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Proteolytic processing of the vitellogenin precursor in the boll weevil Anthonomus grandis.

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Archives of Insect Biochemistry and Physiology 23:125-134 (1 993)
Proteolytic Processing of the Vitellogenin
Precursor in the Boll Weevil, Anthonornus
grandis
Larry J.
Heilmann, Patrick M. Trewitt, and A. Krishna Kumaran
Biosciences Research Laboratory, U.S. Department of Agriculture, Agricultural Research
Service, Fargo, North Dakota (L.J.H . ); Department of Biology, Marquette University,
Milwaukee, Wisconsin ( P . M .T., A .K . K . )
The soluble proteins of the eggs of the coleopteran insect Anthonornus grandis
Boheman, the cotton boll weevil, consist almost entirely of two vitellin types with
M,s of 160,000 and 47,000. We sequenced their N-terminal ends and one internal
cyanogen bromide fragment of the large vitellin and compared these sequences
with the deduced amino acid sequence from the vitellogenin gene. The results
suggest that both the boll weevil vitellin proteins are products of the proteolytic
cleavage of a single precursor protein. The smaller 47,000 h$ vitellin protein i s
derived from the N-terminal portion of the precursor adjacent to an 18 amino
acid signal peptide. The cleavage site between the large and small vitellins at
amino acid 362 is adjacent to a pentapeptide sequence containing two pairs of
arginine residues. Comparison of the boll weevil sequences with limited known
sequences from the single 180,000 M, honey bee protein show that the honey
bee vitellin N-terminal exhibits sequence homology to the N-terminal of the
47,000 Mr boll weevil vitellin. Treatment of the vitellins with an N-glycosidase
results in a decrease in molecular weight of both proteins, from 47,000 to 39,000
and frnm 160,000 to 145,000, indicating that about 10-15% of the molecular
weight of each vitellin consists of N-linked carbohydrate. The molecular weight
of the deglycosylated large vitellin i s smaller than that predicted from the gene
sequence, indicating possible further proteolytic processing at the C-terminal of
that protein. o 1993 Wiley-Liss, \nc.*
Key words: Coleoptera, vitellin, protein sequence, yolk protein, glycosylation, evolution
Acknowledgments: This research was supported by USDA cooperative agreement SCA 58-575961 2, CWU 5442-22000-005-00D, and grant 9001 328. We wish to thank Sheila 5. Degrugillier
(USDA) for excellent technical assistance, Drs. James Courtright (Marquette), Terrance Adams
(USDA), and Ian McDonald (USDA) for advice and suggestions, Leonard W. Cook of the North
Dakota State University Biochemistry Department and Biotechnology Institute for the amino acid
sequencing, and Bonnie Muhl and Dianne Evanson (USDA) for preparation of the figures.
Mention of any trademark or proprietary product does not constitute endorsement of that
product by the USDA.
Received August 18, 1992; accepted November 28, 1992.
Address reprint requests to Larry I. Heilrnann, USDA-Agricultural
Research Laboratory, P.O. Box 5674, Fargo, ND 58105.
0 1993 Wiley-Liss, Inc. 'This article is a US Government work
and, as such, is in the public domain in the United States of America,
Research Service, Biosciences
126
Heilmann et al.
INTRODUCTION
Vitellogenin provides a good model system for studying the control of
synthesis and post-translational modification in proteins [l-31. In insects it is
synthesized in the fat body and transported via the hernolymph to the ovary.
This involves secretion, transport, and specific uptake by receptors in the
target tissue [3]. Vitellogenin is female-specific in its expression and made only
in the reproductive stage. The expression of the vitellogenin genes in insects
is hormonally controlled by ecdysone and/or juvenile hormone. All of these
steps, synthesis, modification, transport, uptake, and degradation, are possible sites of action for control measures targeting female insects. This paper
concentrates on post-translational modifications of vitellogenin.
The vertebrate vitellogenins are known to be proteolytically cleaved into a
number of proteins including the lipovitellins and phosvitins, the latter a
protein not present in insect yolk [4]. Monoclonal antibody monitoring of
vitellogenin biosynthesis [5,6], cell-free translation analysis [7], and in vivo
pulse labeling experiments [6,8] in mosquitoes and cockroaches support the
hypothesis that insect vitellins are also derived by proteolysis of larger
precursor proteins. However, direct sequence evidence for proteolytic cleavage of vitellogenin has not been available in insects.
