5.7.1 Vitellogenins and Yolk Proteins of Vertebrates
197
cylglycerol they contain. This dependence of
receptor affinity on lipid content facilitates lipid
transport from the mid-gut to the fat bodies in the
feeding larvae [259].
In addition to the lipophorins, further speciesand stage-specific lipoproteins are found in insect
haemolymph; at least seven different types can
be distinguished electrophoretically [39, 121].
Vitellogenins play a dominant role in mature
females. A VHDL found in the larva of the honey
bee contains only 10 % lipid and 2.6 % carbohydrate; this is a homodimer with 160-kDa subunits and has no similarity to any other known
haemolymph protein [224, 233]. In the last larval
stage of the butterfly Heliothis zea, the haemolymph takes on a green colour; this is caused by
the appearance of a homotetrameric protein of
560 kDa which contains 8.4 % lipid and biliverdin
as the colour component. The protein, for which
there are no known parallels, is not found in
young larvae, pupae or adults [97, 224].
Very little is known about the lipoproteins in
the haemolymph of other arthropods. All the
examined chilopods, diplopods and arachnids
(scorpions, araneae, solifugae) have two
lipophorin-like lipoproteins in the haemolymph
and these are made up of two types of subunit
with molecular masses of 220-250 and
80-90 kDa. A completely different type of haemolymph lipoprotein with polypeptides of about
100 kDa is found in the crustaceans. Surprisingly,
the lipoprotein of the velvet mite Dinothrombium
pando rae is more similar to that of the crustaceans than to that of other arachnids [99, 141].
Lipoproteins have been detected occasionally in
the body fluids of molluscs and other invertebrates by specific staining reactions on electrophoregrams but have not been investigated further.
5.7 Vitellogenins and Yolk Proteins
The quantity of yolk in the eggs of most nonmammalian species and many invertebrates is so
great that it cannot all be synthesized in the
oocyte alone. In these cases, lipoproteins are synthesized as precursors of yolk proteins outside of
the ovary, transported in the blood to the oocyte
and there incorporated into the yolk. These lipoproteins are known as vitellogenins (VGs); their
formation is mostly under hormonal control. The
proteins are significantly altered between their
synthesis and their incorporation into the yolk
structure of the oocyte; the nature of this posttranslational modification varies greatly between
different groups of animals. Vertebrate VGs
remain almost unchanged from their synthesis in
the liver until they are taken up into the oocyte;
they are then converted by proteolytic cleavage,
phosphorylation and other processes into two
fundamentally different types of yolk protein
(lipovitellogenins and phosvitins).
In many insects, the primary translation products of the VG genes are already greatly modified
by partial proteolysis, glycosylation, phosphorylation and other post-translational processes in
the fat body before they enter the haemolymph;
only minor changes occur after uptake into the
oocytes. The comparative biochemistry of the
VGs is a particularly attractive subject because
their evolution is determined by so many structural/functional requirements: secretion out of
VG-producing cells, transport in the blood
plasma or haemolymph with protection from
hydrolysis, specific recognition and uptake into
oocytes, chemical changes in the oocytes, and
incorporation into the yolk structure, e.g. the
crystalline yolk platelets of the lower vertebrates.
Their investigation, however, is hindered by the
fact that they are usually large and complicated
glycolipophosphoprotein complexes. The VGs
are apparently quite old proteins; at least in the
case of the chicken, the clawed frog Xenopus laevis, the sea urchin Strongylocentrotus purpuratus
and the nematode Caenorhabditis elegans the
similarity in VG gene structure and sequence
indicates a common origin [185, 234]. There is
significant homology between partial VG sequences of the chicken, frog or Caenorhabditis
and the human apoB-100, as well as between
Drosophila VG and human lipoprotein lipase.
This suggests that apolipoproteins and the lipase
evolved from a vitellogenin-like precursor [13].
Use of the yolk proteins during embryogenesis of
vertebrates and arthropods involves lysosomal
enzymes that are under the control of specific
protease inhibitors [70, 163].
5.7.1 ViteUogenins and Yolk Proteins
of Vertebrates
The process of yolk formation has been very conservative throughout the evolution of the vertebrates: in all oviparous amphibians, reptiles and
birds dimeric VGs of 400-600 kDa are synthesized under oestrogen control in the liver, secreted into the blood and taken up by the oocytes,
where they undergo proteolytic cleavage and are
197
cylglycerol they contain. This dependence of
receptor affinity on lipid content facilitates lipid
transport from the mid-gut to the fat bodies in the
feeding larvae [259].
