The variation amongst the keratins is increased
by cell-specific phosphorylation [244]. This heterogeneity of the proteins corresponds to the morphological and evolutionary plasticity of horned
epidermal structures. Usually, the distinction is
drawn between the soft keratins in vertebrate skin
and hard keratin structures like hair, feathers, spines, claws, nails and hooves, horns, beaks and
whalebone. The formation of specialized keratin
structures in various parts of the epidermis first
appeared after the transition of the vertebrates to
a terrestrial existence, i.e. with the reptiles, birds
and mammals; only a few exceptional amphibians
have keratinized warts, claws or horned jaws [20].
The keratinocytes in the stratum germinativum
of human epidermis have up to 30 % of their total
protein in the form of prekeratins, which can be
extracted into acidic buffers. In the space of a few
days, these living cells are converted into the
dead, horny cells of the outer skin layer (stratum
corneum), in which 70 % of the dry weight consists of keratins; here, they are cross-linked into a
fibrous state by disulphide bridges and N'-(yglutamyl)-lysine isodipeptide bonds, and can be
solubilized only by a combination of reducing and
denaturing reagents. The formation of the crosslinks is catalysed by keratinocyte-specific transglutaminases which occur as different isoenzymes
[221]. In the course of its development, each
keratinocyte expresses a series of different cytokeratins. Cytokeratins 1 and 10 predominate at
the start of terminal differentiation. These form a
highly complex aggregate of cross-linked fibrils
beneath the plasma membrane, the cross-linked
cell envelope (CE). Construction of the CE involves the histidine-rich, 26-kDa protein maggrin,
the sequence of which is very similar between the
rat and mouse but is quite different in man. Special granules in immature keratinocytes contain a
high molecular weight (>350 kDa) pro-filaggrin,
made up of numerous filaggrin copies that are
bound together by linkers and phosphorylated on
many serine residues. During the first phase of
terminal differentiation, the pro-filaggrin is
dephosphorylated and then processed further by
specific proteases [237]. In addition to cytokeratins, the CE of terminally differentiated, flattened keratinocytes contains further, cross-linked
proteins, e.g. the small pancornulins, keratolinin,
loricrin [109, 222], and involucrin, the structure
and evolution of which has received particular
attention. A large part of the involucrin chain
consists of a segment of tandem repeats, and this
varies in position and repeat length between different mammals. In the dog and pig and in the
10.10.1 Cytokeratins
357
pro simians Lemur catta and Galago crassicaudatus, there is a "premodern" segment, located at a
site P near the N-terminus, which has 6-13
repeats each of 20-31 amino acids. The higher
primates possess a "modern" segment of 10residue repeats located at a site M nearer the Cterminus. In the tarsioids, which lie close to the
roots of the modern apes, there is in fact a P segment with 14 repeats of 17-18 amino acids, and
also an M segment. The modern segment of the
higher primates can be divided into three regions,
of which the early region is present in all species,
the middle region is found only in the hominoids,
and the late region shows large differences
between man, gorilla and orang-utan [57]. The
continuing evolution of the late region is indicated by the distinct polymorphism of this region
in the gorilla [282].
As well as in the ectodermal epidermis cells,
cytokeratins are also detectable in other multilayered epithelia, for example, in the mouse in the
mucous membranes of the palate and cheeks, on
the tongue and in the lining of the oesophagus and
the cardia portion of the stomach, i.e. in cells of
endodermal origin [137]. The cytokeratins have
the typical structure of IF proteins with a central
rod domain, which can form a coiled-coil structure, and a non-helical terminal domain. Two
types of cytokeratin can be distinguished. Type I
includes the acidic proteins (pI 4.5-5.5) of about
40-60 kDa, and type II the neutral to basic proteins (pI 6.5-7.5) of about 60-70 kDa. The charge
differences are due to the large proportion of
basic amino acids in segments lA and 2B of the
central domain; the size difference results from
the absence from type I of the terminal sub domains Hl and H2 (Fig. 10.8). The keratin filaments are IIII heteropolymers, with the cells
forming one or more specific IIII pairs at each step
in their differentiation [262]. The genes of both
cytokeratin types are arranged in separate clusters
[226]. The immunological similarity within one
type in different species is much greater than
between types I and II in the skin of one animal.
Hybridization experiments with cDNAs for the
keratin mRNAs synthesized in human epidermis
cell cultures have shown that genes for both types
are present in all vertebrates from Myxine to man.
