200
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
5.7.2 Vitellogenins and Yolk Proteins
of Insects
The VG of the haemolymph and the major components of the yolk proteins of all insects so far
examined are essentially identical in terms of
molecular size and both electrophoretic and
immunological properties; in contrast to the vertebrates, the VGs of insects are not significantly
altered after their uptake into the oocytes. Many
authors, therefore, also use the term vitellogenins
to include the yolk proteins. However, there are
often differences in solubility and lipid content
between the VGs of the haemolymph and the corresponding yolk proteins and, in several cases,
proteolytic processing or aggregation to higher
polymers has also been described [27, 121, 204].
It is therefore justifiable to distinguish between
the vitellius (VTs) of the yolk protein and the
VGs of the haemolymph [69, 121].
The VGs are lipoglycoproteins with about
7-16% lipid and 1-14% carbohydrate. In many
insects (e.g. Hyalophora, Apis), VGs are the predominant haemolymph proteins found during
vitellogenesis; in others (e.g. Periplaneta, Leucophaea, Drosophila), the proportion hardly
exceeds 1 %. This is not the result of quantitative
differences in synthesis; in tracer experiments
with labelled leucine, more than 80 % was incorporated into VGs in Leucophaea as well as in Apis
and Locusta. It appears more likely that the
oocyte transport system is much more efficient in
Drosophila and the cockroaches [69]. In contrast
to the lipophorins, the VGs and the VTs contain
more phospholipid than diacylglycerol; they also
contain sterols but never hydrocarbons. The carbohydrate fractions of the VGs are mainly the
classical asparagine-bound chains Man9GlcNAez
which, in this case, can be later modified. Following inhibition of N-glycosylation by tunicamycin,
the fat bodies of the cockroach Blattella germanicus accumulate the primary translation product of
over 200 kDa. The release of VGs into haemolymph is therefore only possible after glycosylation, as has been recorded for other haemolymph
proteins of the wax moth Galleria melonella.
Conversely, Xenopus liver is able to secrete nonglycosylated VG [121, 135, 196]. The VGs and
VTs of many insects contain phosphorylated
serine residues or sulphated tyrosine residues
[10, 45, 116, 249].
The synthesis of VG takes place predominantly, or entirely, in the fat bodies. Before release
into the haemolymph, the precursor formed in
the fat bodies is converted into VG by glycosylation, phosphorylation, addition of lipid and, in
many cases, partial proteolysis. In most insects,
VG synthesis in the fat bodies is regulated by the
juvenile hormone; synthesis declines after
removal of the corpora allata and is restored by
injection of juvenile hormone or analogues
thereof. VG synthesis can also be induced in male
animals by hormone injection. In several dipterans, in addition to, or instead of, juvenile hormone, 20-hydroxyecdysone is responsible for the
control of VG synthesis [29, 35, 69, 84, 121, 135,
278]. VG is also normally found in the haemolymph of males of several insect species (Hyalophora, Tenebrio, Oncopeltus, Rhodnius). VG is
the predominant haemolymph protein in the
honey bee Apis melli/era, not only in the egglaying queens but also in freshly hatched queens
and in workers; it is missing only in the drones. In
the bumble bee Bombus terrestris, the workers
produce VG only when they lack a queen [69].
Uptake into oocytes proceeds via receptormediated endocytosis in coated pits; it was in the
oocytes of Aedes aegypti that this widely distributed mechanism was first found in 1964. Uptake
is highly specific; isolated ovaries of Hyalophora
cecropia take up only the own-species type ofVG
from a mixture of Hyalophora and Blattella VGs.
Small amounts of haemolymph proteins other
than VGs are also found in the oocytes, but only
VG is enriched by 30-fold or more. The oocyte
receptor of Locusta migratoria has been solubilized and the binding constant for VG was found
to be 4.2 . 1O--1l mol/l [211].
Insects may be subdivided into three classes
according to the molecular architecture of the
yolk proteins and their precursors, and to the in
vitro translation products of the corresponding
mRNAs [29,95, 121, 135]. In group 1, the VGs
consist of two polypeptide classes, H
(100-180 kDa) and L (43-86 kDa). As a rule,
they have a molecular mass of 440-560 kDa and
contain two each of these polypeptides according
to the formula H2~; however, the VG of Manduca sexta is only 260 kDa and has the formula
HL. During synthesis in the fat body, a precursor
of 200-260 kDa is formed and this is cleaved into
two unequal fragments prior to release into the
haemolymph. This partial proteolysis is apparently pre-programmed in the sequence of the precursor; this also occurs if the fat-body mRNA of
Locusta is translated in Xenopus oocytes [135].
Group 1 includes the mayflies, cockroaches
[31, 45, 244, 274], locusts, earworms, butterflies
[19, 127], beetles [199], bug Rhodnius prolixus
[159] and bristletail Thermobia domestica [214].
