chapter 1 - Bentham Science

Dynamics of Storage Proteins in Lepidoptera HPFP: Recent Advances in Insects and Other Arthropods Vol. 1 33
survival and reproduction, or enter diapause during an adverse season. In particular, storage proteins
synthesized during the feeding period are expected to have roles as amino acid reservoirs in these
developmental stages and circumstances, not only in the pupal stage. Although many studies have been
carried out on the function of the fat body as the intermediate metabolic site, including synthesis of
hemolymph proteins, rather few researches have been conducted on storage protein function and dynamics
[1-4]. Therefore, in this mini-article, we will focus mostly on the latter subject, with the emphasis on the
lepidoperan species with which Tojo and colleagues have been involved.
1. CLASSIFICATION OF STORAGE PROTEINS
Different groups of storage proteins are known in insects. Most of them are storage hexamers that is, so
called hexamerins [3, 4]. These are evolutionally related to arthropod hemocyanin [4-6]. In addition,
another group of storage protein, biliverdin-binding protein, which is quite different from hexamerin in
chemical nature, is known in some species.
1.1. Hexamerin
Arylphorin
Munn and Greville [7] purified a protein called calliphorin, which is rich in phenylalanine and tyrosine,
constituting over 20 mol % of amino acid composition; it makes upto 60% of the total soluble protein in the
mature larvae of the blowfly, Calliphora erythrocephala. Subsequently, similar kinds of proteins with mol
wt of ~500 kDa, hexamer with subunits of about 70-80 kDa have been isolated from the hemolymph of
other dipterans [2]. These storage proteins are synthesized by the fat body and secreted into the hemolymph
without being maintained, so the rate of increase of these proteins in the hemolymph can be regarded as
reflecting the fat body activity for storage protein synthesis. During the feeding stage of the last instar, in
particular, storage protein level predominantly increases in the hemolymph, in some species so as to
constitute nearly 80% of soluble proteins. In most species, these proteins decline the concentration in the
hemolymph around the period of pupation, so it has been generally accepted that the fat body changes its
role from synthesis to storage, however, studies to prove their uptake into the fat body were rarely
conducted in this earlier period [2]. Clear evidence of calliphorin as the nitrogen reserve for metamorphosis
was first provided by Levenbook and Baure [8], who demonstrated labeling of all tissues of adult flies after
injection of 14 C-calliphorin into mature larvae of the blue blowfly, C. vicia. In Lepidoptera, this type of
hexamerin was first isolated from the tobacco hornworm, Manduca sexta by Telfer et al. [9], who proposed
calling this type of hexamerins, including those of calliphorin-type, as arylphorin, depending on the high
level of aromatic amino acids, namely, phenylalanine and tyrosine. Arylphorins have been identified in
most holometabolous species, so far, and in some hemimetaborous insects [3, 4]. As given in Table 1, the
contents of aromatic amino acids in arylphorin found in lepidopteran species are between 15 and 20 % mol.
At present, arylpholin is considered the predominant storage hexamerin in dipterans, but in lepidopterans
other types of hexamerins appear to be common.
Methionine-Rich Hexamerin
Second and third types of hexamerins are methionine-rich hexamerins (MtH), containing methionine at
very high (H-MtH) or a moderately high level (M-MtH). Both types were first purified from the fat body of
the Cecropia moth, Hyalophora cecropia [28], and then H-MtH-type together with arylphorin from the
silkworm, Bombyx mori [13]. These types of hexamerins were first regarded to be exceptional [2], but as
surveys have extended to other lepidopterans, these have been identified together with arylphorin in most
species surveyed. Table 2 lists up H-MtHs with methionine at 5.8-12 mol % and M-MtHs at methionine of
3.4-5.4 mol %, identified as proteins, or from cDNA in Lepidoptera. In some species for which gene
analysis is not reported, the presence of MtH-type hexamerins has been indicated by biochemical and
immunological surveys, as H-MtH in the red hairy caterpillar, Amsacta albistriga [45]. Not listed in Table
2, in the tobacco hornworm, Manduca sexta, M-MtH was identified by immunological similarity to H.
cecropia M-MtH [58], besides the female-specific hexamerin (H-MtH) [32]. H-MtH and M-MtH were also
shown to be present in a saturniid moth, Actia luna [59], and the monarch butterfly, Danaus plexippus [60].