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© The Rockefeller University Press, 0021-9525/1998//1323 $5.00
The Journal of Cell Biology, Volume 141, Number 6, , 1998 1323-1333

Articles

The Protozoan Parasite Toxoplasma gondii Targets Proteins to Dense Granules and the Vacuolar Space Using Both Conserved and Unusual Mechanisms



Verena Karsten*, Huilin Qi*, Con J.M. Beckers*, Anita Reddy*, Jean-Francois Dubremetz§, Paul Webster{ddagger}, and Keith A. Joiner*

* Section of Infectious Diseases, {ddagger} Center for Cell Imaging, Department of Cell Biology, Yale University School of Medicine, New Haven, Connecticut 06520-8022; and § Inserm U42, F-59650 Villenueve d'Ascq, France

All known proteins that accumulate in the vacuolar space surrounding the obligate intracellular protozoan parasite Toxoplasma gondii are derived from parasite dense granules. To determine if constitutive secretory vesicles could also mediate delivery to the vacuolar space, T. gondii was stably transfected with soluble Escherichia coli alkaline phosphatase and E. coli β-lactamase. Surprisingly, both foreign secretory reporters were delivered quantitatively into parasite dense granules and efficiently secreted into the vacuolar space. Addition of a glycosylphosphatidylinositol membrane anchor rerouted alkaline phosphatase to the parasite surface. Alkaline phosphatase fused to the transmembrane domain and cytoplasmic tail from the endogenous dense granule protein GRA4 localized to dense granules. The protein was secreted into a tuboreticular network in the vacuolar space, in a fashion dependent upon the cytoplasmic tail, but not upon a tyrosine-based motif within the tail. Alkaline phosphatase fused to the vesicular stomatitis virus G protein transmembrane domain and cytoplasmic tail localized primarily to the Golgi, although staining of dense granules and the intravacuolar network was also detected; truncating the cytoplasmic tail decreased Golgi staining and increased delivery to dense granules but blocked delivery to the intravacuolar network. Targeting of secreted proteins to T. gondii dense granules and the plasma membrane uses general mechanisms identified in higher eukaryotic cells but is simplified and exaggerated in scope, while targeting of secreted proteins beyond the boundaries of the parasite involves unusual sorting events.

Abbreviations used in this paper: BAP, bacterial alkaline phosphatase; BFA, brefeldin A; BLA, bacterial β-lactamase; CRD, cross-reacting determinant; CSV, constitutive secretory vesicles; DHFR, dihydrofolate reductase; GPI, glycosylphosphatidylinositol; ISG, immature secretory granules; MSG, mature secretory granules; NTPase, nucleoside triphosphate hydrolase; PI-PLC, phosphatidylinositol-specific phospholipase C; PVM, parasitophorous vacuolar membrane; UTR, untranslated region; VSV-G, vesicular stomatitis virus.

Address all correspondence to Keith Joiner, LCI 808, Section of Infectious Diseases, Yale University School of Medicine, 333 Cedar Street, P.O. Box 208022, New Haven, CT 06520-8022. Tel.: (203) 785-4140. Fax: (203) 785-3864. E-mail: Keith.Joiner{at}yale.edu

Huilin Qi's current address is Department of Cell Biology and Howard Hughes Medical Institute, Yale University School of Medicine.

Con J.M. Beckers' current address is Division of Geographic Medicine, University of Alabama, Birmingham.


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