NMR structure and dynamics of the C‐terminal domain of R‐type lectin from the earthworm Lumbricus terrestris

The C‐terminal domain (Ch; C‐half) of the R‐type earthworm 29‐kDa lectin (EW29), isolated from the earthworm Lumbricus terrestris, has two sugar‐binding sites, in subdomains α and γ, and the protein uses the two sugar‐binding sites for its function as a single domain‐type haemagglutinin. Our previous NMR titration experiments showed that the α sugar‐binding site is a high‐affinity site and the γ sugar‐binding site is a low‐affinity site. However, it remains unclear why the α sugar‐binding site of EW29Ch binds to lactose much more strongly because the crystal structure of lactose‐bound EW29Ch showed that the interaction between the α sugar‐binding site and lactose was almost same as that between the γ sugar‐binding site and lactose. In the present study, we have determined the NMR structure of EW29Ch in the sugar‐free state and performed 15N relaxation experiments for EW29Ch in both the sugar‐free state and the lactose‐bound states. The conformation of EW29Ch in the sugar‐free state was similar to that of EW29Ch in complex with lactose. Conformational changes upon binding of lactose were observed only for the α sugar‐binding site. By contrast, the 15N relaxation experiments revealed a conformational exchange at the α sugar‐binding site in the sugar‐free state, which was suppressed in the lactose‐bound state. The conformational exchange phenomenon observed for the α sugar‐binding site was not observed for the γ sugar‐binding site. Differences in the conformational change and the backbone dynamics between subdomains α and γ may be associated with the difference of the sugar‐binding modes between the two sugar‐binding sites. Database Structural data for the NMR structure of EW29Ch in the sugar‐free state have been deposited in the Protein Data Bank database under accession number 2RST. The sugar‐free NMR structure of the C‐terminal domain of R‐type lectin (EW29Ch) from the earthworm and 15N relaxation data for EW29Ch in both the sugar‐free and the lactose‐bound states suggest that differences in the conformational change and the backbone dynamics between subdomains α and γ may be associated with the difference of the sugar‐binding modes between the two sugar‐binding sites