5.7.1 Vitellogenins and Yolk Proteins of Vertebrates
199
possibly occurred as follows: the AlB duplication
happened about 150 million years ago, and was
followed by the duplication A1/A2; the resulting
A1-A2-B species took over a further B gene by
allopolyploidization with another species [225].
The tetraploid X. borealis also has four VG genes
of which A1-B1 are clustered but A2 and B2 are
not. In contrast, in the diploid X. tropicalis, only
one A and one B gene are found together with a
weakly expressed A* gene [12].
In accordance with the multiplicity of genes,
three different VGs are detectable in the blood of
oestrogen-induced X. laevis females. The yolk
proteins are present as crystalline structures in
the so-called yolk platelets. These mainly consist
of the two protein types vitellin (VT, 200 kDa)
and phosvitin (pY, 35 kDa) as well as lower
amounts of the phosvettes PVT1 (19 kDa) and
PVT2 (13 kDa). Vitellin contains 20 % lipid and
0.5 % protein-bound phosphate and is made up
of two subunits, VT1 (116 kDa) and VTI
(32 kDa). Both VT1 and VT2 separate electrophoretically into three differently sized fractions,
which correspond in fact to the three VGs. PV is
unique in that it contains more than 50 % serine
residues, the majority of which are phosphorylated, and therefore 9.5 % protein-bound phosphorus. Dephosphorylated PV can also be separated into two fractions of different size. In the
1807-amino-acid-Iong chain of VG-A2, the VT1
sequence lies at the N-terminal end, the VT2
sequence lies close to the C-terminal end and the
PV sequence is in the middle region; it is not yet
clear what happens to the C-terminal 20 kDa
encoded by exons 30-35 [264]. The VGs pass into
the oocytes by a specific endocytosis process
which shows a 30- to 50-fold preference for VG
over other proteins. The VG receptor has a molecular mass of 115 kDa and is immunologically
similar to the chicken VG receptor. Each receptor
can bind the VG of the other species [242].
Vitellogenesis of other amphibian species has
also been examined in some detail. A VG of
427 kDa is present in the tetraploid frog Odontophrynus americanus; this VG consists of two
phosphoglycopeptides VGT1 (208 kDa) and
VGT2 (204 kDa). The yolk contains two vitellins
and three phosphoproteins: VT1 is made up of
two glycopeptides of 105 and 93 kDa, and VT2
consists of three different glycopeptides of 32, 30
and 28 kDa. The phosphoproteins are phosphovitin (37 kDa) and the two phosvettes PVT1
(28 kDa) and PVT2 (26 kDa). The VGs from
O. american us are very similar to those of X.laevis, but result in different yolk proteins, probably
because of differences in proteolytic cleavage
caused by deviations in the amino acid sequence
[273]. Very little is known about the yolk proteins
of the urodelans and their ancestors. Four VGs,
which can be electrophoretically defined in the
blood plasma of Pleurodeles waltii, give at least
two different polypeptides of 210 and 180 kDa on
denaturation [37].
The quite variable results so far obtained in
investigations of yolk formation in fish do not
allow firm conclusions to be drawn. The uptake of
female-specific plasma proteins into the oocytes
of the agnathan Eptatretus has been demonstrated
immunologically [281]. However, the possibility of
hormone-regulated VG formation was not investigated in this case; corresponding experiments
with cartilaginous fish were entirely negative and
only with teleosts were the results always positive.
The molecular sizes of the yolk proteins and the
plasma precursors and apoproteins in the teleosts
show great variety. VGs of 550-600 kDa were
found in the rainbow trout Salmo gairdneri, the
flounder Pleuronectes flesus and the winter flounder Pseudopleuronectes americanus; there appear
to be several VGs of 380 kDa in the goldfish
Carassius auratus. The VG of the Japanese eel
Anguilla japonica is reported to be a 350-kDa
homotetramer with subunits of 85 kDa. The VG
ofAmeiurus nebulosus has a mass of 145 kDa and
the corresponding mRNA is also shorter than that
of the chicken and Xenopus [182, 243].
The yolk proteins are just as heterogeneous;
no less than nine VG-derived polypeptides have
been found in the egg yolk of the antarctic teleost
Caenocephalus aceratus and these have sizes
between 13 and 172 kDa and differing phosphate
contents [232]. Denaturation of the VTs of the
rainbow trout and the goldfish yield subunits of
90-140 kDa and 15-25 kDa, whilst the PV subunits have molecular masses of 7.6-15 kDa [208].
Partial proteolysis of teleost VGs seems to produce at first larger and then smaller phosphoproteins that are analogous to the phosvettes of
Xenopus. The cleavage products apparently suffer partial dephosporylation; in any case, the content of protein-bound phosphate in PVs varies
between 0.0012 % in the halibut Hippoglossus
hippoglossus and 0.73 % in the dogfish Scyliorhinus canicula [251, 263]. The yolk platelets of the
Agnatha, of the archaic fish genera Amia, Lepisosteus, Polypterus and Latimeria, and of the
freshwater teleosts are crystalline like those of the
amphibians; the yolk crystals of the Agnatha are
monoclinal whereas the others are of the orthorhombic type [139].
