392
11 Extracellular Structural and Secretory Proteins
values of 3-6 [118]. The amino acid composition
of the cuticula proteins is always rather biased;
this is seen, for example, in the particularly wellinvestigated protein number 38 of Locusta, in
which only 14 of the 20 common protein amino
acids are found. The proteins are consistently rich
in alanine and/or glycine, but are low in charged
amino acids and often entirely lacking in methionine and cysteine [3, 65). Similarities in the
amino acid composition and distribution are
found in both the sclerotinized proteins (oothecins) of cockroach egg capsules ( ootheca) and the
egg-membrane proteins (chorion proteins) of flies
and moths. This does not necessarily indicate
phylogenetic relationships between the proteins
but, more likely, suggests similar functional
adaptations. With the exception of sclerotization,
the cuticula proteins are subject to little posttranslational alteration [22].
Only a few cuticula proteins have been
sequenced so far, e.g. those of the locust Locusta
migratoria, the lepidopterans Bombyx mori,
Manduca sexta and Hyalophora cecropia, and the
dipterans Drosophila melanogaster and Sarcophaga bullata [53, 54, 88, 104, 136]. The data
show quite clearly that the cuticula proteins do
not belong to any of the known protein superfamilies. The partial sequences from the two fly
species agree at 50-60 % of positions, but have
no significant homology to the locust proteins. A
cuticula-gene cluster found in the region 44D of
Drosophila melanogaster chromosome 2 includes
four active genes (I-IV) and one pseudogene.
The active genes are organized in pairs with the
genes of each pair being read in opposite directions. The encoded amino acid sequences agree by
95 % (1/11) and 33 % (1/111). There is significant
sequence agreement between the cuticula proteins of the lepidoptera Manduca and Hyalophora
and those of the Diptera Drosophila and Sarcophaga [104, 136]. A pupal cuticula protein from
the silk moth Bombyx mori, sequenced via the
cDNA, contains nine repeats of the sequence
APAX1X2X3WN in its total sequence of 253
amino acids [88]. Little is known about the cuticular proteins of other arthropod groups. Numerous acidic proteins of 10-70 kDa have been
extracted from the cuticula of the crustacean
Scylla serrata; the cuticula of the crayfish Astacus
leptodactylus contains mainly larger proteins
which do not differ significantly between soft and
calcareous cuticulae. Similarly, the cuticula proteins from different body regions of the shrimp Pandalus borealis appear the same on 2-D electropherograms [4].
11.8 Chorion Proteins
The insect egg is surrounded by an integument
which must fulfil several functional requirements.
It must allow entry of spermatozoa, and for this
there are one or more narrow canals (micropyles). It must be permeable to respiratory gases,
and an example here is the "aeropyles" of silkworm eggs; these are arranged either in bands
(Antheraea polyphemus) or over the entire egg
surface (A. pernyi), and in some species they are
surrounded by a small crown. The egg membrane
must be flexible enough for the egg-laying process
to occur but must be sufficiently protective, especially during the diapause of the winter months.
This membrane is the secreted product of several
thousand cells (follicle epithelium) which surround the oocyte together with its nutritional
cells. Structurally, an inner thin vitellin membrane, the composition of which is not entirely
known, can be distinguished from the outer chorion, which consists almost entirely of proteins.
The chorion has a predominantly lamellar structure, the lamellae being two-dimensional networks of protein fibrils. Formation of the 60-f..I.mthick chorion of Antheraea polyphemus requires
about 2 days, and the 0.8-f..I.ID-thick chorion of
Drosophila melanogaster is synthesized in only
5-6 h; the chorion of Bombyx mori is intermediate between these two extremes and has a thickness of 20-25 f..I.ID. During choriogenesis, the follicle· epithelial cells are given over entirely to the
production of chorion proteins. These proteins,
together with the corresponding genes and their
regulation, were studied first in the tussah moth
Antheraea polyphemus and later in Bombyx mori
and Drosophila melanogaster. The chorion proteins of the silk moth have a characteristic composition of 30-37 % glycine, high proportions of alanine, valine, leucine and tyrosine, and 6-12 %
cysteine. The corresponding Drosophila proteins
have only 16 % glycine and 0.6 % cysteine and,
instead, are high in charged, especially basic
amino acids. Thus, the chorion proteins of the
flies are at best only distantly related to those of
the moths [105].