Distantly related sequences have even been found
in the Drosophila genome. Little is known about
the structure of cytokeratins of the lower vertebrates; nevertheless, a non-epidermal, 56-kDa cytokeratin from the oocytes and embryos and from
cells in the liver, gut and kidney of the clawed frog
Xenopus laevis has been sequenced via the
by cell-specific phosphorylation [244]. This heterogeneity of the proteins corresponds to the morphological and evolutionary plasticity of horned
epidermal structures. Usually, the distinction is
drawn between the soft keratins in vertebrate skin
and hard keratin structures like hair, feathers, spines, claws, nails and hooves, horns, beaks and
whalebone. The formation of specialized keratin
structures in various parts of the epidermis first
appeared after the transition of the vertebrates to
a terrestrial existence, i.e. with the reptiles, birds
and mammals; only a few exceptional amphibians
have keratinized warts, claws or horned jaws [20].
The keratinocytes in the stratum germinativum
of human epidermis have up to 30 % of their total
protein in the form of prekeratins, which can be
extracted into acidic buffers. In the space of a few
days, these living cells are converted into the
dead, horny cells of the outer skin layer (stratum
corneum), in which 70 % of the dry weight consists of keratins; here, they are cross-linked into a
fibrous state by disulphide bridges and N'-(yglutamyl)-lysine isodipeptide bonds, and can be
solubilized only by a combination of reducing and
denaturing reagents. The formation of the crosslinks is catalysed by keratinocyte-specific transglutaminases which occur as different isoenzymes
[221]. In the course of its development, each
keratinocyte expresses a series of different cytokeratins. Cytokeratins 1 and 10 predominate at
the start of terminal differentiation. These form a
highly complex aggregate of cross-linked fibrils
beneath the plasma membrane, the cross-linked
cell envelope (CE). Construction of the CE involves the histidine-rich, 26-kDa protein maggrin,
the sequence of which is very similar between the
rat and mouse but is quite different in man. Special granules in immature keratinocytes contain a
high molecular weight (>350 kDa) pro-filaggrin,
made up of numerous filaggrin copies that are
bound together by linkers and phosphorylated on
many serine residues. During the first phase of
terminal differentiation, the pro-filaggrin is
dephosphorylated and then processed further by
specific proteases [237]. In addition to cytokeratins, the CE of terminally differentiated, flattened keratinocytes contains further, cross-linked
proteins, e.g. the small pancornulins, keratolinin,
loricrin [109, 222], and involucrin, the structure
and evolution of which has received particular
attention. A large part of the involucrin chain
consists of a segment of tandem repeats, and this
varies in position and repeat length between different mammals. In the dog and pig and in the
10.10.1 Cytokeratins
357
pro simians Lemur catta and Galago crassicaudatus, there is a "premodern" segment, located at a
site P near the N-terminus, which has 6-13
repeats each of 20-31 amino acids. The higher
primates possess a "modern" segment of 10residue repeats located at a site M nearer the Cterminus. In the tarsioids, which lie close to the
roots of the modern apes, there is in fact a P segment with 14 repeats of 17-18 amino acids, and
also an M segment. The modern segment of the
higher primates can be divided into three regions,
of which the early region is present in all species,
the middle region is found only in the hominoids,
and the late region shows large differences
between man, gorilla and orang-utan [57]. The
continuing evolution of the late region is indicated by the distinct polymorphism of this region
in the gorilla [282].
As well as in the ectodermal epidermis cells,
cytokeratins are also detectable in other multilayered epithelia, for example, in the mouse in the
mucous membranes of the palate and cheeks, on
the tongue and in the lining of the oesophagus and
the cardia portion of the stomach, i.e. in cells of
endodermal origin [137]. The cytokeratins have
the typical structure of IF proteins with a central
rod domain, which can form a coiled-coil structure, and a non-helical terminal domain. Two
types of cytokeratin can be distinguished. Type I
includes the acidic proteins (pI 4.5-5.5) of about
40-60 kDa, and type II the neutral to basic proteins (pI 6.5-7.5) of about 60-70 kDa. The charge
differences are due to the large proportion of
basic amino acids in segments lA and 2B of the
central domain; the size difference results from
the absence from type I of the terminal sub domains Hl and H2 (Fig. 10.8). The keratin filaments are IIII heteropolymers, with the cells
forming one or more specific IIII pairs at each step
in their differentiation [262]. The genes of both
cytokeratin types are arranged in separate clusters
[226]. The immunological similarity within one
type in different species is much greater than
between types I and II in the skin of one animal.
Hybridization experiments with cDNAs for the
keratin mRNAs synthesized in human epidermis
cell cultures have shown that genes for both types
are present in all vertebrates from Myxine to man.
Distantly related sequences have even been found
in the Drosophila genome. Little is known about
the structure of cytokeratins of the lower vertebrates; nevertheless, a non-epidermal, 56-kDa cytokeratin from the oocytes and embryos and from
cells in the liver, gut and kidney of the clawed frog
Xenopus laevis has been sequenced via the