Keratin (OED 2nd edition, 1989 as Entry "keratin" in Merriam-Webster Online Dictionary .) is one of a family of structural fibrous proteins also known as scleroproteins. It is the key structural material making up scales, hair, nails, feathers, horns, claws, hooves, and the outer layer of skin in tetrapod 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. Excessive keratinization participate in fortification of certain tissues such as in horns of cattle and rhinos, and armadillos' osteoderm. The only other biological matter known to approximate the toughness of keratinized tissue is chitin. Keratin comes in two types: the primitive, softer forms found in all vertebrates and the harder, derived forms found only among sauropsids (reptiles and birds).
Examples of occurrence
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. 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, i.e., all living reptiles and birds. They are found in the nails, scales, and claws of reptiles, in some reptile shells (Testudines), and in the feathers, beaks, and claws of birds. These keratins are formed primarily in beta sheets. However, beta sheets are also found in α-keratins. 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.
Keratins (also described as cytokeratins) are polymers of type I and type II intermediate filaments that have been found only in chordates (vertebrates, amphioxi, urochordates). Nematodes and many other non-chordate animals seem to have only type V intermediate filaments, fibers that structure the nucleus.
Genes
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.
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, KRT80, KRT81, KRT82, KRT83, KRT84, KRT85 and KRT86 have been used to describe keratins past 20. +Table of keratin genes and biological processes (GeneCards)SymbolBiological processKRT1complement activation, lectin pathwayKRT1retina homeostasisKRT1response to oxidative stressKRT1peptide cross-linkingKRT1keratinizationKRT1fibrinolysisKRT1intermediate filament organizationKRT1regulation of angiogenesisKRT1negative regulation of inflammatory responseKRT1protein heterotetramerizationKRT1establishment of skin barrierKRT10morphogenesis of an epitheliumKRT10epidermis developmentKRT10peptide cross-linkingKRT10keratinocyte differentiationKRT10epithelial cell differentiationKRT10positive regulation of epidermis developmentKRT10protein heterotetramerizationKRT12morphogenesis of an epitheliumKRT12visual perceptionKRT12epidermis developmentKRT12epithelial cell differentiationKRT12cornea development in camera-type eyeKRT13cytoskeleton organizationKRT13epithelial cell differentiationKRT13regulation of translation in response to stressKRT13intermediate filament organizationKRT14agingKRT14epidermis developmentKRT14keratinocyte differentiationKRT14epithelial cell differentiationKRT14hair cycleKRT14intermediate filament organizationKRT14intermediate filament bundle assemblyKRT14stem cell differentiationKRT15epidermis developmentKRT15epithelial cell differentiationKRT15intermediate filament organizationKRT16morphogenesis of an epitheliumKRT16inflammatory responseKRT16cytoskeleton organizationKRT16agingKRT16keratinocyte differentiationKRT16negative regulation of cell migrationKRT16epithelial cell differentiationKRT16keratinizationKRT16hair cycleKRT16innate immune responseKRT16intermediate filament cytoskeleton organizationKRT16intermediate filament organizationKRT16keratinocyte migrationKRT16establishment of skin barrierKRT17morphogenesis of an epitheliumKRT17positive regulation of cell growthKRT17epithelial cell differentiationKRT17hair follicle morphogenesisKRT17keratinizationKRT17intermediate filament organizationKRT17positive regulation of translationKRT17positive regulation of hair follicle developmentKRT18cell cycleKRT18anatomical structure morphogenesisKRT18tumor necrosis factor-mediated signaling pathwayKRT18obsolete Golgi to plasma membrane CFTR protein transportKRT18Golgi to plasma membrane protein transportKRT18negative regulation of apoptotic processKRT18intermediate filament cytoskeleton organizationKRT18extrinsic apoptotic signaling pathwayKRT18hepatocyte apoptotic processKRT18cell-cell adhesionKRT19Notch signaling pathwayKRT19epithelial cell differentiationKRT19response to estrogenKRT19intermediate filament organizationKRT19sarcomere organizationKRT19cell differentiation involved in embryonic placenta developmentKRT2keratinocyte developmentKRT2epidermis developmentKRT2peptide cross-linkingKRT2keratinizationKRT2keratinocyte activationKRT2keratinocyte proliferationKRT2intermediate filament organizationKRT2positive regulation of epidermis developmentKRT2keratinocyte migrationKRT20apoptotic processKRT20cellular response to starvationKRT20epithelial cell differentiationKRT20intermediate filament organizationKRT20regulation of protein secretionKRT23epithelial cell differentiationKRT23intermediate filament organizationKRT24biological_processKRT25cytoskeleton organizationKRT25agingKRT25hair follicle morphogenesisKRT25hair