We have been studying the expression of the vitellin proteins of the boll
weevil, Anthonornus grundis Boheman, a major pest of the domesticated cotton
crop in the U.S. and the western hemisphere. In work reported elsewhere we
recently completed cloning and sequencing of the entire vitellogenin gene of
the boll weevil, the first complete non-Drosophila insect vitellogenin gene
sequence and the first coleopteran yolk sequence [9]. It codes for a protein of
1,790 amino acids with significant homology to vitellogenins of Caenorhabditis
elegans and Xenopus laevis.
We report that the N-terminal amino acid sequences of two boll weevil
vitellin proteins as well as that of a cyanogen bromide generated peptide from
one of them match the deduced amino acid sequence from the vitellogenin
gene, demonstrating that these vitellins are derived from a primary vitellogenin translation product by proteolytic cleavage.
MATERIALS AND METHODS
Vitellin Protein Preparation
Boll weevils, Anthonornus grundis Boheman, were of the ebony strain [lo]
reared on artificial diet at the Agricultural Research Service, Biosciences
Research Lab, Fargo, North Dakota. Vitellin proteins were purified from eggs
collected 12-15 h after laying. The eggs were homogenized in five volumes of
50 mM Tris-HC1, pH 6.7, in a Dounce homogenizer and the solution was
centrifuged at 5,OOOg for 5 min to pellet insoluble debris. A lipid layer also
formed on top of the aqueous layer. The aqueous fraction was collected and
used as the vitellin stock solution. Protein concentrations were determined by
the method of Bradford [ l l ] .
Boll Weevil Vitellogenin
127
Amino Acid Sequencing
The individual vitellin proteins were purified by fractionating the vitellin
solution by HPLC using a Beckman TSK 4000SW gel filtration column and a
50 mM Nap04 pH 7, 0.3 M NaCl, 0.1% S D S mobile phase. Fractions
containing the vitellin proteins were identified by SDS polyacrylamide gel
electrophoresis. These protein fractions were pooled and further purified by
reverse phase HPLC using a 0-90% acetonitrile gradient. Protein containing
fractions were pooled and lyophilized. These fractions were used for sequencing. Purified large vitellin protein was digested with cyanogen bromide by
standard methods [12]. The protein was dissolved in 70% formic acid and
CNBr was added to a 50-100-fold molar excess. The reaction was allowed to
proceed for 24 h in the dark at room temperature and the resulting peptides
were separated by reverse phase HPLC. Approximately 70-100 pmol of each
purified protein was sequenced by ten cycles of automated Edman degradation using a Porton PI2090 sequencer (Porton Instruments, Tarzana, CA) and
analysis of the sequences was done using the University of Wisconsin GCG
program package.
Glycosidase Treatment
Vitellin proteins were treated with N-glycosidase F (Boehringer Mannheim) to
remove carbohydrate side-chains. Five micrograms of total egg protein were
incubated with 0.5 units of the enzyme in 100 mM Nap04 buffer, pH 7, 1%
2-mercaptoethanol, 1% n-octylglucoside, and 0.2% sodium dodecylsulfate at
37°C for 12-24 h. Digests were analyzed directly on SDS polyacrylamide gels.
RESULTS AND DISCUSSION
When soluble proteins of the boll weevil egg are separated by SDS-PAGE
almost all of the protein migrates as two distinct bands (Fig. 1). The large
protein has an estimated molecular mass of 160,000 kilodaltons (Vt160)
while the smaller is measured at 47,000 kilodaltons (Vt47). The proteins
purified by HPLC migrate at these same molecular weights (Fig. 1, lanes
C, D). Thus the boll weevil yolk is a representative of the class I vitellins
of Harnish and White [13].
Harnish and White [13] divided insect vitellins into three categories based
on their molecular size. Group I vitellins are characterized by a primary
vitellogenin with a Mr of approximately 200,000 which is cleaved into two or
more vitellin peptides, usually of unequal size. Group TI vitellins are distinguished by a lack of this proteolysis, resulting in a single large vitellin protein.
