by about 83-96 %. In the ciliate Tetrahymena and
the flagellate Trypanosoma the cilia and the cytoplasm contain different tubulins [11, 63, 227].
Multiple tubulin genes are the rule in the eukaryotes. Toxoplasma gondii, on the other hand,
possesses only one a- and one ~-tubulin gene,
and Tetrahymena pyriformis has one a and two ~
genes [11, 303]. The genome of the ciliate Stylonychia lemnae contains two each of the a- and ~tubulin genes, which are amplified 20000- to
150000-fold in the macronucleus [45]. Drosophila
has 4 genes each for a and ~ tubulin, all of which
are active; the sea urchin has about 13 of each,
and mammals more than 20, although in this case
they include many pseudogenes. There are three
active human a genes and four ~ genes; in the
mouse six of each are active, and in the chicken
five of the seven to nine ~ genes are active [45].
The tubulin genes of the Metazoa are always
widely dispersed in the genome, with the exception of several linked genes in the sea urchin. In
contrast, Trypanosoma brucei and T. evansi have
tubulin loci containing numerous a~ pairs in tandem; these loci are apparently transcribed as a
unit, but are then rapidly cleaved into individual
mRNAs with the typical mini-exons. The tubulin
genes of T. cruzi and T. rangeli are not so strictly
organized; in addition to tandem clusters, these
species also have isolated a~ gene pairs
[113, 172]. In the related species Leishmania
major, there are separate a and ~ clusters and also
dispersed tubulin genes [256]. Vertebrate a- and
~-tubulin genes both contain three introns but in
different positions. Comparisons within the eukaryotes show that a genes contain one to seven
introns and ~ genes two to eight introns, the positions of which are extremely variable. Seventeen
different intron positions have been reported for
a-tubulin genes and 20 for ~ genes, only one of
which is common to the two types [56, 303].
The expression of many tubulin genes is tissue
specific and dependent on the developmental stage.
This has been examined in detail in Drosophila.
Here, there are four a and four ~ genes, of which, in
each case, two are expressed constitutively, whereas
a2 is active only in the testes of adult males, a4 in
early embryos and the ovaries of mature females,
one ~ gene only in the testes and early embryos, and
another ~ gene only during the intermediate stages
of embryo development. The sequence differences
between these isotubulins are very large; a1 differs
in only 2 amino acids from a3, but in 21 from a2,
and in 149 from a4 [41, 283]. Several ~-tubulin genes
of the chicken have been sequenced and the
encoded amino acid sequences show up to 8.7% dif10.9 Microtubule Proteins
353
ference. Tubulin c~6 from erythrocytes differs, in
fact, by 17 % from the rest. ~1 is the predominant
tubulin of skeletal muscle, and ~2 predominates in
the brain, which is the only organ to contain ~4. ~3
is found only in the testis; ~5 is everywhere except in
neurons; and ~6 occurs only in erythrocytes
[199, 268]. In the mouse, two of the six a-tubulin
genes are expressed only in the testis [292], and the
blood platelets also contain a specific ~ tubulin [169].
The sequence differences between isotubulins are
much greater than the differences between the same
isoform in different species. The differences are not
distributed randomly over the ~ chain; for example,
the sequences between positions 401 and 425 in all ~
tubulins from mammals to yeast are identical, but
the C-terminal regions from position 430 onwards
are very variable in both sequence and length [41]. It
would appear that the sequence differences between
the isotubulins are the result of adaptive evolution,
but so far very little is known about the functional
differences between microtubules [41, 199].
The heterogeneity of tubulins is increased by
post-translational modifications such as phosphorylation and glycosylation. Tyrosylation of the Cterminal glutamate residue of a tubulin by an
ATP-dependent tubulin tyrosine ligase is very
common, and is detectable in all mammalian cells
[16]. Worthy of comment is the fact that most a
chains are synthesized with a C-terminal tyrosine
which is later cleaved off by a specific carboxypeptidase; this is an enzyme of 90 kDa which has
been examined in detail only in the brain. Tubulin
tyrosylation is found from the mammals to Trypanosoma but not, for example, in Tetrahymena.
The ligases and tubulin from many different species, e.g. mammal and sea urchin, may be combined, but Tetrahymena tubulin is not processed
by the mammalian enzyme [264].
Flagella and cilia (undulipodia) have a typical
9 + 2 structural pattern in cross-section: two
central tubules are surrounded by nine peripheral
doublet-tubules, each of which is made up of a
complete A tubule and an incomplete B tubule;
the A and B tubules have three protofilaments in
common. The A tubules bear two rows of hooklike dynein arms (outer and inner) over their
whole length, and these can form bridges to the B
tubules of the neighbouring duplex (Fig. 10.7).
The dyneins are energy-releasing ATPases, similar in this respect to myosin. They are protein
complexes of 1.2-1.9 MDa, made up of two or
three heavy chains (a, ~ and y) of 350-430 kDa
and with ATPase activity, and several medium
(57-85 kDa) and light (6-22 kDa) chains. It is
not yet understood why the dyneins are so much
the flagellate Trypanosoma the cilia and the cytoplasm contain different tubulins [11, 63, 227].
