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J. Biol. Chem., Vol. 276, Issue 21, 18272-18281, May 25, 2001
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From the Department of Internal Medicine, Section of Infectious
Diseases, Yale University School of Medicine, New Haven, Connecticut
06520-8022
Toxoplasma gondii dense
granules are morphologically similar to dense matrix granules in
specialized secretory cells, yet are secreted in a constitutive,
calcium-independent fashion. We previously demonstrated that secretion
of dense granule proteins in permeabilized parasites was augmented by
the non-hydrolyzable GTP analogue guanosine
5'-3-O-(thio)triphosphate (GTP The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) AF227524.
Toxoplasma gondii ADP-ribosylation Factor 1 Mediates Enhanced Release of Constitutively Secreted Dense Granule
Proteins*
S) (Chaturvedi, S., Qi,
H., Coleman, D. L., Hanson, P., Rodriguez, A., and Joiner, K. A. (1998) J. Biol. Chem. 274, 2424-2431). As now
demonstrated by pharmacological and electron microscopic approaches,
GTPS enhanced release of dense granule proteins in the
permeabilized cell system. To investigate the role of ADP-ribosylation
factor 1 (ARF1) in this process, a cDNA encoding T. gondii ARF1 (TgARF1) was isolated. Endogenous and transgenic
TgARF1 localized to the Golgi of T. gondii, but not to
dense granules. An epitope-tagged mutant of TgARF1 predicted to be
impaired in GTP hydrolysis (Q71L) partially dispersed the Golgi signal,
with localization to scattered vesicles, whereas a mutant impaired in
nucleotide binding (T31N) was cytosolic in location. Both mutants
caused partial dispersion of a Golgi/trans-Golgi network marker.
TgARF1 mutants inhibited delivery of the secretory reporter,
Escherichia coli alkaline phosphatase, to dense granules,
precluding an in vivo assessment of the role of TgARF1 in
release of intact dense granules. To circumvent this limitation,
recombinant TgARF1 was purified using two separate approaches, and used
in the permeabilized cell assay. TgARF1 protein purified on a Cibacron
G3 column and able to bind GTP stimulated dense granule secretion in
the permeabilized cell secretion assay. These results are the first to
show that ARF1 can augment release of constitutively secreted vesicles
at the target membrane.
*
This work was supported by United States Public Health
Service Grant ROI-AI30060 from the National Institutes of Health (NIH) and a Scholar Award in Molecular Parasitology from the Burroughs Wellcome Fund (to K. A. J.), by NIH Training Grants T32-AI 07404-09 (to A. L.) and T32 AI07404 (to H. M. N.), by National Research Service Awards 1F32-AI09938-01 (to T. S.) and 1F32-AI10044-01A1 (to
H. M. N.), and by a South African Foundation for Research Development
postdoctoral fellowship (to H. C. H.).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed: Dept. of Internal
Medicine, Section of Infectious Diseases, Yale University School of
Medicine, LCI 808, 333 Cedar St., New Haven, CT 06520-8022. Tel.:
203-785-4140; Fax: 203-785-3864; E-mail: keith.joiner@yale.edu.
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