Toxoplasma gondii major surface antigen (SAG1): in vitro analysis of host cell binding

Abstract

Article:Robinson, S.A., Smith, J.E. and Millner, P.A. (2004) SUMMARYPrevious studies have indicated that SAG1, the major surface molecule of the protozoan parasite Toxoplasma gondii,isan important attachment ligand for the host cell. However, the research data that supports this claim comes largely from studies investigating tachyzoite binding, and not SAG1 binding per se. In this study we successfully developed an in vitro attachment assay to directly evaluate the mechanism of SAG1-host cell binding. Competition experiments were then performed using SAG1 that had been pre-treated with the neoglycoprotein BSA-glucosamide or with antibody. Soluble BSA-glucosamide blocked SAG1 attachment to MDBK cells in a dose-dependent manner, implying that SAG1 binding is mediated, in part, via attachment to host cell surface glucosamine. Interestingly, pre-incubation of SAG1 in polyclonal sera from chronically infected mice failed to block binding. This challenges the assumption that anti-SAG1 antibodies block parasite attachment through the masking of SAG1 host cell binding domains. Taken together, this evidence presents new strategies for understanding SAG1-mediated attachment.

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

“…16,38 Hence, the characterization of SAG1 immunodominant epitopes and their relations with the ligand-binding site has been a longstanding issue addressed through different approaches. [24][25][26][27][31][32][33]38 The monoclonal antibody used in this study recognizes, in the nanomolar range, the native and recombinant SAG1, and efficiently inhibits SAG1 binding to most of the sera from infected patients that we have tested, suggesting that the mAb 4F11E12 is highly representative of the human anti-Toxoplasma response. Consequently, it is likely that mAb 4F11E12 and human sera recognize overlapping areas on SAG1, a region that is considered to be part of an immunodominant B-cell epitope.…”

Bradyzoite-Specific Surface Antigen SRS9 Plays a Role in Maintaining Toxoplasma gondii Persistence in the Brain and in Host Control of Parasite Replication in the Intestine

“…SRS antigens are thought to facilitate attachment to and invasion of a host cell (6,9,13,18), but the identity of the interacting host cell ligands remains elusive. Recently, X-ray crystallography has revealed that the highly abundant SAG1 specific to the tachyzoite stage forms a homodimer in which the dimeric interface shaped by an extended ␤-sheet forms a deep groove lined with positively charged amino acids (7).…”

The Role of Inositol Acylation and Inositol Deacylation in the Toxoplasma gondii Glycosylphosphatidylinositol Biosynthetic Pathway

“…Toxoplasma as with other Apicomplexa has an obligatory intracellular developmental stage where it multiplies before bursting from a host cell, pre-adapted to infect other host cells. The ability to invade nearly all nucleated cells in an active multistep process (5-7) is thought to be in part due to several surface proteins, among them the major surface antigen SAG1 (P30), which establishes the first contact between the parasite and the host cell (8,9). As in the case with many other parasitic protozoa (10 -14), glycosylphosphatidylinositol (GPI) 3 -anchored proteins like SAG1 dominate the plasma membrane.…”