertebrates. Keratin also protects epithelial cells from damage or stress. Keratin is extremely insoluble in water and organic solvents. Keratin monomers assemble into bundles to form intermediate filaments, which are tough and form strong unmineralized epidermal appendages found in reptiles, birds, amphibians, and mammals.[3][4] Excessive keratinization participate in fortification of certain tissues such as in horns of cattle and rhinos, and armadillos' osteoderm.[5] The only other biological matter known to approximate the toughness of keratinized tissue is chitin.[6][7][8] Keratin comes in two types, the primitive, softer forms found in all vertebrates and harder, derived forms found only among sauropsids (reptiles and birds).

Spider silk is classified as keratin,[9] although production of the protein may have evolved independently of the process in vertebrates.

Examples of occurrence

The horns of the impala are made of keratin covering a core of bone.

Alpha-keratins (α-keratins) are found in all vertebrates. They form the hair (including wool), the outer layer of skin, horns, nails, claws and hooves of mammals, and the slime threads of hagfish.[4] The baleen plates of filter-feeding whales are also made of keratin. Keratin filaments are abundant in keratinocytes in the hornified layer of the epidermis; these are proteins which have undergone keratinization. They are also present in epithelial cells in general. For example, mouse thymic epithelial cells react with antibodies for keratin 5, keratin 8, and keratin 14. These antibodies are used as fluorescent markers to distinguish subsets of mouse thymic epithelial cells in genetic studies of the thymus.

The harder beta-keratins (β-keratins) are found only in the sauropsids, that is all living reptiles and birds. They are found in the nails, scales, and claws of reptiles, in some reptile shells (testudines, such as tortoise, turtle, terrapin), and in the feathers, beaks, and claws of birds.[10] These keratins are formed primarily in beta sheets. However, beta sheets are also found in α-keratins.[11] Recent scholarship has shown that sauropsid β-keratins are fundamentally different from α-keratins at a genetic and structural level. The new term corneous beta protein (CBP) has been proposed to avoid confusion with α-keratins.[12]

Keratins (also described as cytokeratins) are polymers of type I and type II intermediate filaments that have been found only in chordates (vertebrates, amphioxus, urochordates). Nematodes and many other non-chordate animals seem to have only type VI intermediate filaments, fibers that structure the nucleus.

Genes

This section may require cleanup to meet Wikipedia's quality standards. The specific problem is: Not particularly helpful to dump a big list of KRT genes here. Using the source a bit more to explain what each gene and each zone of genes mean will be helpful, as we currently have no particular examples of a hair keratin. Please help improve this section if you can. (October 2022) (Learn how and when to remove this template message)
The neutral–basic keratins are encoded on chromosome 12 (12q13.13).
The acidic keratins are encoded on chromosome 17 (17q21.2).

The human genome encodes 54 functional keratin genes, located in two clusters on chromosomes 12 and 17. This suggests that they originated from a series of gene duplications on these chromosomes.[13]

The keratins include the following proteins of which KRT23, KRT24, KRT25, KRT26, KRT27, KRT28, KRT31, KRT32, KRT33A, KRT33B, KRT34, KRT35, KRT36, KRT37, KRT38, KRT39, KRT40, KRT71, KRT72, KRT73, KRT74, KRT75, KRT76, KRT77, KRT78, KRT79, KRT8, KRT80, KRT81, KRT82, KRT83, KRT84, KRT85 and KRT86 have been used to describe keratins past 20.[14]

