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J. Cell Biol.,
Volume 141, Number 6, June 15, 1998 1323-1333
* Section of Infectious Diseases, 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 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
-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.
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