Biochemical and functional characterization of rMe'exLec1, a recombinant tandem-repeat lectin from the ancient marine chelicerate Limulus polyphemus

Mass extinctions have disrupted the evolutionary history of several species, and with it the discovery of novel proteins with biomedical and biotechnological value. However, Limulus polyphemus, a xiphosuran marine arthropod, is a "living fossil" with a 480-million-year-old lineage. To explore this evolutionary legacy, rMe'exLec1, a 13.15 kDa tandem-repeat lectin from L. polyphemus, was studied. Named after the ancient Maya term "Me'ex" ("spider fish"), it was Identified in the genome, recombinantly expressed in Escherichia coli, and purified via nickel affinity chromatography, yielding 1 g/L of culture. The protein remained stable across a wide pH range and different salts. Far-UV circular dichroism confirmed proper folding and the presence of both α and β secondary structures. rMe'exLec1 exhibited specificity for L-Fuc and D-GalNAc, terminal sugars of the human A-antigen, demonstrating agglutinating activity on type A erythrocytes. It also bound divalent ions such as nickel, and formed monodisperse oligomers. Additionally, this lectin inhibited the growth of Gram-negative (Vibrio parahaemolyticus, E. coli Rosetta(DE3) and DH5α), and Gram-positive (Lactiplantibacillus plantarum) bacteria, highlighting its potential for biomedical applications. Structural and sequence analyses of homologous proteins, along with their limited phyletic distribution and unique evolutionary relationships, support the classification of this protein group as a novel lectin family, named Me'exLec-type. Furthermore, its tandemly repeated, non-identical binding sites are evolutionarily conserved across phylogenetically distant groups.