198
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
used in the production of two types of yolk protein, the vitellins (VIS) and phosvitins (PVs). In
addition to the PVs, there are often other smaller
phosphoproteins, the phosvettes (PVT); PVs and
PVTs are probably alternative cleavage products
of the same VG region. The monotreme mammals, which produce large, abundantly yolked
eggs, have unfortunately not yet been investigated. However, the formation of yolk proteins has
been particularly well-described for the chicken
and the clawed frog Xenopus laevis. General
questions relating to cell biology and biochemistry, e.g. about the glycosylation and phosphorylation of proteins, have been studied using
these model systems. A comparison of the gene
sequences of VG-II from chicken and VG-A2 of
Xenopus illustrates the homology of the vertebrate VGs. The two genes are very alike in structure with 35 often quite similar exons; in other
regions the bird and frog genes are very different,
in particular in the PV region, where numerous
nucleotide exchanges and segment mutations are
recognizable [186].
The VGs of the domestic chicken make up a
family of related proteins encoded by different
genes. They are homodimers with subunits
between 170 and 190 kDa whose molecular mass
is increased by about 10 kDa through glycosylation and phosphorylation. The main components,
VG-I and VG-II, are very similar in their amino
acid composition and each contains 116 phosphate residues. The gene of the minor component
VG-III shows major differences to the VG-II
gene in the region coding for the phosphoproteins
PV and PVT: the agreement in amino acid
sequence between VG-II and VG-III in this
region is only 23 % compared with 40 % for the
whole protein; exon 23 is reduced in length by
more than half and the number of serine codons,
and therefore phosphate-binding sites, is drastically reduced. As a result, VG-III carries only 44
phosphate residues [33]. Three VGs are also distinguishable in the quail Coturnix eoturnix. The
VG receptor on the surface of chicken oocytes
has been solubilized and characterized in detail
[241].
Centrifugation of diluted chicken-egg yolk
leads to the sedimentation of granulae which
make up approximately 20 % of the dry weight.
They correspond to the yolk platelets of the lower
vertebrates but are not crystalline. The major
components of these granulae are two pairs of
VTs and PVs derived from the two major VGs.
The VTs are tetramers of two each of the subunits
VT-I (125 kDa) and VT-II (30 kDa); the PVs
have molecular masses of 34 kDa (with 104
phosphate residues) and 28 kDa (with 85 phosphate residues). In the supernatant there is a lipoprotein fraction with a density of 0.95 glcm 3 ; this
constitutes 70 % of the yolk dry weight and shows
immunological similarity to the plasma VLDL.
The lipid requirement for yolk formation in birds
can apparently not be satisfied by VGs alone; a
considerable part of the yolk lipid originates in
the VLDL of the blood plasma. However, only a
small apoprotein of the plasma VLDL, and not
apoB, is found unchanged in the yolk as apo VT-I.
It is likely that fragments of apoB are present in
the yolk, in particular in apoVT-IVand apoVTVI, which together have approximately the same
size as apoB. There are also proteins present in
the yolk which do not come from the plasma, e.g.
apoVT-III and apoVT-V. The apoVT-I proteins of
many birds have been sequenced and they show a
relatively high rate of evolution, differing
between the chicken and the turkey, for example,
in 12 of the 82 amino acids [118]. Apart from the
VLDL fraction, the yolk supernatant also contains the so-called livetins, a mixture of plasma
proteins (serum albumin, transferrins, yglobulins, etc.) which makes up the remaining
10 % of the yolk. Thus, uptake into the oocyte is
apparently not specifically restricted to VG and
VLDL.
Little is known about the VGs and the yolk
proteins of the reptiles. The VG induced by 17~estradiol in the snake Thamnophis sirtalis does
not have the homodimer structure of other vertebrate VGs but is constructed from a VG2 chain
of 124 kDa bound by a disulphide bridge to a
VGl chain of 149 kDa. In the turtle Chrysemys
pieta, on the other hand, there is a normal homodimeric VG with subunits of 210-220 kDa [18].
The predominant yolk protein of the lizard Anolis
pulehellus, like the VT-I of chicken and Xenopus,
has a molecular mass of 110-120 kDa [172].
The liver of sexually mature females of the
clawed frog Xenopus continuously produces VGs,
whereas in the male VG is induced by oestrogens.
