Structural basis for T cell immunoglobulin and mucin protein 3 and Toxascaris leonina galectin complex.

ABSTRACT

T-cell immunoglobulin and mucin protein 3 (Tim-3), also known as Hepatitis A virus cellular receptor 2, has been discovered to have a negative regulatory effect on murine T-cell responses. Galectin-9 exhibits various biological effects, including cell aggregation, eosinophil chemoattraction, activation, and apoptosis, observed in murine thymocytes, T-cells, and human melanoma cells. Such approach demonstrated that Galectin-9 acts as a binding partner on Tim-3 and mediates the T-cell inhibitory effects. Tl-gal is a homologous protein to galectin-9, isolated from the adult stage of the canine gastrointestinal nematode parasite Toxascaris leonina. However, molecular mechanism between Tim-3 and galectin-9 is still remain unknown. Here, we describe the cryo-electron microscopy and X-ray structures and interactions of the Tim-3 and Tl-gal complex as well as their biochemical and biophysical characterization. In the structure, Ser46 residue of Tl-gal NCRD was bound to Asp25 residue of hTim-3. Compared to our previous study, the binding site of the complex is the same as the sugar binding site (the Ser46 residue) of Tl-gal. In addition, analysis of the complex structure revealed that the four Tl-gal molecules were in an open form packing and one mTim-3 peptide was bound to one Tl-gal molecule. These observations suggest that how Tl-gal binds hTim3 is essential to understanding the molecular mechanism for the Tim-3-galectin 9 interaction that regulates immune responses. This could potentially serve as a therapeutic target for inflammatory diseases.