Structural and Functional Characterization of SporoSAG

Abstract

Toxoplasma gondii, the etiological agent of toxoplasmosis, utilizes stage-specific expression of antigenically distinct glycosylphosphatidylinositol-tethered surface coat proteins to promote and establish chronic infection. Of the three infective stages of T. gondii, sporozoites are encapsulated in highly infectious oocysts that have been linked to large scale outbreaks of toxoplasmosis. SporoSAG (surface antigen glycoprotein) is the dominant surface coat protein expressed on the surface of sporozoites. Using a bioinformatic approach, we show that SporoSAG clusters with the SAG2 subfamily of the SAG1-related superfamily (SRS) and is non-polymorphic among the 11 haplogroups of T. gondii strains. In contrast to the immunodominant SAG1 protein expressed on tachyzoites, SporoSAG is non-immunogenic during natural infection. We report the 1.60 Å resolution crystal structure of SporoSAG solved using cadmium single anomalous dispersion. SporoSAG crystallized as a monomer and displays unique features of the SRS ␤-sandwich fold relative to SAG1 and BSR4. Intriguingly, the structural diversity is localized to the upper sheets of the ␤-sandwich fold and may have important implications for multimerization and host cell ligand recognition. The structure of SporoSAG also reveals an unexpectedly acidic surface that contrasts with the previously determined SAG1 and BSR4 structures where a basic surface is predicted to play a role in binding negatively charged glycosaminoglycans. Our structural and functional characterization of SporoSAG provides a rationale for the evolutionary divergence of this key SRS family member.

Integrated Bioinformatic and Targeted Deletion Analyses of the SRS Gene Superfamily Identify SRS29C as a Negative Regulator ofToxoplasmaVirulence

“…S1 in the supplemental material). However, in subfamilies 1 and 2, a subfamily-specific cysteine upstream of Cys-5 replaces the absent Cys-3 to form the third disulfide bond (16). Members of subfamilies 3 and 6 are also missing middle cysteine residues but do not have a compensatory residue to rescue the disulfide bond.…”

The s48/45 six-cysteine proteins: mediators of interaction throughout the Plasmodium life cycle

“…The SRS superfamily comprises a group of more than 180 genes found in different stages of the life cycle of Toxoplasma that also tend to arrange in tandem and share the structural SRS domain (Jung et al, 2004; Wasmuth et al, 2012). The SRS proteins that have been characterized to date show that these are glycosylphosphatidylinositol (GPI)-anchored surface proteins with important functions in cell adhesion, immune evasion or virulence attenuation (Dzierszinski et al, 2000; Jacquet et al, 2001; Lekutis et al, 2001; Jung et al, 2004; Radke et al, 2004; Crawford et al, 2010; Wasmuth et al, 2012). …”

Analysis of Structures and Epitopes of Surface Antigen Glycoproteins Expressed in Bradyzoites ofToxoplasma gondii

“…However, among 160 members of the SRS family, only three proteins' structures were reported. They are (i) the tachyzoite-expressed SAG1 [11], (ii) the bradyzoite-expressed BSR4 [12], and (iii) the sporoSAG [13]. The structure and function domains of SAG2C, -2D, -2X, and -2Y are still not very clear.…”