Modification by site-directed mutagenesis of the specificity of Erythrina corallodendron lectin for galactose derivatives with bulky substituents at C-2

Examination of the three-dimensional structure of Erythrina corallodendron lectin (ECorL) in complex with a ligand (lactose), the first of its kind for a Gal/GalNAc-specific lectin [(1991) Science 254, 862-866], revealed the presence of a hydrophobic cavity, surrounded by Tyr108 and Pro134-Trp135, which can accommodate bulky substituents such as acetamido or dansylamido (NDns) at C-2 of the lectin-bound galactose. Comparison of the primary sequence of ECorL with that of soybean agglutinin, specific for galactose and its C-2 substituted derivatives, and of peanut agglutinin, specific for galactose only, showed that in soybean agglutinin, Tyr108 is retained, and Pro134-Trp135 is replaced by Ser-Trp, whereas in peanut agglutinin, the former residue is replaced by Thr and the dipeptide by Ser-Glu-Tyr-Asn. Three mutants of ECorL were therefore constructed: L2, in which Pro134-Trp135 was replaced by Ser-Glu-Tyr-Asn; Y108T, in which Tyr108 was replaced by Thr and the double mutant L2; Y108T. They were expressed in Escherichia coli, as done for recombinant ECorL [(1992) Eur. J. Biochem. 205, 575-581]. The mutants had the same hemagglutinating activity as native or rECorL. Their specificity for galactose, GalNAc and MeβGalNDns was examined by inhibition of hemagglutination and of the binding of the lectin to immobilized asialofetuin; in addition, their association constants with MeαGalNDns and MeβGalNDns were measured by spectrofluorimetric titration. The results showed that Y108T had essentially similar specificity as the native and recombinant lectins. The affinity of L2 and L2;Y108T for galactose was also the same as ECorL, but they had a lower affinity for GalNAc and markedly diminished affinity for the dansyl sugars (up to 43 times, or 2 kcal, less). This appears to be largely due to steric hindrance by the two additional amino acids present in the cavity region in these mutants. Our findings also provide an explanation for the inability of PNA to accommodate C-2-substituted galactose derivatives at its primary subsite.