Acute myeloid leukemia (AML) is an aggressive blood/bone marrow cancer originating from self-renewing malignant immature myeloid cells. AML rapidly becomes a systemic malignancy and is often a fatal disease since cancerous cells are suppressing anti-cancer immunity by impairing the functional activity of natural killer (NK) cells and T cells with cytotoxic activity1.
Synchrotron radiation circular dichroism (SRCD) spectroscopy at beamline B23 was used to characterise the secondary structure of Tim-3, galectin-9 and the complex made by these two proteins presented in Figure 2a. Structural organisation of both Tim-3 and galectin-9 are presented in the Figure 2a. It was found in earlier studies that galectin-9 interacts with non-glycosylated Tim-3 with high affinity (Kd = 2.8 × 10−8 M)4. However, this binding is further strengthened by interaction of galectin-9 with sugars of glycosylated Tim-3. In AML cells Tim-3 undergoes glycosylation2 which means that its affinity to galectin-9 is very high in AML cells. This assumption was confirmed by the fact that the complex is detectable by Western blot analysis, which suggests a strong high affinity binding between the two proteins2,5. SRCD spectroscopy was also applied to galectin-9 and Tim-3 mixed to a stoichiometry of 1:1 molar ratio (Fig. 2b). When mixed together with Tim-3, galectin-9 showed a CD spectrum which was clearly different from the simulated spectrum. This suggested that the interaction of Tim-3 with galectin-9 caused a conformational change of both proteins. An obvious increase in β-strand component was observed. Based on these findings, one may suggest that Tim-3 binding possibly alters the conformation of galectin-9. This results in increased capability of galectin-9 to interact with receptors in target cells. Galectin-9 is a protein, which contains two sugar-binding domains. Therefore, one domain could bind Tim-3 (or other proteins/receptor/trafficker) and leave the other domain open for interaction with a receptor molecule (for example Tim-3) associated with the plasma membrane of a target cell (for example NK cell or cytotoxic T cell).
Gonçalves Silva I, Yasinska IM, Sakhnevych SS, Fiedler W, Wellbrock J, Bardelli M, Varani L, Hussain R, Siligardi G, Ceccone G, Berger SM, Ushkaryov YA, Gibbs BF, Fasler-Kan E, Sumbayev VV. The Tim-3-galectin-9 Secretory Pathway is Involved in the Immune Escape of Human Acute Myeloid Leukemia Cells. EBioMedicine 22, 44 – 57, doi:10.1016/j.ebiom.2017.07.018 (2017).
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