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
5.7.2 Vitellogenins and Yolk Proteins
of Insects
The VG of the haemolymph and the major components of the yolk proteins of all insects so far
examined are essentially identical in terms of
molecular size and both electrophoretic and
immunological properties; in contrast to the vertebrates, the VGs of insects are not significantly
altered after their uptake into the oocytes. Many
authors, therefore, also use the term vitellogenins
to include the yolk proteins. However, there are
often differences in solubility and lipid content
between the VGs of the haemolymph and the corresponding yolk proteins and, in several cases,
proteolytic processing or aggregation to higher
polymers has also been described [27, 121, 204].
It is therefore justifiable to distinguish between
the vitellius (VTs) of the yolk protein and the
VGs of the haemolymph [69, 121].
The VGs are lipoglycoproteins with about
7-16% lipid and 1-14% carbohydrate. In many
insects (e.g. Hyalophora, Apis), VGs are the predominant haemolymph proteins found during
vitellogenesis; in others (e.g. Periplaneta, Leucophaea, Drosophila), the proportion hardly
exceeds 1 %. This is not the result of quantitative
differences in synthesis; in tracer experiments
with labelled leucine, more than 80 % was incorporated into VGs in Leucophaea as well as in Apis
and Locusta. It appears more likely that the
oocyte transport system is much more efficient in
Drosophila and the cockroaches [69]. In contrast
to the lipophorins, the VGs and the VTs contain
more phospholipid than diacylglycerol; they also
contain sterols but never hydrocarbons. The carbohydrate fractions of the VGs are mainly the
classical asparagine-bound chains Man9GlcNAez
which, in this case, can be later modified. Following inhibition of N-glycosylation by tunicamycin,
the fat bodies of the cockroach Blattella germanicus accumulate the primary translation product of
over 200 kDa. The release of VGs into haemolymph is therefore only possible after glycosylation, as has been recorded for other haemolymph
proteins of the wax moth Galleria melonella.
Conversely, Xenopus liver is able to secrete nonglycosylated VG [121, 135, 196]. The VGs and
VTs of many insects contain phosphorylated
serine residues or sulphated tyrosine residues
[10, 45, 116, 249].
The synthesis of VG takes place predominantly, or entirely, in the fat bodies. Before release
into the haemolymph, the precursor formed in
the fat bodies is converted into VG by glycosylation, phosphorylation, addition of lipid and, in
many cases, partial proteolysis. In most insects,
VG synthesis in the fat bodies is regulated by the
juvenile hormone; synthesis declines after
removal of the corpora allata and is restored by
injection of juvenile hormone or analogues
thereof. VG synthesis can also be induced in male
animals by hormone injection. In several dipterans, in addition to, or instead of, juvenile hormone, 20-hydroxyecdysone is responsible for the
control of VG synthesis [29, 35, 69, 84, 121, 135,
278]. VG is also normally found in the haemolymph of males of several insect species (Hyalophora, Tenebrio, Oncopeltus, Rhodnius). VG is
the predominant haemolymph protein in the
honey bee Apis melli/era, not only in the egglaying queens but also in freshly hatched queens
and in workers; it is missing only in the drones. In
the bumble bee Bombus terrestris, the workers
produce VG only when they lack a queen [69].
Uptake into oocytes proceeds via receptormediated endocytosis in coated pits; it was in the
oocytes of Aedes aegypti that this widely distributed mechanism was first found in 1964. Uptake
is highly specific; isolated ovaries of Hyalophora
cecropia take up only the own-species type ofVG
from a mixture of Hyalophora and Blattella VGs.
Small amounts of haemolymph proteins other
than VGs are also found in the oocytes, but only
VG is enriched by 30-fold or more. The oocyte
receptor of Locusta migratoria has been solubilized and the binding constant for VG was found
to be 4.2 . 1O--1l mol/l [211].
Insects may be subdivided into three classes
according to the molecular architecture of the
yolk proteins and their precursors, and to the in
vitro translation products of the corresponding
mRNAs [29,95, 121, 135]. In group 1, the VGs
consist of two polypeptide classes, H
(100-180 kDa) and L (43-86 kDa). As a rule,
they have a molecular mass of 440-560 kDa and
contain two each of these polypeptides according
to the formula H2~; however, the VG of Manduca sexta is only 260 kDa and has the formula
HL. During synthesis in the fat body, a precursor
of 200-260 kDa is formed and this is cleaved into
two unequal fragments prior to release into the
haemolymph. This partial proteolysis is apparently pre-programmed in the sequence of the precursor; this also occurs if the fat-body mRNA of
Locusta is translated in Xenopus oocytes [135].
Group 1 includes the mayflies, cockroaches
[31, 45, 244, 274], locusts, earworms, butterflies
[19, 127], beetles [199], bug Rhodnius prolixus
[159] and bristletail Thermobia domestica [214].