In addition to the lipophorins, further speciesand stage-specific lipoproteins are found in insect
haemolymph; at least seven different types can
be distinguished electrophoretically [39, 121].
Vitellogenins play a dominant role in mature
females. A VHDL found in the larva of the honey
bee contains only 10 % lipid and 2.6 % carbohydrate; this is a homodimer with 160-kDa subunits and has no similarity to any other known
haemolymph protein [224, 233]. In the last larval
stage of the butterfly Heliothis zea, the haemolymph takes on a green colour; this is caused by
the appearance of a homotetrameric protein of
560 kDa which contains 8.4 % lipid and biliverdin
as the colour component. The protein, for which
there are no known parallels, is not found in
young larvae, pupae or adults [97, 224].
Very little is known about the lipoproteins in
the haemolymph of other arthropods. All the
examined chilopods, diplopods and arachnids
(scorpions, araneae, solifugae) have two
lipophorin-like lipoproteins in the haemolymph
and these are made up of two types of subunit
with molecular masses of 220-250 and
80-90 kDa. A completely different type of haemolymph lipoprotein with polypeptides of about
100 kDa is found in the crustaceans. Surprisingly,
the lipoprotein of the velvet mite Dinothrombium
pando rae is more similar to that of the crustaceans than to that of other arachnids [99, 141].
Lipoproteins have been detected occasionally in
the body fluids of molluscs and other invertebrates by specific staining reactions on electrophoregrams but have not been investigated further.
5.7 Vitellogenins and Yolk Proteins
The quantity of yolk in the eggs of most nonmammalian species and many invertebrates is so
great that it cannot all be synthesized in the
oocyte alone. In these cases, lipoproteins are synthesized as precursors of yolk proteins outside of
the ovary, transported in the blood to the oocyte
and there incorporated into the yolk. These lipoproteins are known as vitellogenins (VGs); their
formation is mostly under hormonal control. The
proteins are significantly altered between their
synthesis and their incorporation into the yolk
structure of the oocyte; the nature of this posttranslational modification varies greatly between
different groups of animals. Vertebrate VGs
remain almost unchanged from their synthesis in
the liver until they are taken up into the oocyte;
they are then converted by proteolytic cleavage,
phosphorylation and other processes into two
fundamentally different types of yolk protein
(lipovitellogenins and phosvitins).
In many insects, the primary translation products of the VG genes are already greatly modified
by partial proteolysis, glycosylation, phosphorylation and other post-translational processes in
the fat body before they enter the haemolymph;
only minor changes occur after uptake into the
oocytes. The comparative biochemistry of the
VGs is a particularly attractive subject because
their evolution is determined by so many structural/functional requirements: secretion out of
VG-producing cells, transport in the blood
plasma or haemolymph with protection from
hydrolysis, specific recognition and uptake into
oocytes, chemical changes in the oocytes, and
incorporation into the yolk structure, e.g. the
crystalline yolk platelets of the lower vertebrates.
Their investigation, however, is hindered by the
fact that they are usually large and complicated
glycolipophosphoprotein complexes. The VGs
are apparently quite old proteins; at least in the
case of the chicken, the clawed frog Xenopus laevis, the sea urchin Strongylocentrotus purpuratus
and the nematode Caenorhabditis elegans the
similarity in VG gene structure and sequence
indicates a common origin [185, 234]. There is
significant homology between partial VG sequences of the chicken, frog or Caenorhabditis
and the human apoB-100, as well as between
Drosophila VG and human lipoprotein lipase.
This suggests that apolipoproteins and the lipase
evolved from a vitellogenin-like precursor [13].
Use of the yolk proteins during embryogenesis of
vertebrates and arthropods involves lysosomal
enzymes that are under the control of specific
protease inhibitors [70, 163].
5.7.1 ViteUogenins and Yolk Proteins
of Vertebrates
The process of yolk formation has been very conservative throughout the evolution of the vertebrates: in all oviparous amphibians, reptiles and
birds dimeric VGs of 400-600 kDa are synthesized under oestrogen control in the liver, secreted into the blood and taken up by the oocytes,
where they undergo proteolytic cleavage and are