chapter 1 - Bentham Science

Hemolymph Lipoproteins: Role in Insect Reproduction HPFP: Recent Advances in Insects and Other Arthropods Vol. 1 5
Until now, Vg primary proteins have been reported from several insect species (see for references a recent review
by Tufail and Takeda [15]. Vgs are members of a larger superfamily of molecules known as large lipid transfer
proteins [50]. Despite their overall low conservation across major taxonomic groups, their common function and
disproportionate conservation of certain amino acids suggest a common phylogenetic ancestor [15, 13, 51].
2.3. Structural Characteristics
The Vg sequences are now available from 31 insect species (see 2.4, below). The sequence alignment shows
that they can be aligned confidently along their entire lengths, with five particular subdomains (data not shown)
where the sequences are well conserved [2, 15, 51, 52]. The most striking feature of all insect Vgs is the
existence of polyserine stretches, which are at least conserved at the N-terminus of almost all insect Vgs [except
Anthonomus grandis [53], Lymantria dispar [54], Pteromalus puparum [55] and Spodoptera litura [56]
sequenced outside the apocritan Hymenoptera (data not shown). The cockroach, brown planthopper and
mosquito Vgs are still unique in harboring these domains also at the C-termini [14, 15, 33-35, 57, 59].
However, the role of polyserine regions remains unclear. Their presence might serve as good substrates for
kinases. In P. americana and L. maderae, the polyserine domains were found to be highly phosphorylated [14,
34, 59, 60] Phosphoserine tract represents extreme concentrations of negative charge [61], which may promote
solubility of the Vg [62] or provide a region for chelating essential metal ions such as Ca 2+ and Fe 3+ [63, 64].
There is evidence that dephosphorylation of Vg reduces its uptake by oocytes [65, 66] which suggests that
phosphorylated residues may contribute to the interaction between Vg and its receptor (VgR).
The comparison of consensus cleavage site motif (RXXR) indicates that its position is particularly conserved
near the N-terminus in nearly all non-apocritan insect species [see review: 15], and is flanked by polyserine
domains (data not shown), except L. dispar Vg where the cleavage signal (RXXR) exists at the C-terminus [see
reviews: 2, 14, 15]. The cleavage in these sites would give polypeptides corresponding to the large and small
subunits as is the case of most insect Vgs [see reviews: 14, 15]. Neverthless, in the Hemimetabola members
(like cockroaches and the bean bug, R. clavatus) where the pro-Vg is cleaved into several subunits, additional
RXXR cleavage motifs resided near the C-terminus or at the center of the primary product are utilized [15, 34,
45]. The cleavage is accomplished by the subtilisin-like endoproteases, proprotein convertases [67, 68], which
recognize a consensus R/KXXR/K sequence motif immediately preceding the cleavage site. However, the
reason why Vgs are differentially cleaved in different insect species remains unclear.
The other common features emerged in insect Vgs include the existence of a GL/ICG motif and cysteine
residues that follow at conserved locations near the C-termini [15, 33, 52, 69, 70]. In addition, a DGXR motif
has also been observed 17-19 residues upstream of the GL/ICG in all insect Vgs except for that of L. maderae
Vgs [34, 60]. In Nilaparvata lugens, Homalodisca coagulata and Solenopsis invicta arginine was missing [see
15]. The GL/ICG motif and cysteine residues were necessary for oligomerization of vertebrate Vns [71, 72]. It
was also demonstrated that the DG residues located near the cysteine-rich repeat contributed to establishing
appropriate folding of the LDL receptor protein [73]. One possible function of insect Vns oligomerized in eggs
would be binding lipids, in which inactive ecdysteroids are enclosed. The ecdysteroids are then released as Vns
and undergo proteolysis during embryogenesis [11, 12]. It was proposed that the DG residues (of DGXR motif)
together with the GL/ICG motif and cysteine residues might participate in forming the structure necessary for
Vns to function properly during embryogenesis [33].
2.4. Phylogenetic Relationship
The phylogenetic relationship of 60 Vgs from 46 arthropod and non-arthropod species was examined based
on the distances of Vg sequences (excluding the signal peptide) conducting a phylogenetic analysis using a
neighbbour-joining tree construction method with MEGA version 3.1 [74] (Fig. 1). The Vg sequences used
were from the following species (accession number in parentheses): Periplaneta americana (Vg1:
AB034804; Vg2: AB047401), Leucophaea maderae (Vg1: AB052640; Vg2: AB194976), Blattella
germanica (AJ005115), Lethocerus deyrollei (AB425334), Graptopsaltria nigrofuscata (AB026848),
Plautia stali (Vg1: AB033498; Vg2: AB033499; Vg3: AB033500), Riptortus clavatus (U97277),
Homalodisca coagulata (DQ118408), Nilaparvata lugens (AB353856), Anthonomus grandis (M72980),