Group 111 vitellins, found only in some higher dipterans, appear to be quite
different, consisting of several small proteins which are deposited into yolk
without extensive post-translational modification. Indeed, the three Drosophila yolk proteins are individually encoded by separate genes that each bear
resemblance to the triacylglycerol lipase gene family rather than to other
vitellogenin genes [14].
*Abbreviations used: SDS = sodium dodecylsulfate; Vt47 = the M, 47,000 vitellin protein of A.
grandis; Vtl60 = the M, 160,000 vitellin protein of A. grandis.
128
Heilmann et al.
Fig. 1. A 7% SDS-polyacrylamide, Coomassie blue stained gel of proteins isolated from eggs of
the boll weevil. lane A: Protein molecular weight markers. Lane B: Total protein from homogenized
eggs. Lane C: Purified large subunit (Vt160) vitellin. lane D: Purified small subunit (Vt47) vitellin.
In order to test the prediction of this classification that both of the vitellin
proteins would be derived from a single precursor protein we determined the
amino acid sequence of the N-terminal ends of each of these two proteins and
compared them to the deduced amino acid sequence of the boll weevil
vitellogenin gene that we recently isolated and sequenced [9]. The boll weevil
vitellogenin gene is single copy, spans nearly 8,000 base pairs, and contains
an open reading frame interrupted by six introns and capable of coding for a
protein of 1,790 amino acids (M, = 205,857). The size of this deduced
polypeptide is obviously too large to consist solely of the large vitellin protein
unless it is processed to yield the Vt160 protein with the remainder degraded.
Alternatively the precursor could be proteolytically cleaved to yield both Vt47
and Vt160.
Boll Weevil Vitellogenin
129
The N-terminal amino acid sequence data of Vt47 and Vt160 are presented
in Figure 2a,b. In addition the N-terminal sequence of an internal cyanogen
bromide fragment from Vt160 is presented in Figure 2c. Both N-terminal
sequences and the cyanogen bromide fragment sequence match precisely with
different sections of the deduced sequence. The N-terminal sequence of the
Vt47 yolk protein matched a sequence beginning at amino acid 19 of the
deduced sequence (Fig. 2a). The preceding 18 amino acids match the characteristics of a signal sequence as expected in this secreted protein. The cleavage
site of the signal peptide appears to be the middle of a run of four consecutive
serines. The Vt160 protein N-terminal amino acid sequence matches a sequence beginning at amino acid 363 of the deduced sequence (Fig. 2b). Like
the smaller vitellin it also begins with a serine. The N-terminal sequence of
the cyanogen bromide peptide derived from the Vt160 vitellin mapped to a
sequence beginning at amino acid 1556 (Fig. 2c). As expected from a cyanogen
bromide digest fragment it is preceded by a methionine residue.
The results clearly show that both of the vitellin proteins are derived from
the same precursor protein. The smaller Vt47 protein is derived from the
N-terminal portion of the molecule and the Vt160 protein from the C-terminal
portion. Whether any further proteolytic processing of these proteins occurs
b)
S A N S L T K Q X R
...354E M T H R R F R R S A N S L T K Q W R E S ...
1549
... K T V K I
dl
K P S E D R Q
E R K P S E D R Q K S
...
AUG
TAA
I
I
I'= V t 4 7
Vt160
3'
Fig. 2. Comparison of the amino acid sequences of the boll weevil vitellins with portions of the
derived amino acid sequence from the gene sequence 191. X represents amino acids not determined.
Numbering is from the first methionine residue of the derived sequence. a: Sequence of the
N-terminal of the Vt47 vitellin protein (top) compared to the N-terminal sequence of the open
reading frame of the boll weevil vitellogenin gene (bottom). b: Sequence of the Vtl60 vitellin protein
(top) compared to the open reading frame of the boll weevil vitellogenin gene (bottom).The putative
protease recognition site is underlined. c: Sequence of the N-terminal of a cyanogen bromide
fragment of the Vt160 vitellin protein (top) compared to the open reading frame of the boll weevil
vitellogenin starting from residue 1,549 (bottom). d: Diagram of the structure of the boll weevil
vitellogenin gene. The heavy bars represent the exons and the thin lines the introns. The vertical
bar marks the protease cleavage site of the encoded protein.
