Theoretical investigation on structural, functional and epitope of a 12 kDa excretory-secretory protein from Toxoplasma gondii

Abstract

Background: Toxoplasma gondii is an intracellular coccidian parasite that causes toxoplasmosis. It was estimated that more than one third of the world population is infected by T. gondii, and the disease is critical in fetuses and immunosuppressed patients. Thus, early detection is crucial for disease diagnosis and therapy. However, the current available toxoplasmosis diagnostic tests vary in their accuracy and the better ones are costly.

Results: An earlier published work discovered a highly antigenic 12 kDa excretory-secretory (ES) protein of T. gondii which may potentially be used for the development of an antigen detection test for toxoplasmosis. However, the three-dimensional structure of the protein is unknown. Since epitope identification is important prior to designing of a specific antibody for an antigen-detection based diagnostic test, the structural elucidation of this protein is essential. In this study, we constructed a three dimensional model of the 12 kDa ES protein. The built structure possesses a thioredoxin backbone which consists of four α-helices flanking five β-strands at the center. Three potential epitopes (6-8 residues) which can be combined into one "single" epitope have been identified from the built structure as the most potential antibody binding site.

Conclusion: Together with specific antibody design, this work could contribute towards future development of an antigen detection test for toxoplasmosis.

Figure 1

Verify3D results for final built 12 kDa ES protein model. All the residues 3D-1D score are higher than 0.2 with 0.23 as the lowest score of the residues.

Figure 2

Built structure of 12 kDa ES protein and it’s predicted epitopes. (a) The 3-dimensional structure of the built 12 kDa ES protein model. Purple represents α-helices, yellow represents β-sheets, white represents coils and cyan represents turns. Disulfide bond between Cys31 and Cys34 is shown in red dotted line. (b) The predicted epitopes, represented in different colors as below (Figure 2c) of the T. gondii 12 kDa ES protein. The conserved region Trp-Cys-Gly-Pro-Cys is in stick representation. (c) Summary of the epitope prediction results of the T. gondii 12 kDa ES protein from four sequential and conformational epitope prediction servers and protein binding site prediction results from ProBis.

Figure 3

The confirmation of the best ranked ASK1-ES protein complex from ZDOCK. ASK1 and ES protein is in grey and yellow cartoon representation, respectively. The residues in ES protein which is important (Gly-32, Pro-33, Ile-74, Pro-76, Val-91, Gly-92, Ala-93) in the binding with ASK1 are in yellow surface representation.