or GFAP; gene expression is therefore not as
strictly regulated as for other IF proteins. There is
apparently only one vimentin gene in birds and
mammals, and from this several mRNAs are transcribed with varying 3' regions [244]. The evolution of the vimentins is relatively slow, e.g. the
sequences of chicken and hamster agree in 80 %
of positions [320]. The species-specific differences
lie mainly in the N-terminal, non-helical
domains, as is commonly found for IF proteins.
In addition to the filamentous, insoluble vimentins, some cells also contain soluble, tetrameric
vimentins at lower concentrations [253]. Desmin
fIlaments are found in many different types of
vertebrate muscle. Extraction of the contractile
proteins by use of high-ionic-strength buffers leaves behind desmin in the Z lines of striated
muscle, in the intercalated discs of heart muscle,
and in the dense bodies of smooth muscle. In
embryonic muscle, desmin filaments are found as
a cytoplasmic network [244].
The IF proteins of the glial cells (GFAPs) vary
in size in different species; e.g. human 49 kDa,
bovine 54 kDa, and murine 45 and 55 kDa
[244, 283]. In contrast, the nenrofIlaments of
mammals, birds and reptiles apparently always
contain the same protein triplet of NF -L
(62 kDa), NF-M (102 kDa) and NF-H (110116 kDa), although the relative proportions vary
with the species. The corresponding human and
murine genes have been sequenced. The organization of the three genes is very similar (although
with two introns in the NF-M gene and three
introns in NF-L and NF-H), and quite different
from the other IF genes. Thus, the NF proteins
appear to have separated at quite an early stage
from the evolutionary line of the IF proteins.
Compatible with this concept is the fact that neuronal IF proteins have been found in all invertebrates examined [121, 162]. NF-L is necessary but
sufficient for the formation of filaments in vitro;
the other two proteins are integrated into the filament at regular intervals as required [105]. NF-M
and NF-H have C-terminal extensions which
build cross-bridges between the filaments. These
extensions contain numerous Lys-Ser-Pro triplets
and the serines are sometimes phosphorylated.
This high degree of phosphorylation explains the
discrepancy between the molecular masses measured by gel filtration (150 kDa for NF-M and
220 kDa for NF-H) and those, given above,
which have been calculated from sequence data
[121, 162]. NFs may contain other proteins in
addition to the three NF proteins, e.g. periferin
(57 kDa) in the peripheral nervous system, and
10.10.3 Lamins
359
a-internexin in the brain (66 kDa). Both the latter proteins belong to the IF family but do not
form filaments in vitro [70, 157].
10.10.3 Lamins
The nuclear envelope consists of an inner and an
outer membrane and a fibrillar layer (lamina) that
is 10-20 nm thick. The lamina serves to anchor
the chromosomes during interphase and it is broken down at prophase. In vertebrates, the lamina
is built mainly from polypeptides of 60-80 kDa,
the lamins. Mammalian nuclei have three lamins:
A (70 kDa), B (68 kDa) and C (60 kDa). The
sequences of A (702 amino acids) and C (572
amino acids) are identical up to amino acid 566; C
then has only 6 further amino acids whilst A has
136. Lamins A and C are so similar to the IF proteins in structure, having a central rod domain with
a coiled-coil structure and flanking non-helical
domains, that they must surely be considered
members of the same family (Fig. 10.8). On the
other hand, they show significant differences to
the other IF proteins and very probably departed
quite early from the main line of IF evolution. For
example, the central domains of A and Care 42
amino acids longer than other IF proteins, and the
rod region is consequently 6 nm longer (53 nm).
Cross-linking occurs via the terminal domains.
Two further lamins, D and E, have been found in
several very different types of cells in the rat,
including lymphoid cells which lack A and C. The
lamins B, D and E apparently also belong to the
IF family but are structurally and immunologically
quite different to lamins A and C [126].
Two or three lamins are also detected electrophoretically in the cell nuclei of birds and reptiles; e.g. lamin A and two different forms of Bare
found in the chicken [219]. Five lamins have been
described in the clawed frog Xenopus laevis and
these are distinctly cell and developmental-stage
specific. The lamins L-I and L-II are present in
almost all somatic cells, L-I from the blastula
stage onwards, and L-II from the gastrula stage.
