Biology Reference
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inherent ability of proteins and peptides to form amyloid structures
and generate novel and diverse biological functions.
2-4
This was
noted after our proposal that natural, protective amyloids do, indeed,
exist.
5
Apparently, there is no similarity between amyloidogenic
proteins or peptides in terms of amino acid sequence, molecular
weight, morphology, or surrounding conditions.
2
In addition, many
proteins, not implicated in amyloidoses, also called conformational
diseases, have also been shown to form amyloids
1,6
which
led to the hypothesis that the potential for amyloidogenesis may be
a near universal feature of proteins
in vitro
,
6
. However, recent data indicate
that amyloid formation is inherent in short stretches of amino
acid sequences, called amyloidogenic determinants
7,8
and we have
shown that it was derived, in certain cases, after millions of years of
molecular evolution.
5,9
The insect eggshell (chorion and vitelline membrane) and the
associated follicular epithelium, which secretes its constituent
layers, have been the subject of numerous investigations at the
cellular and molecular level. These structures have provided a model
system in several areas of current biological research, for example,
for the physiology of the eggshell layers and morphogenesis of
supramolecular structure, control of gene expression in diferen-
tiating cells, evolution of multigene families, and structural protein
folding and organization.
10,12
The ovary of an adult female silkmoth
usually consists of ovarioles, that is, strings of follicles attached to
each other in linear arrays (Fig. 4.1a). The schematic diagram of each
follicle is shown in Fig. 4.1b. The follicular cells, which surround the
oocyte, synthesize, and secrete, according to a precise spatial and
temporal program, a set of structural proteins onto the surface of
the oocyte, which self-assemble to form the multilayered eggshell
(Fig. 4.1b).
10,12
The major component (90-95%) of the eggshell of many insect
and fish eggs is chorion. Proteins account for more than 95% of its dry
mass. This proteinaceous shell forms the outer layer of the eggshell
and has extraordinary mechanical and physiological properties,
protecting the oocyte and the developing embryo from a series of
environmental hazards such as temperature variations, mechanical
pressures, proteases, bacteria, and viruses.
10,12
It also allows for
sperm entry and fertilization and for the exchange of the respiratory
gases.
10,12
Figures 4.2 and 4.3 show electron micrographs of thin
transverse sections of a silkmoth chorion. A lamellar ultrastructure
of packed fibrils is seen: silkmoth chorion is a biological analogue of
a cholesteric liquid crystal.
11,12
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