Structural basis of surface antigen glycoprotein mediated virulence in Toxoplasma gondii




Bruic, Ekaterina

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Toxoplasma gondii is a eukaryotic, intracellular parasite capable of infecting any vertebrate animal and establishing a life-long latent infection. Despite the prevalence of T. gondii infections, the molecular mechanisms by which these parasites gain access to the host cell remain largely unknown. Recent knockout studies have implicated a select group of T. gondii surface proteins, termed SRSs (Surface Antigen Glycoprotein Related Sequences), in directing parasite attachment and persistence. Follow-up structural studies with the prototypical SRS antigen, SAG1, revealed a novel fold, termed the SRS fold, and a dimeric structure with a topologically defined basic groove predicted to play a role in ligand binding. While these initial results were very exciting, follow-up work has failed to identify a host cell ligand for SAG1, and no other members out of more than 160 members of the SRS superfamily have been structurally characterized. As a result, conservation of the SRS fold and, more specifically, structural determinants of molecular recognition remain elusive. While sequence alignments of the SRS superfamily suggested conservation of the SRS fold, several insertions and deletions presented the possibility of localized structural elements that may be essential in molecular recognition. To characterize how these insertions/deletions are represented at the structural level, the X-ray crystal structures of two members of the SRS superfamily, BSR4 and SRS2, were solved. Structural analysis revealed an unexpected degree of diversity in the SRS fold. Divergent connectivity of the beta-strands in studied proteins indicates that the SRS superfamily may be more structurally diverse than previously thought, while structural variations in the beta-strands and the loops of D1 domain suggests a possible mechanism to recognize diverse host cell ligands, such as heparan sulfate proteoglycans (HSPGs). To probe HSPGs binding and determine the role of homodimerization, the dimer constructs of SRS2 and BSR4 were engineered, produced and tested in a carbohydrate binding macro-array. Selective binding of the SRS2 dimer to heparin was detected during screening and validated using heparin-agarose pull-down and native gel shift assays. Possible molecular mechanism for SRS-HPGS interaction and the implications in T. gondii virulence are also discussed.



Toxoplasma, surface proteins