199
possibly occurred as follows: the AlB duplication
happened about 150 million years ago, and was
followed by the duplication A1/A2; the resulting
A1-A2-B species took over a further B gene by
allopolyploidization with another species [225].
The tetraploid X. borealis also has four VG genes
of which A1-B1 are clustered but A2 and B2 are
not. In contrast, in the diploid X. tropicalis, only
one A and one B gene are found together with a
weakly expressed A* gene [12].
In accordance with the multiplicity of genes,
three different VGs are detectable in the blood of
oestrogen-induced X. laevis females. The yolk
proteins are present as crystalline structures in
the so-called yolk platelets. These mainly consist
of the two protein types vitellin (VT, 200 kDa)
and phosvitin (pY, 35 kDa) as well as lower
amounts of the phosvettes PVT1 (19 kDa) and
PVT2 (13 kDa). Vitellin contains 20 % lipid and
0.5 % protein-bound phosphate and is made up
of two subunits, VT1 (116 kDa) and VTI
(32 kDa). Both VT1 and VT2 separate electrophoretically into three differently sized fractions,
which correspond in fact to the three VGs. PV is
unique in that it contains more than 50 % serine
residues, the majority of which are phosphorylated, and therefore 9.5 % protein-bound phosphorus. Dephosphorylated PV can also be separated into two fractions of different size. In the
1807-amino-acid-Iong chain of VG-A2, the VT1
sequence lies at the N-terminal end, the VT2
sequence lies close to the C-terminal end and the
PV sequence is in the middle region; it is not yet
clear what happens to the C-terminal 20 kDa
encoded by exons 30-35 [264]. The VGs pass into
the oocytes by a specific endocytosis process
which shows a 30- to 50-fold preference for VG
over other proteins. The VG receptor has a molecular mass of 115 kDa and is immunologically
similar to the chicken VG receptor. Each receptor
can bind the VG of the other species [242].
Vitellogenesis of other amphibian species has
also been examined in some detail. A VG of
427 kDa is present in the tetraploid frog Odontophrynus americanus; this VG consists of two
phosphoglycopeptides VGT1 (208 kDa) and
VGT2 (204 kDa). The yolk contains two vitellins
and three phosphoproteins: VT1 is made up of
two glycopeptides of 105 and 93 kDa, and VT2
consists of three different glycopeptides of 32, 30
and 28 kDa. The phosphoproteins are phosphovitin (37 kDa) and the two phosvettes PVT1
(28 kDa) and PVT2 (26 kDa). The VGs from
O. american us are very similar to those of X.laevis, but result in different yolk proteins, probably
because of differences in proteolytic cleavage
caused by deviations in the amino acid sequence
[273]. Very little is known about the yolk proteins
of the urodelans and their ancestors. Four VGs,
which can be electrophoretically defined in the
blood plasma of Pleurodeles waltii, give at least
two different polypeptides of 210 and 180 kDa on
denaturation [37].
The quite variable results so far obtained in
investigations of yolk formation in fish do not
allow firm conclusions to be drawn. The uptake of
female-specific plasma proteins into the oocytes
of the agnathan Eptatretus has been demonstrated
immunologically [281]. However, the possibility of
hormone-regulated VG formation was not investigated in this case; corresponding experiments
with cartilaginous fish were entirely negative and
only with teleosts were the results always positive.
The molecular sizes of the yolk proteins and the
plasma precursors and apoproteins in the teleosts
show great variety. VGs of 550-600 kDa were
found in the rainbow trout Salmo gairdneri, the
flounder Pleuronectes flesus and the winter flounder Pseudopleuronectes americanus; there appear
to be several VGs of 380 kDa in the goldfish
Carassius auratus. The VG of the Japanese eel
Anguilla japonica is reported to be a 350-kDa
homotetramer with subunits of 85 kDa. The VG
ofAmeiurus nebulosus has a mass of 145 kDa and
the corresponding mRNA is also shorter than that
of the chicken and Xenopus [182, 243].
The yolk proteins are just as heterogeneous;
no less than nine VG-derived polypeptides have
been found in the egg yolk of the antarctic teleost
Caenocephalus aceratus and these have sizes
between 13 and 172 kDa and differing phosphate
contents [232]. Denaturation of the VTs of the
rainbow trout and the goldfish yield subunits of
90-140 kDa and 15-25 kDa, whilst the PV subunits have molecular masses of 7.6-15 kDa [208].
Partial proteolysis of teleost VGs seems to produce at first larger and then smaller phosphoproteins that are analogous to the phosvettes of
Xenopus. The cleavage products apparently suffer partial dephosporylation; in any case, the content of protein-bound phosphate in PVs varies
between 0.0012 % in the halibut Hippoglossus
hippoglossus and 0.73 % in the dogfish Scyliorhinus canicula [251, 263]. The yolk platelets of the
Agnatha, of the archaic fish genera Amia, Lepisosteus, Polypterus and Latimeria, and of the
freshwater teleosts are crystalline like those of the
amphibians; the yolk crystals of the Agnatha are
monoclinal whereas the others are of the orthorhombic type [139].