cycleKRT25intermediate filament organizationKRT26KRT27biological_processKRT27hair follicle morphogenesisKRT27intermediate filament organizationKRT28biological_processKRT3epithelial cell differentiationKRT3keratinizationKRT3intermediate filament cytoskeleton organizationKRT3intermediate filament organizationKRT31epidermis developmentKRT31epithelial cell differentiationKRT31intermediate filament organizationKRT32epidermis developmentKRT32epithelial cell differentiationKRT32intermediate filament organizationKRT33Aepithelial cell differentiationKRT33Aintermediate filament organizationKRT33BagingKRT33Bepithelial cell differentiationKRT33Bhair cycleKRT33Bintermediate filament organizationKRT34epidermis developmentKRT34epithelial cell differentiationKRT34intermediate filament organizationKRT35anatomical structure morphogenesisKRT35epithelial cell differentiationKRT35intermediate filament organizationKRT36biological_processKRT36epithelial cell differentiationKRT36intermediate filament organizationKRT36regulation of keratinocyte differentiationKRT37epithelial cell differentiationKRT37intermediate filament organizationKRT38epithelial cell differentiationKRT38intermediate filament organizationKRT39epithelial cell differentiationKRT39intermediate filament organizationKRT4cytoskeleton organizationKRT4epithelial cell differentiationKRT4keratinizationKRT4intermediate filament organizationKRT4negative regulation of epithelial cell proliferationKRT40epithelial cell differentiationKRT40intermediate filament organizationKRT5epidermis developmentKRT5response to mechanical stimulusKRT5regulation of cell migrationKRT5keratinizationKRT5regulation of protein localizationKRT5intermediate filament polymerizationKRT5intermediate filament organizationKRT6Aobsolete negative regulation of cytolysis by symbiont of host cellsKRT6Amorphogenesis of an epitheliumKRT6Apositive regulation of cell population proliferationKRT6Acell differentiationKRT6AkeratinizationKRT6Awound healingKRT6Aintermediate filament organizationKRT6Adefense response to Gram-positive bacteriumKRT6Acytolysis by host of symbiont cellsKRT6Aantimicrobial humoral immune response mediated by antimicrobial peptideKRT6Anegative regulation of entry of bacterium into host cellKRT6Bectoderm developmentKRT6BkeratinizationKRT6Bintermediate filament organizationKRT6CkeratinizationKRT6Cintermediate filament cytoskeleton organizationKRT6Cintermediate filament organizationKRT7keratinizationKRT7intermediate filament organizationKRT71hair follicle morphogenesisKRT71keratinizationKRT71intermediate filament organizationKRT72biological_processKRT72keratinizationKRT72intermediate filament organizationKRT73biological_processKRT73keratinizationKRT73intermediate filament organizationKRT74keratinizationKRT74intermediate filament cytoskeleton organizationKRT74intermediate filament organizationKRT75hematopoietic progenitor cell differentiationKRT75keratinizationKRT75intermediate filament organizationKRT76cytoskeleton organizationKRT76epidermis developmentKRT76keratinizationKRT76pigmentationKRT76intermediate filament organizationKRT76sebaceous gland developmentKRT77biological_processKRT77keratinizationKRT77intermediate filament organizationKRT78keratinizationKRT78intermediate filament organizationKRT79keratinizationKRT79intermediate filament organizationKRT8keratinizationKRT8tumor necrosis factor-mediated signaling pathwayKRT8intermediate filament organizationKRT8sarcomere organizationKRT8response to hydrostatic pressureKRT8response to other organismKRT8cell differentiation involved in embryonic placenta developmentKRT8extrinsic apoptotic signaling pathwayKRT8hepatocyte apoptotic processKRT80keratinizationKRT80intermediate filament organizationKRT81keratinizationKRT81intermediate filament organizationKRT82biological_processKRT82keratinizationKRT82intermediate filament organizationKRT83agingKRT83epidermis developmentKRT83keratinizationKRT83hair cycleKRT83intermediate filament organizationKRT84hair follicle developmentKRT84keratinizationKRT84nail developmentKRT84intermediate filament organizationKRT84regulation of keratinocyte differentiationKRT85epidermis developmentKRT85keratinizationKRT85intermediate filament organizationKRT86keratinizationKRT86intermediate filament organizationKRT9spermatogenesisKRT9epidermis developmentKRT9epithelial cell differentiationKRT9skin developmentKRT9intermediate filament organization
Protein structure The first sequences of keratins were determined by Israel Hanukoglu and Elaine Fuchs (1982, 1983). These sequences revealed that there are two distinct but homologous keratin families, which were named type I and type II keratins. 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. This model has been confirmed by the determination of the crystal structure of a helical domain of keratins.
Type I and II keratins The human genome has 54 functional annotated keratin genes, of which 28 are type I keratins and 26 are type II keratins.
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.