Comparative investigations on the amino-acid sequences of different isolectins from the sponge Axinella polypoides (Schmidt)

Abstract

The sponge Axinella polypoides contains four different d-galactose binding lectins and one, termed lectin IV, which is specific for hexuronic acids. Only the d-galactose binding lectins were investigated in this study. The complete amino-acid sequence of lectin I, the main component in the crude extract was determined. Lectin I is a homodimer and each subunit comprises 144 amino acids with a M r of 15847 ± 10 , as calculated from the sequence data and determined by mass spectrometry. Each subunit contains one intrachain disulfide bridge between positions 4 and 46. Of lectin II, only the first 49 amino acids of the NH 2-terminal end were analysed. This part has 29 amino acids in common with lectin I, including a cysteine residue at position 4, also suggesting an intrachain loop in a identical position as in lectin I. The molecular mass of its subunit is 16235 ± 10 Da . Only the first 15 NH 2-terminal amino acids of lectins III and V could be sequenced. Lectin V was identical to lectin II in all positions, whereas lectin III showed only 5 residues identical to lectins I or II. Thus, lectins I, II and III are derived from three different genes, whereas lectin V may either be a proteolytic cleavage product, or result from different splicing events or may be derived also from a separate gene. Neither of the four lectins showed any similarity to known lectin sequences of animal or plant origin.