**5. Conclusions**

In this report we presented new details of structure-function relationships for a novel lectin from the mussel *C. grayanus*. In silico analysis of CGL complexes with galactose, globotriose and PSM-trisaccharide helped us to suggest the binding mechanisms of CGL. For the first time, it was shown that point mutation of residues that form hydrogen bonds with a terminal monosaccharide and not included in the conservative motif HPY(K)G, led to a change in the mucin-binding activity of mutants. The maximal decrease in the mucin-binding activity of the mutants Asn119Ala in Site 1 and Asn27Ala in Site 2 was due to the loss of all three hydrogen bonds with two terminal galactose residues of oligosaccharides in comparison with the wild type CGL. However, the efficiency of CGL binding depends on the composition of at least three terminal monosaccharide units in oligosaccharides. The amino acid residue Asp127 in Site 3 (and similar residues Asp35 and Asp83 in Sites 1 and 2) was found to play a decisive role in the higher lectin affinity to mucin due to forming an additional bond with the third fucose.

The ability of CGL to recognize Gb3 on the surface of breast cancer cells and bind mucin-type glycoproteins, which are often associated with oncogenic transformation, make it prospect in construction of a biosensor for cancer diagnostics. In this regard, the results elicited the individual contribution of His37, His129, Glu75, Asp127, His85, Asn27 and Asn119 amino acid residues from carbohydrate-binding sites to CGL activity could be helpful for designing an artificial analog of CGL with enhanced Gb3- and mucin-binding properties for applying in cancer diagnostics or anticancer therapy.

**Author Contributions:** S.N.K., N.S.B., G.N.L. and L.A.B. contributed equally to this work; V.A.R., O.M.S. and L.A.T. improved the manuscript through careful review and helpful suggestions.

**Funding:** This research was funded by Ministry of Education and Science of Russia (Agreement 02.G25.31.0172, 01.12.2015), and the APC was partially funded by the program "Far East" grant number 18-4-051.

**Conflicts of Interest:** The authors declare no conflict of interest.