130
Heilmann et al.
is not known. This will have to await determination of the C-terminal
sequence of each of the vitellins.
Assuming no further processing, the derived amino acid sequence of the
small vitellin gives a molecular weight of 38,000 and the large protein sequence
164,000. The difference in the calculated size of the smaller protein and its
measured size on SDS gels (Fig. 1) indicates possible further modification.
This is to be expected as all other vitellins are heavily modified by glycosylation
and lipidation [1,2, and see below].
Figure 2b (underlined) shows the probable cleavage site between the Vt47
and Vt160 domains of the precursor protein. The putative cleavage site is
immediately preceded by a five amino acid sequence containing four arginines
arranged in two pairs. Although this tentative identification of the proteolytic
cleavage site is based solely on the N-terminal amino acid sequences of Vt160,
the adjoining amino acid sequence suggests homology to cleavage sites in
other proteins. Clusters of arginine and/or lysine residues have been reported
at protease cleavage sites in neuropeptide and neurohormone precursors and
a wide variety of other proteins in many species ranging from yeast to humans
[15].The proteases that cleave these sites have recently been shown to belong
to the subtilisin protease family [16,17].Thus, this insect vitellogenin appears
to be processed by proteases related to those that cleave many neuropeptide
and hormone precursors.
Glycosylation
The nucleotide sequence data predicts the presence of three glycosylation
sites (Asn-X-Ser or Asn-X-Thr) in Vt47 and nine sites in Vt160 [9]. The
apparent difference in the predicted molecular weight and the actual size as
determined on SDS gels (Table 1)could be due to glycosylation at some or all
of these sites. In order to test this possibility we treated the vitellin protein
extract with an N-glycosidase enzyme. In insects it has been shown that most
if not all glycosyl modifications of the yolk proteins consist of N-linked high
mannose side chains [I81 so it would be expected that this enzyme would
remove most of the carbohydrate groups. This treatment resulted in a significant decrease in the molecular weight of both vitellins (Fig. 3).
It has been previously reported in two species of lepidoptera that only the
large vitellin is glycosylated [19,20]. In mosquitoes, on the other hand, both
TABLE 1. Molecular Weights of Native and Deglycosylated
Vitellins and Molecular Weights Calculated From the
Gene-Derived Amino Acid Sequence [9]*
Native
N-deglycosylated
Calculated
HMWa
LMW~
160
145
47
39
38
164
*The calculated molecular weights were derived from SDS polyacrylamide gels.
aHMW = high molecular weight.
hLMW = low molecular weight.
Boll Weevil Vitellogenin
131
Fig. 3. Clycosylated and deglycosylated boll weevil vitellin proteins. Vitellin extracts were treated
with N-glycosidase F and separated o n an SDS polyacrylarnide gel. lane M: Molecular weight
markers. lane - : Untreated vitellin protein. lane : Glycosidase treated vitellin protein.
+
large and small vitellins appear to be glycosylated [6]. Our results clearly show
that in the boll weevil both vitellins contain significant amounts of carbohydrate. The 10-15% N-linked carbohydrate content is also larger than that
reported for most other insect vitellins [l]but, again, similar to that of Aedes
mosquitoes [6].
The measured size of the deglycosylated Vt47 vitellin closely matches the
deduced molecular weight (39,000 and 38,000, respectively) indicating that
there is probably little further proteolytic processing of this protein. The
derived sequence of this vitellin does, however, contain the basic dipeptide
flanking the proteolytic cleavage site. In many hormones containing such
sites a specific carboxypeptidase removes the basic amino acids from the
C-terminal[21].
From the size of the deglycosylated Vt160 protein and the size calculated
for a protein consisting of amino acids 363-1,790 (145,000 and 164,000) it is
possible that a considerable portion of the precursor portion of this molecule
132
Heilmann et al.
might be lost. We determined only the amino acid sequence of the N-terminals
and have no information on the C-terminal sequences. The sequence of the
internal cyanogen bromide fragment of Vt160 covers a segment from amino
acid 1,556 to 1,562 defining a minimum size of 1,193amino acids for the Iarger
vitellin. Whether any amino acids have been cleaved from the C-terminal
portion of this molecule will have to be determined by C-terminal sequencing.