SDS electrophoresis separates the chorion proteins of Antheraea pernyi into four classes with
different molecular masses: 38 % A proteins
(9-12 kDa), 50% B (12-14 kDa), 9% C
(16-20 kDa) and trace amounts of the much larger D proteins. A fifth type, the E protein, is
unusual in that it is not involved in the crosslinking of A-D and can therefore still be
11 Extracellular Structural and Secretory Proteins
values of 3-6 [118]. The amino acid composition
of the cuticula proteins is always rather biased;
this is seen, for example, in the particularly wellinvestigated protein number 38 of Locusta, in
which only 14 of the 20 common protein amino
acids are found. The proteins are consistently rich
in alanine and/or glycine, but are low in charged
amino acids and often entirely lacking in methionine and cysteine [3, 65). Similarities in the
amino acid composition and distribution are
found in both the sclerotinized proteins (oothecins) of cockroach egg capsules ( ootheca) and the
egg-membrane proteins (chorion proteins) of flies
and moths. This does not necessarily indicate
phylogenetic relationships between the proteins
but, more likely, suggests similar functional
adaptations. With the exception of sclerotization,
the cuticula proteins are subject to little posttranslational alteration [22].
Only a few cuticula proteins have been
sequenced so far, e.g. those of the locust Locusta
migratoria, the lepidopterans Bombyx mori,
Manduca sexta and Hyalophora cecropia, and the
dipterans Drosophila melanogaster and Sarcophaga bullata [53, 54, 88, 104, 136]. The data
show quite clearly that the cuticula proteins do
not belong to any of the known protein superfamilies. The partial sequences from the two fly
species agree at 50-60 % of positions, but have
no significant homology to the locust proteins. A
cuticula-gene cluster found in the region 44D of
Drosophila melanogaster chromosome 2 includes
four active genes (I-IV) and one pseudogene.
The active genes are organized in pairs with the
genes of each pair being read in opposite directions. The encoded amino acid sequences agree by
95 % (1/11) and 33 % (1/111). There is significant
sequence agreement between the cuticula proteins of the lepidoptera Manduca and Hyalophora
and those of the Diptera Drosophila and Sarcophaga [104, 136]. A pupal cuticula protein from
the silk moth Bombyx mori, sequenced via the
cDNA, contains nine repeats of the sequence
APAX1X2X3WN in its total sequence of 253
amino acids [88]. Little is known about the cuticular proteins of other arthropod groups. Numerous acidic proteins of 10-70 kDa have been
extracted from the cuticula of the crustacean
Scylla serrata; the cuticula of the crayfish Astacus
leptodactylus contains mainly larger proteins
which do not differ significantly between soft and
calcareous cuticulae. Similarly, the cuticula proteins from different body regions of the shrimp Pandalus borealis appear the same on 2-D electropherograms [4].
11.8 Chorion Proteins
The insect egg is surrounded by an integument
which must fulfil several functional requirements.
It must allow entry of spermatozoa, and for this
there are one or more narrow canals (micropyles). It must be permeable to respiratory gases,
and an example here is the "aeropyles" of silkworm eggs; these are arranged either in bands
(Antheraea polyphemus) or over the entire egg
surface (A. pernyi), and in some species they are
surrounded by a small crown. The egg membrane
must be flexible enough for the egg-laying process
to occur but must be sufficiently protective, especially during the diapause of the winter months.
This membrane is the secreted product of several
thousand cells (follicle epithelium) which surround the oocyte together with its nutritional
cells. Structurally, an inner thin vitellin membrane, the composition of which is not entirely
known, can be distinguished from the outer chorion, which consists almost entirely of proteins.
The chorion has a predominantly lamellar structure, the lamellae being two-dimensional networks of protein fibrils. Formation of the 60-f..I.mthick chorion of Antheraea polyphemus requires
about 2 days, and the 0.8-f..I.ID-thick chorion of
Drosophila melanogaster is synthesized in only
5-6 h; the chorion of Bombyx mori is intermediate between these two extremes and has a thickness of 20-25 f..I.ID. During choriogenesis, the follicle· epithelial cells are given over entirely to the
production of chorion proteins. These proteins,
together with the corresponding genes and their
regulation, were studied first in the tussah moth
Antheraea polyphemus and later in Bombyx mori
and Drosophila melanogaster. The chorion proteins of the silk moth have a characteristic composition of 30-37 % glycine, high proportions of alanine, valine, leucine and tyrosine, and 6-12 %
cysteine. The corresponding Drosophila proteins
have only 16 % glycine and 0.6 % cysteine and,
instead, are high in charged, especially basic
amino acids. Thus, the chorion proteins of the
flies are at best only distantly related to those of
the moths [105].
SDS electrophoresis separates the chorion proteins of Antheraea pernyi into four classes with
different molecular masses: 38 % A proteins
(9-12 kDa), 50% B (12-14 kDa), 9% C
(16-20 kDa) and trace amounts of the much larger D proteins. A fifth type, the E protein, is
unusual in that it is not involved in the crosslinking of A-D and can therefore still be