Oocytes contain L-III, which is also found in
highly specialized cells of adult animals, e.g.
muscle and nerve cells. L-IV is restricted to cells
of the male germline. The fifth lamin of this series
to be identified from the gene sequence is
expressed in all somatic cells except erythrocytes
[59, 126]. In contrast to the situation in vertebrates, the molluscs and arthropods appear to possess only one lamin. Sequence analysis is only
available for Drosophila melanogaster; in this
strictly regulated as for other IF proteins. There is
apparently only one vimentin gene in birds and
mammals, and from this several mRNAs are transcribed with varying 3' regions [244]. The evolution of the vimentins is relatively slow, e.g. the
sequences of chicken and hamster agree in 80 %
of positions [320]. The species-specific differences
lie mainly in the N-terminal, non-helical
domains, as is commonly found for IF proteins.
In addition to the filamentous, insoluble vimentins, some cells also contain soluble, tetrameric
vimentins at lower concentrations [253]. Desmin
fIlaments are found in many different types of
vertebrate muscle. Extraction of the contractile
proteins by use of high-ionic-strength buffers leaves behind desmin in the Z lines of striated
muscle, in the intercalated discs of heart muscle,
and in the dense bodies of smooth muscle. In
embryonic muscle, desmin filaments are found as
a cytoplasmic network [244].
The IF proteins of the glial cells (GFAPs) vary
in size in different species; e.g. human 49 kDa,
bovine 54 kDa, and murine 45 and 55 kDa
[244, 283]. In contrast, the nenrofIlaments of
mammals, birds and reptiles apparently always
contain the same protein triplet of NF -L
(62 kDa), NF-M (102 kDa) and NF-H (110116 kDa), although the relative proportions vary
with the species. The corresponding human and
murine genes have been sequenced. The organization of the three genes is very similar (although
with two introns in the NF-M gene and three
introns in NF-L and NF-H), and quite different
from the other IF genes. Thus, the NF proteins
appear to have separated at quite an early stage
from the evolutionary line of the IF proteins.
Compatible with this concept is the fact that neuronal IF proteins have been found in all invertebrates examined [121, 162]. NF-L is necessary but
sufficient for the formation of filaments in vitro;
the other two proteins are integrated into the filament at regular intervals as required [105]. NF-M
and NF-H have C-terminal extensions which
build cross-bridges between the filaments. These
extensions contain numerous Lys-Ser-Pro triplets
and the serines are sometimes phosphorylated.
This high degree of phosphorylation explains the
discrepancy between the molecular masses measured by gel filtration (150 kDa for NF-M and
220 kDa for NF-H) and those, given above,
which have been calculated from sequence data
[121, 162]. NFs may contain other proteins in
addition to the three NF proteins, e.g. periferin
(57 kDa) in the peripheral nervous system, and
10.10.3 Lamins
359
a-internexin in the brain (66 kDa). Both the latter proteins belong to the IF family but do not
form filaments in vitro [70, 157].
10.10.3 Lamins
The nuclear envelope consists of an inner and an
outer membrane and a fibrillar layer (lamina) that
is 10-20 nm thick. The lamina serves to anchor
the chromosomes during interphase and it is broken down at prophase. In vertebrates, the lamina
is built mainly from polypeptides of 60-80 kDa,
the lamins. Mammalian nuclei have three lamins:
A (70 kDa), B (68 kDa) and C (60 kDa). The
sequences of A (702 amino acids) and C (572
amino acids) are identical up to amino acid 566; C
then has only 6 further amino acids whilst A has
136. Lamins A and C are so similar to the IF proteins in structure, having a central rod domain with
a coiled-coil structure and flanking non-helical
domains, that they must surely be considered
members of the same family (Fig. 10.8). On the
other hand, they show significant differences to
the other IF proteins and very probably departed
quite early from the main line of IF evolution. For
example, the central domains of A and Care 42
amino acids longer than other IF proteins, and the
rod region is consequently 6 nm longer (53 nm).
Cross-linking occurs via the terminal domains.
Two further lamins, D and E, have been found in
several very different types of cells in the rat,
including lymphoid cells which lack A and C. The
lamins B, D and E apparently also belong to the
IF family but are structurally and immunologically
quite different to lamins A and C [126].
Two or three lamins are also detected electrophoretically in the cell nuclei of birds and reptiles; e.g. lamin A and two different forms of Bare
found in the chicken [219]. Five lamins have been
described in the clawed frog Xenopus laevis and
these are distinctly cell and developmental-stage
specific. The lamins L-I and L-II are present in
almost all somatic cells, L-I from the blastula
stage onwards, and L-II from the gastrula stage.
Oocytes contain L-III, which is also found in
highly specialized cells of adult animals, e.g.
muscle and nerve cells. L-IV is restricted to cells
of the male germline. The fifth lamin of this series
to be identified from the gene sequence is
expressed in all somatic cells except erythrocytes
[59, 126]. In contrast to the situation in vertebrates, the molluscs and arthropods appear to possess only one lamin. Sequence analysis is only
available for Drosophila melanogaster; in this