The VGs have molecular masses of 460 kDa and
contain 12 % lipid, 1.3 % protein-bound phosphorous, 1 % carbohydrate and two identical
polypeptides of about 200 kDa. X. laevis possesses four active VG genes coding for two VG pairs,
AlIA2 and BIIB2. The amino acid sequences
vary at about 20 % of positions between the pairs
and at about 5 % within each pair. The genes for
AI-A2-B2 are clustered, but Bl is located separately. Taking into account that X. laevis is a tetraploid species, the evolution of the VG genes
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
used in the production of two types of yolk protein, the vitellins (VIS) and phosvitins (PVs). In
addition to the PVs, there are often other smaller
phosphoproteins, the phosvettes (PVT); PVs and
PVTs are probably alternative cleavage products
of the same VG region. The monotreme mammals, which produce large, abundantly yolked
eggs, have unfortunately not yet been investigated. However, the formation of yolk proteins has
been particularly well-described for the chicken
and the clawed frog Xenopus laevis. General
questions relating to cell biology and biochemistry, e.g. about the glycosylation and phosphorylation of proteins, have been studied using
these model systems. A comparison of the gene
sequences of VG-II from chicken and VG-A2 of
Xenopus illustrates the homology of the vertebrate VGs. The two genes are very alike in structure with 35 often quite similar exons; in other
regions the bird and frog genes are very different,
in particular in the PV region, where numerous
nucleotide exchanges and segment mutations are
recognizable [186].
The VGs of the domestic chicken make up a
family of related proteins encoded by different
genes. They are homodimers with subunits
between 170 and 190 kDa whose molecular mass
is increased by about 10 kDa through glycosylation and phosphorylation. The main components,
VG-I and VG-II, are very similar in their amino
acid composition and each contains 116 phosphate residues. The gene of the minor component
VG-III shows major differences to the VG-II
gene in the region coding for the phosphoproteins
PV and PVT: the agreement in amino acid
sequence between VG-II and VG-III in this
region is only 23 % compared with 40 % for the
whole protein; exon 23 is reduced in length by
more than half and the number of serine codons,
and therefore phosphate-binding sites, is drastically reduced. As a result, VG-III carries only 44
phosphate residues [33]. Three VGs are also distinguishable in the quail Coturnix eoturnix. The
VG receptor on the surface of chicken oocytes
has been solubilized and characterized in detail
[241].
Centrifugation of diluted chicken-egg yolk
leads to the sedimentation of granulae which
make up approximately 20 % of the dry weight.
They correspond to the yolk platelets of the lower
vertebrates but are not crystalline. The major
components of these granulae are two pairs of
VTs and PVs derived from the two major VGs.
The VTs are tetramers of two each of the subunits
VT-I (125 kDa) and VT-II (30 kDa); the PVs
have molecular masses of 34 kDa (with 104
phosphate residues) and 28 kDa (with 85 phosphate residues). In the supernatant there is a lipoprotein fraction with a density of 0.95 glcm 3 ; this
constitutes 70 % of the yolk dry weight and shows
immunological similarity to the plasma VLDL.
The lipid requirement for yolk formation in birds
can apparently not be satisfied by VGs alone; a
considerable part of the yolk lipid originates in
the VLDL of the blood plasma. However, only a
small apoprotein of the plasma VLDL, and not
apoB, is found unchanged in the yolk as apo VT-I.
It is likely that fragments of apoB are present in
the yolk, in particular in apoVT-IVand apoVTVI, which together have approximately the same
size as apoB. There are also proteins present in
the yolk which do not come from the plasma, e.g.
apoVT-III and apoVT-V. The apoVT-I proteins of
many birds have been sequenced and they show a
relatively high rate of evolution, differing
between the chicken and the turkey, for example,
in 12 of the 82 amino acids [118]. Apart from the
VLDL fraction, the yolk supernatant also contains the so-called livetins, a mixture of plasma
proteins (serum albumin, transferrins, yglobulins, etc.) which makes up the remaining
10 % of the yolk. Thus, uptake into the oocyte is
apparently not specifically restricted to VG and
VLDL.
Little is known about the VGs and the yolk
proteins of the reptiles. The VG induced by 17~estradiol in the snake Thamnophis sirtalis does
not have the homodimer structure of other vertebrate VGs but is constructed from a VG2 chain
of 124 kDa bound by a disulphide bridge to a
VGl chain of 149 kDa. In the turtle Chrysemys
pieta, on the other hand, there is a normal homodimeric VG with subunits of 210-220 kDa [18].
The predominant yolk protein of the lizard Anolis
pulehellus, like the VT-I of chicken and Xenopus,
has a molecular mass of 110-120 kDa [172].
The liver of sexually mature females of the
clawed frog Xenopus continuously produces VGs,
whereas in the male VG is induced by oestrogens.
The VGs have molecular masses of 460 kDa and
contain 12 % lipid, 1.3 % protein-bound phosphorous, 1 % carbohydrate and two identical
polypeptides of about 200 kDa. X. laevis possesses four active VG genes coding for two VG pairs,
AlIA2 and BIIB2. The amino acid sequences
vary at about 20 % of positions between the pairs
and at about 5 % within each pair. The genes for
AI-A2-B2 are clustered, but Bl is located separately. Taking into account that X. laevis is a tetraploid species, the evolution of the VG genes