Multiple tubulin genes are the rule in the eukaryotes. Toxoplasma gondii, on the other hand,
possesses only one a- and one ~-tubulin gene,
and Tetrahymena pyriformis has one a and two ~
genes [11, 303]. The genome of the ciliate Stylonychia lemnae contains two each of the a- and ~tubulin genes, which are amplified 20000- to
150000-fold in the macronucleus [45]. Drosophila
has 4 genes each for a and ~ tubulin, all of which
are active; the sea urchin has about 13 of each,
and mammals more than 20, although in this case
they include many pseudogenes. There are three
active human a genes and four ~ genes; in the
mouse six of each are active, and in the chicken
five of the seven to nine ~ genes are active [45].
The tubulin genes of the Metazoa are always
widely dispersed in the genome, with the exception of several linked genes in the sea urchin. In
contrast, Trypanosoma brucei and T. evansi have
tubulin loci containing numerous a~ pairs in tandem; these loci are apparently transcribed as a
unit, but are then rapidly cleaved into individual
mRNAs with the typical mini-exons. The tubulin
genes of T. cruzi and T. rangeli are not so strictly
organized; in addition to tandem clusters, these
species also have isolated a~ gene pairs
[113, 172]. In the related species Leishmania
major, there are separate a and ~ clusters and also
dispersed tubulin genes [256]. Vertebrate a- and
~-tubulin genes both contain three introns but in
different positions. Comparisons within the eukaryotes show that a genes contain one to seven
introns and ~ genes two to eight introns, the positions of which are extremely variable. Seventeen
different intron positions have been reported for
a-tubulin genes and 20 for ~ genes, only one of
which is common to the two types [56, 303].
The expression of many tubulin genes is tissue
specific and dependent on the developmental stage.
This has been examined in detail in Drosophila.
Here, there are four a and four ~ genes, of which, in
each case, two are expressed constitutively, whereas
a2 is active only in the testes of adult males, a4 in
early embryos and the ovaries of mature females,
one ~ gene only in the testes and early embryos, and
another ~ gene only during the intermediate stages
of embryo development. The sequence differences
between these isotubulins are very large; a1 differs
in only 2 amino acids from a3, but in 21 from a2,
and in 149 from a4 [41, 283]. Several ~-tubulin genes
of the chicken have been sequenced and the
encoded amino acid sequences show up to 8.7% dif10.9 Microtubule Proteins
353
ference. Tubulin c~6 from erythrocytes differs, in
fact, by 17 % from the rest. ~1 is the predominant
tubulin of skeletal muscle, and ~2 predominates in
the brain, which is the only organ to contain ~4. ~3
is found only in the testis; ~5 is everywhere except in
neurons; and ~6 occurs only in erythrocytes
[199, 268]. In the mouse, two of the six a-tubulin
genes are expressed only in the testis [292], and the
blood platelets also contain a specific ~ tubulin [169].
The sequence differences between isotubulins are
much greater than the differences between the same
isoform in different species. The differences are not
distributed randomly over the ~ chain; for example,
the sequences between positions 401 and 425 in all ~
tubulins from mammals to yeast are identical, but
the C-terminal regions from position 430 onwards
are very variable in both sequence and length [41]. It
would appear that the sequence differences between
the isotubulins are the result of adaptive evolution,
but so far very little is known about the functional
differences between microtubules [41, 199].
The heterogeneity of tubulins is increased by
post-translational modifications such as phosphorylation and glycosylation. Tyrosylation of the Cterminal glutamate residue of a tubulin by an
ATP-dependent tubulin tyrosine ligase is very
common, and is detectable in all mammalian cells
[16]. Worthy of comment is the fact that most a
chains are synthesized with a C-terminal tyrosine
which is later cleaved off by a specific carboxypeptidase; this is an enzyme of 90 kDa which has
been examined in detail only in the brain. Tubulin
tyrosylation is found from the mammals to Trypanosoma but not, for example, in Tetrahymena.
The ligases and tubulin from many different species, e.g. mammal and sea urchin, may be combined, but Tetrahymena tubulin is not processed
by the mammalian enzyme [264].
Flagella and cilia (undulipodia) have a typical
9 + 2 structural pattern in cross-section: two
central tubules are surrounded by nine peripheral
doublet-tubules, each of which is made up of a
complete A tubule and an incomplete B tubule;
the A and B tubules have three protofilaments in
common. The A tubules bear two rows of hooklike dynein arms (outer and inner) over their
whole length, and these can form bridges to the B
tubules of the neighbouring duplex (Fig. 10.7).
The dyneins are energy-releasing ATPases, similar in this respect to myosin. They are protein
complexes of 1.2-1.9 MDa, made up of two or
three heavy chains (a, ~ and y) of 350-430 kDa
and with ATPase activity, and several medium
(57-85 kDa) and light (6-22 kDa) chains. It is
not yet understood why the dyneins are so much