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Infect Immun, May 1998, p. 2237-2244, Vol. 66, No. 5
Department of Microbiology and Immunology,
Stanford University School of Medicine, Stanford, California 94305
Received 5 November 1997/Returned for modification 17 December
1997/Accepted 2 March 1998
Toxoplasma gondii is an Apicomplexan parasite with a
complex life cycle that includes a rapidly dividing asexual stage known as the tachyzoite. The tachyzoite surface has been reported to comprise
five major antigens, the most abundant of which is designated SAG1 (for
surface antigen 1). At least one of the other four (SAG3) and another
recently described minor antigen (SRS1 [for SAG1-related sequence 1])
have previously been shown to be structurally related to SAG1. To
determine if further SAG1 homologs exist, we searched a
Toxoplasma expressed sequence tag (EST) database and found
numerous ESTs corresponding to at least three new genes related to
SAG1. Like SAG1, these new SRS
genes encode apparently glycosylphosphatidylinositol-anchored proteins
that share several motifs and a set of conserved cysteine residues.
This family appears to have arisen by divergence from a common ancestor
under selection for the conservation of overall topology. The products
of two of these new genes (SRS2 and SRS3) are
shown to be expressed on the surface of Toxoplasma
tachyzoites by immunofluorescence. We also identified strain-specific
differences in relative expression levels. A total of 10 members of the
SAG1 gene family have now been identified, which apparently
include three of the five major surface antigens previously described and one antigen expressed only in bradyzoites. The function of this
family may be to provide a redundant system of receptors for
interaction with host cells and/or to direct the immune responses that
limit acute T. gondii infections.
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
The Surface of Toxoplasma Tachyzoites Is
Dominated by a Family of Glycosylphosphatidylinositol-Anchored
Antigens Related to SAG1
*
Corresponding author. Mailing address: Department of
Microbiology and Immunology, Fairchild Science Building, D305, Stanford University School of Medicine, Stanford, CA 94305-5124. Phone: (650)
723-7984. Fax: (650) 723-6853. E-mail:
john.boothroyd{at}stanford.edu.
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