Table of Keratin Genes and Biological Processes (GeneCards)[15]
Symbol Biological Process
KRT1 complement activation, lectin pathway
KRT1 retina homeostasis
KRT1 response to oxidative stress
KRT1 peptide cross-linking
KRT1 keratinization
KRT1 fibrinolysis
KRT1 intermediate filament organization
KRT1 regulation of angiogenesis
KRT1 negative regulation of inflammatory response
KRT1 protein heterotetramerization
KRT1 establishment of skin barrier
KRT10 morphogenesis of an epithelium
KRT10 epidermis development
KRT10 peptide cross-linking
KRT10 keratinocyte differentiation
KRT10 epithelial cell differentiation
KRT10 positive regulation of epidermis development
KRT10 protein heterotetramerization
KRT12 morphogenesis of an epithelium
KRT12 visual perception
KRT12 epidermis development
KRT12 epithelial cell differentiation
KRT12 cornea development in camera-type eye
KRT13 cytoskeleton organization
KRT13 epithelial cell differentiation
KRT13 regulation of translation in response to stress
KRT13 intermediate filament organization
KRT14 aging
KRT14 epidermis development
KRT14 keratinocyte differentiation
KRT14 epithelial cell differentiation
KRT14 hair cycle
KRT14 intermediate filament organization
KRT14 intermediate filament bundle assembly
KRT14 stem cell differentiation
KRT15 epidermis development
KRT15 epithelial cell differentiation
KRT15 intermediate filament organization
KRT16 morphogenesis of an epithelium
KRT16 inflammatory response
KRT16 cytoskeleton organization
KRT16 aging
KRT16 keratinocyte differentiation
KRT16 negative regulation of cell migration
KRT16 epithelial cell differentiation
KRT16 keratinization
KRT16 hair cycle
KRT16 innate immune response
KRT16 intermediate filament cytoskeleton organization
KRT16 intermediate filament organization
KRT16 keratinocyte migration
KRT16 establishment of skin barrier
KRT17 morphogenesis of an epithelium
KRT17 positive regulation of cell growth
KRT17 epithelial cell differentiation
KRT17 hair follicle morphogenesis
KRT17 keratinization
KRT17 intermediate filament organization
KRT17 positive regulation of translation
KRT17 positive regulation of hair follicle development
KRT18 cell cycle
KRT18 anatomical structure morphogenesis
KRT18 tumor necrosis factor-mediated signaling pathway
KRT18 obsolete Golgi to plasma membrane CFTR protein transport
KRT18 Golgi to plasma membrane protein transport
KRT18 negative regulation of apoptotic process
KRT18 intermediate filament cytoskeleton organization
KRT18 extrinsic apoptotic signaling pathway
KRT18 hepatocyte apoptotic process
KRT18 cell-cell adhesion
KRT19 Notch signaling pathway
KRT19 epithelial cell differentiation
KRT19 response to estrogen
KRT19 intermediate filament organization
KRT19 sarcomere organization
KRT19 cell differentiation involved in embryonic placenta development
KRT2 keratinocyte development
KRT2 epidermis development
KRT2 peptide cross-linking
KRT2 keratinization
KRT2 keratinocyte activation
KRT2 keratinocyte proliferation
KRT2 intermediate filament organization
KRT2 positive regulation of epidermis development
KRT2 keratinocyte migration
KRT20 apoptotic process
KRT20 cellular response to starvation
KRT20 epithelial cell differentiation
KRT20 intermediate filament organization
KRT20 regulation of protein secretion
KRT23 epithelial cell differentiation
KRT23 intermediate filament organization
KRT24 biological_process
KRT25 cytoskeleton organization
KRT25 aging
KRT25 hair follicle morphogenesis
KRT25 hair cycle
KRT25 intermediate filament organization
KRT26
KRT27 biological_process
KRT27 hair follicle morphogenesis
KRT27 intermediate filament organization
KRT28 biological_process
KRT3 epithelial cell differentiation
KRT3 keratinization
KRT3 intermediate filament cytoskeleton organization
KRT3 intermediate filament organization
KRT31 epidermis development
KRT31 epithelial cell differentiation
KRT31 intermediate filament organization
KRT32 epidermis development
KRT32 epithelial cell differentiation
KRT32 intermediate filament organization
KRT33A epithelial cell differentiation
KRT33A intermediate filament organization
KRT33B aging
KRT33B epithelial cell differentiation
KRT33B hair cycle
