H2A.X and the appearance of the testis-specific
H3 variant, TH3; the sperm-specific histones
H1t, TH2A and TH2B are produced during meiosis. In the intermediate spermatid stages, all histone and non-histone proteins are replaced by the
transitional proteins TP-1 and TP-2, which are no
longer organized into nucleosomes. The protamines appear 2-3 days later; there is one form in
the rat and several forms in other mammals, and
these persist into the mature sperms [165, 379].
Similar alterations in nuclear proteins are seen
during spermatogenesis in other vertebrates;
thus, in the dogfish, Scyliorhinus caniculus, for
example, two transitional proteins are followed
by four protamines [32].
In sea urchin sperm there are no protamines
but late H4 and H3, particular H2A subtypes and
sperm-specific HI and H2B variants. The latter
are interesting in that the N-terminus is extended
by 20-26 amino acids with highly basic, repetitive
tetrapeptides (HI) or pentapeptides (H2B); the
many positive charges neutralize the negative
charges on the DNA and thus allow dense packing of the chromatin [348]. The sequence of the
repeats shows species-specific variation [246].
H2B variants with N-terminal extensions appear
to be limited to sea urchin sperm; the H2B from
the sperm of the starfish, Asterias rubens, consists
of only 121 amino acids without N-terminal
repeats [283] ; and the histones in the sperm of the
holothurians are also not significantly different
from these of the somatic cells [492]. The sperms
of the polychaete Platynereis dumerilii contain
the same core histones as other cells, but they
have four specific HI variants and two protamines. Two of the HI variants are unusually small,
with only 119 and 121 amino acids. Only one HI
and one protamine are found in Chaetopterus
variopedatus [234]. The micronucleus of the Ciliophora contains genetically inactive chromatin that
is, nevertheless, organized into nucleosomes. In
Tetrahymena this includes, in addition to the histone H3S that is also present in the macronucleus,
a special H3F and three polypeptides a-y, which
are not HI variants, in the linker region [484].
The unusual structure of the HI in the macronucleus has already been mentioned (p. 29); in Tetrahymena, the macronucleus also contains a specific
H2A variant [474].
The variability of the his tones is also increased
by post-translational modification, such as phosphorylation, acetylation, methylation and covalent binding to poly-ADP-ribose. The significance
of these processes is to be found in the fine
adaptation of chromatin structure to changing
2.5.4 Protamines
33
internal and external conditions. uH2A is a particularly interesting post-translationally altered
histone that is a conjugate of H2A with ubiquitin.
This polypeptide, with a length of 76 amino acids
(see Fig. 3.5; p. 88), occurs in all organisms from
bacteria to mammals and apparently functions as
a signal for protein degradation. In uH2A, the Cterminal glycine of ubiquitin is attached via an
isopeptide bond to the E-amino group of the H2A
lysine-119, and the conjugated molecule has a
forked "Y" configuration that is very unusual for
a protein. About 10 % of H2A is found in this
form. uH2A should probably not be considered
an intermediate of histone degradation, and ubiquitin apparently has more than one function in
the cell [320].
2.5.4 Protamines
The protamines are small, extremely basic nuclear proteins with 30-70 % arginine and were first
discovered in mature fish sperm, although they
are also be found in the sperm of birds, mammals,
and several invertebrates. As has already been
mentioned, protamines are first produced during
sperm maturation and they replace the preexisting histones and transition proteins. Fish
protamines are known after the genus or species
name: "scyliorhinin" from the dogfish, Scyliorhinus canicula; "clupein" from the herring, Clupea
harengus; "salmin" from salmon of the genera
Salmo and Oncorhynchus; "iridin" from the trout
Salmo irideus, etc. The sperm of anyone species
contains several protamines which are designated, for example, as iridin la, Ib and II. Protamines have been sequenced from more than 50 species of cartilaginous and bony fish (Fig. 2.8). Due
to their unusual amino acid composition, the protamine sequences are difficult to compare; analysis of their relationships and evolution is best
carried out using the gene sequences [332].
The teleost protamines are, without exception,
very small polypeptides of less than 35 amino
acids, in which arginine predominates and cysteine is absent. In the particularly well-investigated
rainbow trout, Salmo gairdneri, and in other teleosts, the protamines show pronounced heterogeneity; six different protamines, albeit with
quite similar sequences, can be isolated from a
single trout testis (Fig. 2.8) [295]. Protamines are
not found in all teleosts; they are lacking, for
example, in carp and in the winter flounder, Pseudopleuronectes american us . The discovery that a
protamine gene of the rainbow trout, Salmo
H3 variant, TH3; the sperm-specific histones
H1t, TH2A and TH2B are produced during meiosis. In the intermediate spermatid stages, all histone and non-histone proteins are replaced by the
transitional proteins TP-1 and TP-2, which are no
longer organized into nucleosomes. The protamines appear 2-3 days later; there is one form in
the rat and several forms in other mammals, and
these persist into the mature sperms [165, 379].
