Caracterização estrutural das formas silvestre e recombinante de uma lectina de sementes de Vatairea macrocarpa Benth e análise das suas bases moleculares de ligação ao antígeno Tn

Abstract

Lectins are a very diverse class of proteins able to bind specific sugar structures reversibly and with high specificity, but without enzymatically modifying them, triggering several important cellular processes. Among the different lectin families, legume lectins are the most thoroughly studied and have been widely reported to exhibit a number of links to many pathological processes, including carcinogenesis. The remarkable anti-tumor properties of some legume lectins, resulting from their ability to induce programmed cell death and/or autophagocytosis in cancer cells have attracted much attention for biomedical applications. Moreover, a few galactose/N-acetylgalactosamine (Gal/GalNAc)-binding lectins from this group have proven to be useful markers for cancer histochemistry, and the structural characterization of these lectins bound to specific cancer epitopes has been carried out successfully. The seed lectin isolated from the legume tree Vatairea macrocarpa (VML) is a well characterized Gal/GalNAc-binding protein able to specifically recognize naturally occurring O-mucins presenting the carcinoma epitope Tn antigen (GalNAc-α-O-Ser). The crystal structures of VML in complex with Tn antigen and GalNAc have been determined at the resolution of 1.4 and 1.7 Å, respectively. Unfortunately, most of lectins obtained from natural sources consist in a mixture of forms, which is an undesired feature for biomedical applications. Thus, the recombinant form of VML (rVML) was expressed in Escherichia coli, being obtained soluble and wih high yielding. The crystal structure for rVML, as well as for the complex with Tn antigen, GalNAc and α-Lactose have been determined at resolutions of 1.7, 2.7, 2 and 1.8 Å, respectively, presenting the same overall structure and binding patterns as the wild lectin. Molecular docking analysis of this new structure and other Tn-binding legume lectins to O-mucin fragments differently decorated with this antigen provides a comparative binding profile among these proteins, stressing that subtle alterations that may not influence monosaccharide binding can, nonetheless, directly impact the ability of these lectins to recognize naturally occurring antigens. In addition to the specific biological effects of VML, the structural and binding similarities between it and other lectins commonly used as histological markers (e.g., VVLB4 and SBA) strongly suggest that VML can be used as a new tool for cancer research. This is the first report of crystal structures of a Gal/GalNAc-binding legume lectin from the Dalbergieae tribe.