50
2 Nucleic Acids and Nuclear Proteins
only approximately but is clearly larger than in
the prokaryotes. For example, 84 proteins have
been isolated from the cytoplasmic ribosomes of
rat liver [64]; with the help of 2-D electrophoresis, 35-45 proteins were identified in the large
subunits of the insects Acheta, Drosophila and
Bombyx and 30-32 were identified in the small
subunits [41, 278]. Of these, at least 26 of the
Drosophila proteins have already been purified
[74]. The ribosomal proteins from rat liver have
molecular masses between 11 and 41 kDa, and
those of Drosophila are between 11.5 and 61 kDa.
There is apparently only one molecule of each
protein species per ribosome, except for the protein type that corresponds to L71L12 of E. coli;
this is present in several copies. The proteins of
the small subunit are designated "S" and those of
the large subunit "L", and they are numbered
rather variably by different authors according to
their location in 2-D gels. Because comparisons
between proteins isolated by different authors are
so difficult, attempts have been made to develop
a uniform nomenclature [294]. The 53 ribosomal
proteins of E. coli, with altogether 7336 amino
acids, have now been completely sequenced. The
sequencing of all the ribosomal proteins of the
rat, involving about 16000 amino acids, is now
the subject of intense efforts; by 1989 more than
30 had been examined [65, 339, 426]. Although
altogether about 500 ribosomal proteins have
been sequenced so far, the information obtained
is still insufficient to allow firm statements to be
made about species-specific sequence differences
and the evolution of the ribosomal proteins.
However, it is already quite clear that they are
very conserved proteins. Direct sequence comparisons show that, for example, the proteins S14
and L31 of the rat and man differ by less than
1 %, protein S19 differs by just as little between
Xenopus and the golden hamster, 22 % difference
exists between S26 in the rat and the homologous
protein in Drosophila, 25 % difference exists
between S6 in the rat and yeast, and there is
about 50 % identity between L44 in Trypanosoma
brucei, baker's yeast and man [19, 207, 231, 255,
435].
Although in vitro many ribosomal proteins are
phosphorylated by cAMP-dependent or -independent protein kinases, in vivo one particular
basic protein is phosphorylated in all eukaryotes:
the protein from rat liver is designated S6 and has
within its sequence of 249 amino acids 15 serine
residues that possibly act as a phosphate receptor
[187,473]. S6-like proteins have been detected in
various animals, even down to the Protozoa; the
protein from ovaries of the clawed frog, Xenopus
laevis, shows immunological cross-reactivity with
the S6 from chicken liver. In all cells that have
been examined, the phosphorylation of S6 increases in connection with cell division and with the
association of ribosomes into polysomes [217].
The ribosomes of yeast, higher plants, the crustacean Artemia salina, Drosophila melanogaster,
and various mammalian organs all contain two
strongly acidic proteins with pI values less than
4.5 and molecular masses from 13.5 to 17 kDa; in
Artemia and Drosophila these are singly phosphorylated, and in mammals they are multiply
phosphorylated. The two proteins in Artemia,
known as eL12 and eL12', are found in several
copies in the large ribosomal subunit and are possibly involved in the elongation step of protein
biosynthesis. The sequences of eL12 and eL12'
are known; they are identical in the 22 amino
acids of the C-terminus but otherwise show no
significant homology. Although the protein L 7 /
L12 from E. coli shows little similarity to eL12, it
can replace eL12 in the ribosomes of Artemia
[275].
The proteins of the mitochondrial ribosomes
form a distinct group, although they are synthesized in the cytoplasm and only later introduced
into the mitochondria. Comparisons, by 2-D gel
electrophoresis, of mitochondrial and cytoplasmic ribosomal proteins from the same tissue
reveal only very limited similarities between species from the vertebrates to Neurospora [393].
Just like the rRNAs, the proteins of the mitochondrial ribosomes show a much greater rate of
evolution than do the components of the cytoplasmic ribosomes. In 2-D gels of rat and cow
ribosomal proteins, only 15 % of the spots are different for cytoplasmic ribosomes, whereas 85 %
differ for the mitochondrial ribosomes [344].
Even between the closely related clawed frog species Xenopus laevis and X. mulleri, the mitochondrial ribosomes show seven differences. It
appears that only in the mammals are there multiple genes for the ribosomal proteins, e.g. 7-20
copies per protein in the mouse. In contrast,
there are only one or two copies of each gene in
yeast, insects and amphibians. Surprisingly, only
300-400 relatively stable mRNAs are present for
each ribosomal protein in rapidly growing mouse
cells; the reason for the large number of genes is
therefore not clear [25, 304].