KRT33B intermediate filament organization
KRT34 epidermis development
KRT34 epithelial cell differentiation
KRT34 intermediate filament organization
KRT35 anatomical structure morphogenesis
KRT35 epithelial cell differentiation
KRT35 intermediate filament organization
KRT36 biological_process
KRT36 epithelial cell differentiation
KRT36 intermediate filament organization
KRT36 regulation of keratinocyte differentiation
KRT37 epithelial cell differentiation
KRT37 intermediate filament organization
KRT38 epithelial cell differentiation
KRT38 intermediate filament organization
KRT39 epithelial cell differentiation
KRT39 intermediate filament organization
KRT4 cytoskeleton organization
KRT4 epithelial cell differentiation
KRT4 keratinization
KRT4 intermediate filament organization
KRT4 negative regulation of epithelial cell proliferation
KRT40 epithelial cell differentiation
KRT40 intermediate filament organization
KRT5 epidermis development
KRT5 response to mechanical stimulus
KRT5 regulation of cell migration
KRT5 keratinization
KRT5 regulation of protein localization
KRT5 intermediate filament polymerization
KRT5 intermediate filament organization
KRT6A obsolete negative regulation of cytolysis by symbiont of host cells
KRT6A morphogenesis of an epithelium
KRT6A positive regulation of cell population proliferation
KRT6A cell differentiation
KRT6A keratinization
KRT6A wound healing
KRT6A intermediate filament organization
KRT6A defense response to Gram-positive bacterium
KRT6A cytolysis by host of symbiont cells
KRT6A antimicrobial humoral immune response mediated by antimicrobial peptide
KRT6A negative regulation of entry of bacterium into host cell
KRT6B ectoderm development
KRT6B keratinization
KRT6B intermediate filament organization
KRT6C keratinization
KRT6C intermediate filament cytoskeleton organization
KRT6C intermediate filament organization
KRT7 keratinization
KRT7 intermediate filament organization
KRT71 hair follicle morphogenesis
KRT71 keratinization
KRT71 intermediate filament organization
KRT72 biological_process
KRT72 keratinization
KRT72 intermediate filament organization
KRT73 biological_process
KRT73 keratinization
KRT73 intermediate filament organization
KRT74 keratinization
KRT74 intermediate filament cytoskeleton organization
KRT74 intermediate filament organization
KRT75 hematopoietic progenitor cell differentiation
KRT75 keratinization
KRT75 intermediate filament organization
KRT76 cytoskeleton organization
KRT76 epidermis development
KRT76 keratinization
KRT76 pigmentation
KRT76 intermediate filament organization
KRT76 sebaceous gland development
KRT77 biological_process
KRT77 keratinization
KRT77 intermediate filament organization
KRT78 keratinization
KRT78 intermediate filament organization
KRT79 keratinization
KRT79 intermediate filament organization
KRT8 keratinization
KRT8 tumor necrosis factor-mediated signaling pathway
KRT8 intermediate filament organization
KRT8 sarcomere organization
KRT8 response to hydrostatic pressure
KRT8 response to other organism
KRT8 cell differentiation involved in embryonic placenta development
KRT8 extrinsic apoptotic signaling pathway
KRT8 hepatocyte apoptotic process
KRT80 keratinization
KRT80 intermediate filament organization
KRT81 keratinization
KRT81 intermediate filament organization
KRT82 biological_process
KRT82 keratinization
KRT82 intermediate filament organization
KRT83 aging
KRT83 epidermis development
KRT83 keratinization
KRT83 hair cycle
KRT83 intermediate filament organization
KRT84 hair follicle development
KRT84 keratinization
KRT84 nail development
KRT84 intermediate filament organization
KRT84 regulation of keratinocyte differentiation
KRT85 epidermis development
KRT85 keratinization
KRT85 intermediate filament organization
KRT86 keratinization
KRT86 intermediate filament organization
KRT9 spermatogenesis
KRT9 epidermis development
KRT9 epithelial cell differentiation
KRT9 skin development
KRT9 intermediate filament organization
Protein sequence alignment of human keratin 1, 2A, 3,4, 5, 6A, 7, and 8 (KRT1 – KRT8). Only the first rod domain is shown above. Alignment was created using Clustal Omega.

