Small-angle X-ray scattering study on CEL-III, a hemolytic lectin from Holothuroidea Cucumaria echinata, and its oligomer induced by the binding of specific carbohydrate.

Hemolytic lectin CEL-III from a marine invertebrate Cucumaria echinata forms an oligomer upon binding of specific carbohydrate such as lactose at high pH values and in the presence of high concentrations of salt. In this study, using small-angle X-ray scattering, we characterized CEL-III and its oligomer induced by the binding of lactose. The molecular mass of the oligomer was determined as 1019 kDa from its forward scattering value, compared with 47,490 Da for the monomer. This oligomer size is much larger than that estimated using SDS-polyacrylamide gel electrophoresis (SDS-PAGE, 270 kDa). The monomer has a 24.6 A radius of gyration and can be approximated by a rod which has a 20 A radius and a height of 75 A, while the oligomer has a 101.4 A radius of gyration. Together with the comparison of the radii of gyration and the forward scattering of the cross-section of the monomer and oligomer, it is suggested that in aqueous solution the oligomer comprises three or four molecules of a smaller unit which was observed by SDS-PAGE (270 kDa), held by a relatively weak interaction. The scattering profile also suggests that the oligomer has a hole in its central axis which might be associated with the formation of ion-permeable pores in the erythrocyte membrane by CEL-III during the hemolytic process.