Molecular
cloning of a rhoptry protein (ROP6)
secreted
from Toxoplasma gondii
Hye-Jin AHN, Sehra KIM and
Ho-Woo
Department of Parasitology
and the Catholic Institute of Parasitic Diseases,
Abstract: Monoclonal antibody
(mAb) Tg786 against Toxoplasma gondii has been found to detect a 42-kDa rhoptry
protein (ROP6) which showed
protease activity and host cell binding characteristics after secretion. Using
the mAb, a
colony containing a 3’-UTR
was probed in a T. gondii cDNA expression library. A full length cDNA sequence
of the
rhoptry protein was completed
after 5’-RACE, which consisted of 1,908 bp with a 1,443 bp ORF. The deduced
amino
acid sequence of ROP6
consisted of a polypeptide of 480 amino acids without significant homology to
any other
known proteins. This sequence
contains an amino terminal stop transfer sequence downstream of a short neutral
sequence, hydrophilic middle
sequence, and hydrophobic carboxy terminus. It is suggested that the ROP6 is
inserted
into the rhoptry membrane
with both N- and C-termini.
Key words: Toxoplasma gondii,
excretory/secretory proteins, ROP6, cDNA sequence, hydrophilic domain
Korean Journal of
Parasitology
Vol. 44, No. 3: 251-254,
September 2006
Among the 3 proteases found
in the excretory/
secretory proteins (ESP) of
Toxoplasma gondii, a 42 kDa
protease was immunoprecipitated
by a monoclonal
antibody (mAb), Tg786, which
detected the protein in
the rhoptry organelles of T.
gondii by immunofluorescence
assay (Ahn et al., 2001). The
secreted protease
targeted to the plasma
membrane of host cells, which
was suggested to favor the
appropriate environment
for the entry of the parasite
into host cells. Using the
mAb, a T. gondii cDNA
expression library was
screened in order to obtain
the genetic information of
the 42 kDa rhoptry protease.
A T. gondii λ ZAPII cDNA expression library
obtained through the AIDS
Research and Reference
Reagent Program, Division of
AIDS, NIAID, National
Institutes of Health
(McKesson Biosciences,
Blue
MRF’ (Stratagene,
using mAb Tg786 in
PBS/Tween-20 containing 1%
(w/v) BSA. Positive plaques
were detected using an
ECL Detection System
(Amersham Phamacia Biotech,
isolated by co-infection of
the λ ZAPII phage and the
ExAssist helper phage
(Stratagene). The excised
phagemids were further
propagated in the E. coli
SOLR host strain (Stratagene)
to purify the phagemid
DNA. All DNA sequencing was
performed using a
dye terminator
fluorescent-based sequence analysis
on an Applied Biosystems 373
automated sequencer
using primers directed to the
vector T7 and T3 promoter
sequences. The longest cDNA
clone containing
a poly (A+) tail was selected
for the design of the
� Brief
Communication �
�Received 14 July
2006, accepted after revision 17 August
2006.
�This work was
supported by the Korea Research
Foundation Grant funded by
the Korean Government
(MOEHRD)
(KRF-2005-041-E00128).
*Corresponding author
(e-mail: howoo@catholic.ac.kr)
internal gene-specific
primers (Bioneer Co., Daejeon,
The RH strain of T. gondii
was maintained via peritoneal
passage in BALB/c mice. Prior
to use, tachyzoites
were purified by
centrifugation over 40%
Percoll (Amersham Phamacia
Biotech) in PBS solution.
Total T. gondii tachyzoite
RNA was extracted
using Tri reagent (Sigma
Chemical Co.,
et al., 1998). First strand
cDNA was synthesized from
252
Korean J. Parasitol. Vol. 44, No. 3: 251-254, September 2006
Fig. 1. The cDNA and deduced
amino acid sequences of the ROP6 of Toxoplasma gondii. Nucleotide sequence: the
sequence of ROP6 contains an
open reading frame of 1,443 bp downstream of the Tg consensus translation
initiation
sequence of gtcaaa, as
indicated by stars. The nucleotide numbers are shown on the right. Nucleotide
sequence is available
in the GenBank under the
accession number of AY792971, and the deduced amino acid sequence: the ORF of
1,443
bp encodes a polypeptide of
480 amino acids.
