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

The C‐terminal domain (Ch; C‐half) of the R‐type earthworm 29‐ kD a lectin ( EW 29), isolated from the earthworm L umbricus 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 EW 29 C h binds to lactose much more strongly because the crystal structure of lactose‐bound EW 29 C h 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 EW 29 C h in the sugar‐free state and performed 15 N relaxation experiments for EW 29 C h in both the sugar‐free state and the lactose‐bound states. The conformation of EW 29 C h in the sugar‐free state was similar to that of EW 29 C h in complex with lactose. Conformational changes upon binding of lactose were observed only for the α sugar‐binding site. By contrast, the 15 N 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 EW 29Ch in the sugar‐free state have been deposited in the Protein Data Bank database under accession number 2RST .