Crystal Structure of the Complex between the Monomeric Form of Toxoplasma gondii Surface Antigen 1 (SAG1) and a Monoclonal Antibody that Mimics the Human Immune Response Marc Graille 1 †, Enrico A. Stura 1 * , Marc Bossus 2,3 , Bruno H. Muller 2,3 Odile Letourneur 3 , Nicole Battail-Poirot 3 , Genevie `ve Sibaı ¨ 3 , Marie Gauthier 3 , Dominique Rolland 3 , Marie-He ´le `ne Le Du 1 and Fre ´de ´ ric Ducancel 1,2 * 1 De ´partement d’Inge ´nierie et d’E ´ tudes des Prote ´ines, Centre d’E ´ tudes de Saclay, 91191 Gif-sur-Yvette, France 2 Unite ´ Mixte CEA/bioMe ´rieux in the De ´partement d’Inge ´nierie et d’E ´ tudes des Prote ´ines Centre d’E ´ tudes de Saclay 91191 Gif-sur-Yvette, France 3 De ´partement R.&D. Immunoessais et Prote ´omique bioMe ´rieux, Chemin de l’Orme 69280 Marcy l’E ´ toile, France Toxoplasma gondii, the intracellular parasite responsible for toxoplasmosis infects more than one-third of the world population and can be life- threatening for fetuses and immunocompromised patients. The surface protein SAG1 is an important immune target, which provides a strong immune response against the invasive tachyzoite while the other forms of the parasite, devoid of SAG1 at their surface, are multiplying. In addition to this role as a “hot spot” decoy, SAG1 is predicted to act as an adhesin during host-cell attachment through its binding to proteoglycans. To begin to understand the relationships between SAG1 epitopes and the ligand- binding site, we have solved the crystal structure of the monomeric form of T. gondii SAG1 complexed to a Fab derived from a monoclonal antibody raised against tachyzoite particles. This antibody competes strongly with human Toxoplasma-specific sera, suggesting that its epitope is part of an immunodominant region present on the surface of SAG1. The structure reveals that this conformational epitope, located within the SAG1 N-terminal domain, does not overlap with the proposed ligand-binding pocket. This study provides the first structural description of the monomeric form of SAG1, and significant insights into its dual role of adhesin and immune target during parasite infection. q 2005 Elsevier Ltd. All rights reserved. Keywords: Toxoplasma gondii recombinant SAG1; conformational epitope; crystal structure *Corresponding authors Introduction Toxoplasma gondii is a pervasive protozoan parasite of warm-blooded vertebrates that produces life-long chronic infections that can result in congenital disease with severe consequences for fetuses, 1 and immunocompromised individuals. 2,3 The efficiency of T. gondii infection results from its ability to invade any cell, to proliferate and to disseminate throughout the body as tachyzoites, and to reside inside the cells via the dormant bradyzoite stage. 4 The ability of T. gondii to survive and replicate, and thus to cause chronic and persistent infections, relies also on its capacity to resist natural host defences. Indeed, protozoa in general and T. gondii in particular have evolved different mechanisms for evading and resisting specific immunity that lead to a weak natural immunity against them. The immune response to T. gondii involves both humoral and cell-mediated 0022-2836/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. † Present address: M. Graille, Institut de Biochimie et de Biophysique Mole ´culaire et Cellulaire (IBBMC), (CNRS-UMR 8619), Universite ´ Paris-Sud, Ba ˆt. 430, 91405 Orsay, France. Abbreviations used: GPI, glycosylphosphatidylinositol; SAG1, surface antigen 1; SRS, SAG1-related sequence; mAb, monoclonal antibody; D1, N-terminal domain; D2, C-terminal domain; PpL, protein L; EIA, enzyme immuno assay. E-mail addresses of the corresponding authors: estura@cea.fr; frederic.ducancel@cea.fr doi:10.1016/j.jmb.2005.09.028 J. Mol. Biol. (2005) 354, 447–458