1 μg of total RNA by using a Superscript
Preamplification
System (Life Technologies,
non-coding cDNA using
terminal deoxynucleotide
transferase (Life
Technologies). PCR amplification of
C-tailed cDNA was performed
with an anchor primer
(5’-CTA
ATA CGA CTC ACT ATA GGG CAA GCA
GTG GTA TCA ACG CAG AGT-
primer (5’-CGT TCG AGA CTT
GAG TCC CAG
GCT-
further with the abridged
universal anchor primer (5’-
AAG CAG TGG TAT CAA CGC AGA
GT-
with an internal
gene-specific primer (5’-GCG AAA
ACC GAA TTT TGC ACC GAG-
PCR product was cloned into
the pGEM-T
EASY vector (Promega,
sequence using T7 and SP6
primers. The complete
1,908 bp sequence was
constructed by the 5’-RACE
method containing an 1,443 bp
open reading frame
(Fig. 1). A search for
homologous sequences in the
Toxoplasma dbEST (Database of
Expressed Sequence
Tags) using the BLASTn
algorithm with default settings
resulted in the match of a
contig assembly containing
20 ESTs with high BLAST
scores. A full cDNA
sequence was registered in
the GenBank (Accession
No. AY792971) as a 42 kDa
rhoptry protein (ROP6) of
T. gondii. Using the second
in-frame ATG as a starting
site downstream of the
typical Tg consensus translation
initiation sequence of GTCAAA
(Seeber, 1997),
the ROP6 gene encoded a
polypeptide of 480 amino
acids with a molecular mass
of 42 kDa. The protein
sequence was then compared to
entries in the
GenBank database using BLASTp
search, which
resulted in no matches to any
previously known proteins.
The hydropathicity of the
amino acids sequence
was obtained from ExPASy
using the Kyte and
Doolittle (1982) calculation.
The sequence contains
amino terminal stop transfer
sequence downstream of
a short neutral sequence, a
long middle hydrophilic
sequence, and a hydrophobic
carboxy terminus (Fig.
2), thereby suggesting that
the ROP6 is inserted into
the rhoptry membrane with
both the N- and C-termini
in a similar fashion to that
of other ROP proteins
except for ROP1.
The membrane insertion of
ROP6 was confirmed by
differential centrifugation
and the Triton X-114 phase
partitioning (Bordier, 1981)
of the protein. When the
supernatants of the
tachyzoite extracts in PBS at 2,500
Ahn et al.: ROP6 gene of T.
gondii 253
Fig. 2. Hydropathicity of
ROP6 sequence and the N’-terminal
amino acid sequences of ROP
proteins. The internal
hydrophilic sequence is
lodged in the rhoptry membrane
in both the N’- and
C’-termini. Potential stop transfer
sequences (the signal
sequence in case of the ROP1) are
indicated with boxes.
Fig. 3. Physical
sedimentation of RH tachyzoite extracts
(A) and Triton X-114 phase
partitioning of the extracts (B).
S: supernatant of RH
tachyzoite extracts after centrifugation,
and P: precipitant. Total
parasite proteins (lane 1), A:
proteins recovered in the
aqueous phase (lane 2), and D:
detergent (lane 3) phase
after partitioning were blotted
with by mAb Tg786 and mAb to
SAG1 as a control.
g centrifugation was
sequentially centrifuged at
mAb Tg786 in the precipitant
with a smudge in the
supernatant (Fig. 3A). The
tachyzoite extracts in the
extraction buffer (
NaCl,
0.5% (v/v) precondensed Triton X-114 (Pierce,
aqueous and DMSO protease
inhibitor stocks) were
centrifuged at
detergent phase and not in
the aqueous phase (Fig.
3B). A major surface antigen
of T. gondii (SAG1)
behaved as a control of the
typical membrane inserting
protein in both
centrifugations.
Until now, 9 rhoptry proteins
(ROP1 - ROP9) have
been found to contribute to
the formation of the parasitophorous
vacuole (Ngo et al., 2004).
We have added
the genetic information of
ROP6 to the genes of ROP1,
2, 4, 8, and 9, but still no
clues to the properties from
the gene sequences or deduced
amino acid sequences,
even protease moiety of ROP6.
These are similar to
other secretory proteins from
secretory organelles,
such as, the MIC proteins of
micronemes and the GRA
proteins of dense granules.
ROP proteins are potent
antigens that can induce
strong parasite-directed Tcell
and B-cell responses
(Reichman et al., 2002).
Therefore, ROPs deserve to be
considered as promising
candidates for vaccine
development. ROP6
appears to constitute an
excellent candidate for a
potential vaccine, because it
is secreted as ESP and
exhibits protease activity
essential for the entry of the
parasite into host cells.
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