Similar alterations in nuclear proteins are seen
during spermatogenesis in other vertebrates;
thus, in the dogfish, Scyliorhinus caniculus, for
example, two transitional proteins are followed
by four protamines [32].
In sea urchin sperm there are no protamines
but late H4 and H3, particular H2A subtypes and
sperm-specific HI and H2B variants. The latter
are interesting in that the N-terminus is extended
by 20-26 amino acids with highly basic, repetitive
tetrapeptides (HI) or pentapeptides (H2B); the
many positive charges neutralize the negative
charges on the DNA and thus allow dense packing of the chromatin [348]. The sequence of the
repeats shows species-specific variation [246].
H2B variants with N-terminal extensions appear
to be limited to sea urchin sperm; the H2B from
the sperm of the starfish, Asterias rubens, consists
of only 121 amino acids without N-terminal
repeats [283] ; and the histones in the sperm of the
holothurians are also not significantly different
from these of the somatic cells [492]. The sperms
of the polychaete Platynereis dumerilii contain
the same core histones as other cells, but they
have four specific HI variants and two protamines. Two of the HI variants are unusually small,
with only 119 and 121 amino acids. Only one HI
and one protamine are found in Chaetopterus
variopedatus [234]. The micronucleus of the Ciliophora contains genetically inactive chromatin that
is, nevertheless, organized into nucleosomes. In
Tetrahymena this includes, in addition to the histone H3S that is also present in the macronucleus,
a special H3F and three polypeptides a-y, which
are not HI variants, in the linker region [484].
The unusual structure of the HI in the macronucleus has already been mentioned (p. 29); in Tetrahymena, the macronucleus also contains a specific
H2A variant [474].
The variability of the his tones is also increased
by post-translational modification, such as phosphorylation, acetylation, methylation and covalent binding to poly-ADP-ribose. The significance
of these processes is to be found in the fine
adaptation of chromatin structure to changing
2.5.4 Protamines
33
internal and external conditions. uH2A is a particularly interesting post-translationally altered
histone that is a conjugate of H2A with ubiquitin.
This polypeptide, with a length of 76 amino acids
(see Fig. 3.5; p. 88), occurs in all organisms from
bacteria to mammals and apparently functions as
a signal for protein degradation. In uH2A, the Cterminal glycine of ubiquitin is attached via an
isopeptide bond to the E-amino group of the H2A
lysine-119, and the conjugated molecule has a
forked "Y" configuration that is very unusual for
a protein. About 10 % of H2A is found in this
form. uH2A should probably not be considered
an intermediate of histone degradation, and ubiquitin apparently has more than one function in
the cell [320].
2.5.4 Protamines
The protamines are small, extremely basic nuclear proteins with 30-70 % arginine and were first
discovered in mature fish sperm, although they
are also be found in the sperm of birds, mammals,
and several invertebrates. As has already been
mentioned, protamines are first produced during
sperm maturation and they replace the preexisting histones and transition proteins. Fish
protamines are known after the genus or species
name: "scyliorhinin" from the dogfish, Scyliorhinus canicula; "clupein" from the herring, Clupea
harengus; "salmin" from salmon of the genera
Salmo and Oncorhynchus; "iridin" from the trout
Salmo irideus, etc. The sperm of anyone species
contains several protamines which are designated, for example, as iridin la, Ib and II. Protamines have been sequenced from more than 50 species of cartilaginous and bony fish (Fig. 2.8). Due
to their unusual amino acid composition, the protamine sequences are difficult to compare; analysis of their relationships and evolution is best
carried out using the gene sequences [332].
The teleost protamines are, without exception,
very small polypeptides of less than 35 amino
acids, in which arginine predominates and cysteine is absent. In the particularly well-investigated
rainbow trout, Salmo gairdneri, and in other teleosts, the protamines show pronounced heterogeneity; six different protamines, albeit with
quite similar sequences, can be isolated from a
single trout testis (Fig. 2.8) [295]. Protamines are
not found in all teleosts; they are lacking, for
example, in carp and in the winter flounder, Pseudopleuronectes american us . The discovery that a
protamine gene of the rainbow trout, Salmo