Structural insights and antimicrobial synergy of a proto-galectin from the marine sponge Aiolochroia crassa

In this study, we isolated a novel lectin from the marine sponge Aiolochroia crassa, named AcrL. The lectin showed a preference for glycans containing sialic acid terminal residues, as indicated by the strongest inhibition with fetuin and bovine submaxillary mucin. Primary structure determination by mass spectrometry revealed that AcrL is a galectin with conserved amino acid residues typically involved in carbohydrate binding. Structural modeling indicated that AcrL adopts a typical galectin β-sandwich motif, featuring two anti-parallel β-sheets with five strands each. Docking calculations revealed a carbohydrate-binding site composed of a main site, capable of hosting galactopyranosides, and an extended site, facilitating the binding of complex carbohydrates. AcrL inhibited significant biofilm formation against Staphylococcus aureus, S. epidermidis, and Escherichia coli with concentrations ranging from 500 to 15.6 μg.mL⁻¹ for S. aureus, 7.8 μg.mL⁻¹ for S. epidermidis, and 500 μg.mL⁻¹ for E. coli. Furthermore, when combined with different antibiotics, AcrL potentiated their effect against pathogenic bacteria. The antimicrobial mechanism of AcrL was investigated using scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). The analysis indicates that AcrL induces damage to the bacterial membrane. These findings underscore the discovery of a novel galectin in a basal organism and the comprehensive biochemical characterization conducted in this research, highlighting the potential of AcrL as a novel antibacterial agent and emphasizing its importance in combating bacterial infections.