Protein structure

The first sequences of keratins were determined by Israel Hanukoglu and Elaine Fuchs (1982, 1983).[16][17] These sequences revealed that there are two distinct but homologous keratin families, which were named type I and type II keratins.[17] By analysis of the primary structures of these keratins and other intermediate filament proteins, Hanukoglu and Fuchs suggested a model in which keratins and intermediate filament proteins contain a central ~310 residue domain with four segments in α-helical conformation that are separated by three short linker segments predicted to be in beta-turn conformation.[17] This model has been confirmed by the determination of the crystal structure of a helical domain of keratins.[18]

Type 1 and 2 Keratins

The human genome has 54 functional annotated Keratin genes, 28 are in the Keratin type 1 family, and 26 are in the Keratin type 2 family. [19]

Keratin (high molecular weight) in bile duct cell and oval cells of horse liver.

Fibrous keratin molecules supercoil to form a very stable, left-handed superhelical motif to multimerise, forming filaments consisting of multiple copies of the keratin monomer.[20]

The major force that keeps the coiled-coil structure is hydrophobic interactions between apolar residues along the keratins helical segments.[21]

Limited interior space is the reason why the triple helix of the (unrelated) structural protein collagen, found in skin, cartilage and bone, likewise has a high percentage of glycine. The connective tissue protein elastin also has a high percentage of both glycine and alanine. Silk fibroin, considered a β-keratin, can have these two as 75–80% of the total, with 10–15% serine, with the rest having bulky side groups. The chains are antiparallel, with an alternating C → N orientation.[22] A preponderance of amino acids with small, nonreactive side groups is characteristic of structural proteins, for which H-bonded close packing is more important than chemical specificity.

Disulfide bridges

In addition to intra- and intermolecular hydrogen bonds, the distinguishing feature of keratins is the presence of large amounts of the sulfur-containing amino acid cysteine, required for the disulfide bridges that confer additional strength and rigidity by permanent, thermally stable crosslinking[23]—in much the same way that non-protein sulfur bridges stabilize vulcanized rubber. Human hair is approximately 14% cysteine. The pungent smells of burning hair and skin are due to the volatile sulfur compounds formed. Extensive disulfide bonding contributes to the insolubility of keratins, except in a small number of solvents such as dissociating or reducing agents.

The more flexible and elastic keratins of hair have fewer interchain disulfide bridges than the keratins in mammalian fingernails, hooves and claws (homologous structures), which are harder and more like their analogs in other vertebrate classes.[24] Hair and other α-keratins consist of α-helically coiled single protein strands (with regular intra-chain H-bonding), which are then further twisted into superhelical ropes that may be further coiled. The β-keratins of reptiles and birds have β-pleated sheets twisted together, then stabilized and hardened by disulfide bridges.

Thiolated polymers (=thiomers) can form disulfide bridges with cysteine substructures of keratins getting covalently attached to these proteins.[25] Thiomers exhibit therefore high binding properties to keratins found in hair,[26] on skin[27][28] and on the surface of many cell types.[29]

Filament formation

It has been proposed that keratins can be divided into 'hard' and 'soft' forms, or 'cytokeratins' and 'other keratins'.[clarification needed][dubious ] That model is now understood to be correct. A new nuclear addition in 2006 to describe keratins takes this into account.[14]

Keratin filaments are intermediate filaments. Like all intermediate filaments, keratin proteins form filamentous polymers in a series of assembly steps beginning with dimerization; dimers assemble into tetramers and octamers and eventually, if the current hypothesis holds, into unit-length-filaments (ULF) capable of annealing end-to-end into long filaments.