2 Nucleic Acids and Nuclear Proteins
only approximately but is clearly larger than in
the prokaryotes. For example, 84 proteins have
been isolated from the cytoplasmic ribosomes of
rat liver [64]; with the help of 2-D electrophoresis, 35-45 proteins were identified in the large
subunits of the insects Acheta, Drosophila and
Bombyx and 30-32 were identified in the small
subunits [41, 278]. Of these, at least 26 of the
Drosophila proteins have already been purified
[74]. The ribosomal proteins from rat liver have
molecular masses between 11 and 41 kDa, and
those of Drosophila are between 11.5 and 61 kDa.
There is apparently only one molecule of each
protein species per ribosome, except for the protein type that corresponds to L71L12 of E. coli;
this is present in several copies. The proteins of
the small subunit are designated "S" and those of
the large subunit "L", and they are numbered
rather variably by different authors according to
their location in 2-D gels. Because comparisons
between proteins isolated by different authors are
so difficult, attempts have been made to develop
a uniform nomenclature [294]. The 53 ribosomal
proteins of E. coli, with altogether 7336 amino
acids, have now been completely sequenced. The
sequencing of all the ribosomal proteins of the
rat, involving about 16000 amino acids, is now
the subject of intense efforts; by 1989 more than
30 had been examined [65, 339, 426]. Although
altogether about 500 ribosomal proteins have
been sequenced so far, the information obtained
is still insufficient to allow firm statements to be
made about species-specific sequence differences
and the evolution of the ribosomal proteins.
However, it is already quite clear that they are
very conserved proteins. Direct sequence comparisons show that, for example, the proteins S14
and L31 of the rat and man differ by less than
1 %, protein S19 differs by just as little between
Xenopus and the golden hamster, 22 % difference
exists between S26 in the rat and the homologous
protein in Drosophila, 25 % difference exists
between S6 in the rat and yeast, and there is
about 50 % identity between L44 in Trypanosoma
brucei, baker's yeast and man [19, 207, 231, 255,
435].
Although in vitro many ribosomal proteins are
phosphorylated by cAMP-dependent or -independent protein kinases, in vivo one particular
basic protein is phosphorylated in all eukaryotes:
the protein from rat liver is designated S6 and has
within its sequence of 249 amino acids 15 serine
residues that possibly act as a phosphate receptor
[187,473]. S6-like proteins have been detected in
various animals, even down to the Protozoa; the
protein from ovaries of the clawed frog, Xenopus
laevis, shows immunological cross-reactivity with
the S6 from chicken liver. In all cells that have
been examined, the phosphorylation of S6 increases in connection with cell division and with the
association of ribosomes into polysomes [217].
The ribosomes of yeast, higher plants, the crustacean Artemia salina, Drosophila melanogaster,
and various mammalian organs all contain two
strongly acidic proteins with pI values less than
4.5 and molecular masses from 13.5 to 17 kDa; in
Artemia and Drosophila these are singly phosphorylated, and in mammals they are multiply
phosphorylated. The two proteins in Artemia,
known as eL12 and eL12', are found in several
copies in the large ribosomal subunit and are possibly involved in the elongation step of protein
biosynthesis. The sequences of eL12 and eL12'
are known; they are identical in the 22 amino
acids of the C-terminus but otherwise show no
significant homology. Although the protein L 7 /
L12 from E. coli shows little similarity to eL12, it
can replace eL12 in the ribosomes of Artemia
[275].
The proteins of the mitochondrial ribosomes
form a distinct group, although they are synthesized in the cytoplasm and only later introduced
into the mitochondria. Comparisons, by 2-D gel
electrophoresis, of mitochondrial and cytoplasmic ribosomal proteins from the same tissue
reveal only very limited similarities between species from the vertebrates to Neurospora [393].
Just like the rRNAs, the proteins of the mitochondrial ribosomes show a much greater rate of
evolution than do the components of the cytoplasmic ribosomes. In 2-D gels of rat and cow
ribosomal proteins, only 15 % of the spots are different for cytoplasmic ribosomes, whereas 85 %
differ for the mitochondrial ribosomes [344].
Even between the closely related clawed frog species Xenopus laevis and X. mulleri, the mitochondrial ribosomes show seven differences. It
appears that only in the mammals are there multiple genes for the ribosomal proteins, e.g. 7-20
copies per protein in the mouse. In contrast,
there are only one or two copies of each gene in
yeast, insects and amphibians. Surprisingly, only
300-400 relatively stable mRNAs are present for
each ribosomal protein in rapidly growing mouse
cells; the reason for the large number of genes is
therefore not clear [25, 304].