Unraveling the Effect of the C-terminal Tail of Agaricus bisporus Mannose-Binding Protein and Discovery of a Second Functional Sugar-Binding Site

The effect of the C-terminal tail on the bioactivity of <i>Agaricus bisporus</i> mannose-binding protein (Abmb) was investigated. Based on the earlier obtained crystal structure of Abmb, it was suggested that the additional C-terminal tail can modulate the binding of sugars to the protein. According to glycan microarray, Abmb can bind β-Gal sugars, which contradicted the results of SPR analysis showing that Abmb only interacts with α-Man and not with α-Gal. Here, we used MCF-7 and MDA-MB-231 breast cancer cells to demonstrate that the presence of the C-terminal tail decreased the anti-proliferative activity of Abmb. Pre-incubating Abmb with α-Gal did not eliminate the anti-proliferative activity, while pre-incubation with α-Man attenuated it. At the same time, preincubation with a mixture of α-Gal and α-Man strongly promoted the anti-proliferative activity of Abmb. <i>In silico</i> analysis using molecular docking suggested the presence of a second functional sugar-binding site for Gal, which had not been identified previously. The study provides new insights into the structure of lectins and their interaction with sugars.