Pairing

A (neutral-basic) B (acidic) Occurrence
keratin 1, keratin 2 keratin 9, keratin 10 stratum corneum, keratinocytes
keratin 3 keratin 12 cornea
keratin 4 keratin 13 stratified epithelium
keratin 5 keratin 14, keratin 15 stratified epithelium
keratin 6 keratin 16, keratin 17 squamous epithelium
keratin 7 keratin 19 ductal epithelia
keratin 8 keratin 18, keratin 20 simple epithelium

Cornification

Cornification is the process of forming an epidermal barrier in stratified squamous epithelial tissue. At the cellular level, cornification is characterised by:

Metabolism ceases, and the cells are almost completely filled by keratin. During the process of epithelial differentiation, cells become cornified as keratin protein is incorporated into longer keratin intermediate filaments. Eventually the nucleus and cytoplasmic organelles disappear, metabolism ceases and cells undergo a programmed death as they become fully keratinized. In many other cell types, such as cells of the dermis, keratin filaments and other intermediate filaments function as part of the cytoskeleton to mechanically stabilize the cell against physical stress. It does this through connections to desmosomes, cell–cell junctional plaques, and hemidesmosomes, cell-basement membrane adhesive structures.

Cells in the epidermis contain a structural matrix of keratin, which makes this outermost layer of the skin almost waterproof, and along with collagen and elastin gives skin its strength. Rubbing and pressure cause thickening of the outer, cornified layer of the epidermis and form protective calluses, which are useful for athletes and on the fingertips of musicians who play stringed instruments. Keratinized epidermal cells are constantly shed and replaced.

These hard, integumentary structures are formed by intercellular cementing of fibers formed from the dead, cornified cells generated by specialized beds deep within the skin. Hair grows continuously and feathers molt and regenerate. The constituent proteins may be phylogenetically homologous but differ somewhat in chemical structure and supermolecular organization. The evolutionary relationships are complex and only partially known. Multiple genes have been identified for the β-keratins in feathers, and this is probably characteristic of all keratins.

Silk

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The silk fibroins produced by insects and spiders are often classified as keratins, though it is unclear whether they are phylogenetically related to vertebrate keratins.

Silk found in insect pupae, and in spider webs and egg casings, also has twisted β-pleated sheets incorporated into fibers wound into larger supermolecular aggregates. The structure of the spinnerets on spiders’ tails, and the contributions of their interior glands, provide remarkable control of fast extrusion. Spider silk is typically about 1 to 2 micrometers (µm) thick, compared with about 60 µm for human hair, and more for some mammals. The biologically and commercially useful properties of silk fibers depend on the organization of multiple adjacent protein chains into hard, crystalline regions of varying size, alternating with flexible, amorphous regions where the chains are randomly coiled.[30] A somewhat analogous situation occurs with synthetic polymers such as nylon, developed as a silk substitute. Silk from the hornet cocoon contains doublets about 10 µm across, with cores and coating, and may be arranged in up to 10 layers, also in plaques of variable shape. Adult hornets also use silk as a glue, as do spiders.

Glue

Glues made from partially-hydrolysed keratin include hoof glue and horn glue.

Clinical significance

Abnormal growth of keratin can occur in a variety of conditions including keratosis, hyperkeratosis and keratoderma.

Mutations in keratin gene expression can lead to, among others:

Several diseases, such as athlete's foot and ringworm, are caused by infectious fungi that feed on keratin.[33]

Keratin is highly resistant to digestive acids if ingested. Cats regularly ingest hair as part of their grooming behavior, leading to the gradual formation of hairballs that may be expelled orally or excreted. In humans, trichophagia may lead to Rapunzel syndrome, an extremely rare but potentially fatal intestinal condition.

Diagnostic use

Keratin expression is helpful in determining epithelial origin in anaplastic cancers. Tumors that express keratin include carcinomas, thymomas, sarcomas and trophoblastic neoplasms. Furthermore, the precise expression-pattern of keratin subtypes allows prediction of the origin of the primary tumor when assessing metastases. For example, hepatocellular carcinomas typically express CK8 and CK18, and cholangiocarcinomas express CK7, CK8 and CK18, while metastases of colorectal carcinomas express CK20, but not CK7.[34]

See also

References

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  2. ^ Entry "keratin" in Merriam-Webster Online Dictionary.
  3. ^ Fraser, R.D.B. (1972). Keratins: Their composition, structure and biosynthesis. Bannerstone House: Charles C Thomas. pp. 3–6. ISBN 978-0-398-02283-9.
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