Carbohyrate recognition factors of a Tα (Galβ1→3GalNAcα1→Ser/Thr) and Tn (GalNAcα1→Ser/Thr) specific lectin isolated from the seeds of Artocarpus lakoocha

Abstract

Artocarpus lakoocha agglutinin (ALA), isolated from the seeds of A. lakoocha fruit, is a galactose-binding lectin and a potent mitogen of T and B cells. Knowledge obtained from previous studies on the affinity of ALA was limited to molecular and submolecular levels of Galβ1→3GalNAc (T) and its derivatives. In the present study, the carbohydrate specificity of ALA was characterized at the macromolecular level according to the mammalian Gal/GalNAc structural units and corresponding glycoconjugates by an enzyme-linked lectinosorbent (ELLSA) and inhibition assays. The results indicate that ALA binds specifically to tumor-associated carbohydrate antigens GalNAcα1→Ser/Thr (Tn) and Galβ1→3 GalNAcα1→Ser/Thr (Tα). It barely cross-reacts with other common glycotopes on glycoproteins, including ABH blood group antigens, Galβ1→3/4GlcNAc (I/II) determinants, T/Tn covered by sialic acids, and N-linked plasma glycoproteins. Dense clustering structure of Tn/Tα-containing glycoproteins tested resulted in 2.4 × 105-6.7 × 105-fold higher affinities to ALA than the respective GalNAc and Gal monomer. According to our results, the overall affinity of ALA for glycans can be ranked respectively: polyvalent Tn/Tα glycotopes ≫ monomeric Tα and simple clustered Tn ≫ monomeric Tn > GalNAc > Gal; while other glycotopes: Galα1→3/4Gal (B/E), Galβ1→3/4GlcNAc (I/II), GalNAcα1→3Gal/GalNAc (A/F), and GalNAcβ1→3/4Gal (P/S) were inactive. The strong specificity of ALA for Tn/Tα cluster suggests the importance of glycotope polyvalency during carbohydrate-receptor interactions and emphasizes its value as an anti-Tn/T lectin for analysis of glycoconjugate mixtures or transformed carbohydrates. © Oxford University Press 2005; all rights reserved.