Sequence Homology Between Class I and Class I1 Vitellins
In the honey bee only a single 180 kilodalton yolk protein is present [22]
and it is thus regarded as a class I1 vitellin according to Harnish and White
[13].Wheeler and Kawooya [23] determined the amino acid sequences of the
N-terimnal end and an internal fragment of this single peptide. Figure 4a
shows a comparison of this sequence with that of the N-terminal sequence of
the Vt47 protein from boll weevil. With the insertion of one amino acid in the
honey bee sequence, five of the first twelve amino acids match with the boll
weevil sequence; in addition the tyrosine present at position 13 of the boll
weevil sequence (position 14 of the honey bee sequence) has been found to
be invariant in all species so far sequenced. Thus the N-terminal sequence of
the honey bee M, 180,000 protein is homologous to the N-terminal of the Vt47
protein of the boll weevil.
A comparison of the sequence of the honey bee vitellin internal fragment
with a segment of the deduced boll weevil Vt160 sequence starting from amino
acid residue 1,108 (residue 745 of the Vt160 sequence) is shown in Figure 4b.
Five out of twelve residues match. The length of this honey bee sequence is
too short to say definitively that it is homologous only to this boll weevil
sequence. Lesser homology is detectable with other portions of the boll weevil
vitellogenin sequence as well as with other sequences in the data banks. The
honey bee N-terminal sequence, however, does bear a high degree of homology to the N-terminal of the Vt47 protein of the boll weevil.
1
a)
Boll Weevil Vt47
Honey Bee Vtl
tig 4. Comparison of the derived boll weevil vitellogenin sequence [9] with the honey bee
vitellogenin sequence of Wheeler and Kawooya 1231. a: Comparison of the N-terminal sequences.
Amino acids 1-1 Oof the boll weevil sequence were from the amino acid sequence and the remainder
from the derived amino acid sequence from the gene. b: Comparison of an internal honey bee
sequence with a portion of the boll weevil derived sequence showing high similarity. The numbering
is from the initiator methionine of the boll weevil derived sequence.
Boll Weevil Vitellogenin
133
Harnish and White [13] postulated that the three different size classes of
insect vitellins represent different domains of the common ancestral gene and
that the domains have been retained or lost during evolution giving the
different classes. According to this hypothesis class I1 insect vitellins (honey
bee) retained only the domain of the large vitellin protein while class I11
vitellins (Drosophilu) retained only the small vitellin domain. Class I vitellins
retained both. However, the class I11 Drosophilu yolk proteins show no homology
with known vitellogenin sequences and, in fact, have been shown to be related
to a lipase enzyme [14].
We have shown here that the class I1 vitellogenin from the honey bee, Apis
melliferu, appears to exhibit homology to both the Vt47 and Vt160 vitellins from
the boll weevil. As shown in Figure 4 the N-terminal of the honey bee protein
has definite homology with the small boll weevil vitellin while an internal
fragment sequence is similar to a boll weevil sequence more than 1,000 amino
acids downstream from the initiator methionine, well into the large vitellin
sequence, thereby suggesting the presence of small and large vitellin protein
sequences in the class I1 honey bee protein. Hence, evolution of class I1
vitellins may be explained as a result of the loss of the protease cleavage
recognition site or the loss of the enzyme activity itself rather than the result
of loss of the small yolk protein domain. It has been shown that a single amino
acid change in the recognition site can eliminate activity of a processing
protease [24]. These predictions should be readily testable as more sequence
data becomes available.
This is the first report correlating both amino acid and nucleotide sequence
for a nondrosophilid insect vitellin. When sequence data for other class I and
class I1 insect vitellogenins become available it will be possible to determine
the conservation of the position of the smaller and larger vitellins in the gene
and the importance of the cleavage site in its processing. It will also be
interesting to determine the molecular basis of the lack of proteolytic processing of class I1 vitellins. If the honey bee does not possess the enzyme involved
in this process it might be possible to devise methods of control involving
vitellogenin processing that would affect many pest insects but not the
beneficial honey bee.
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