A novel
lectin structure was found for a 17-kDa α-
D-galactose-binding
lectin (termed "MytiLec") isolated from the Mediterranean mussel, Mytilus galloprovincialis. The complete primary structure of the
lectin was determined by Edman degradation and mass spectrometric analysis. MytiLec was found to consist of 149
amino acids with a total molecular mass of 16,812.59 Da by Fourier transform-ion
cyclotron resonance mass spectrometry, in good agreement with the calculated value of 16,823.22 Da. MytiLec had an N terminus of acetylthreonine and a primary structure that was highly novel in comparison with those of all known
lectins in the structure database. The
polypeptide structure consisted of three tandem-repeat domains of ∼50
amino acids each having 45-52% homology with each other. Frontal affinity chromatography technology indicated that MytiLec bound specifically to
globotriose (Gb3; Galα1-4Galβ1-4Glc), the
epitope of
globotriaosylceramide. MytiLec showed a dose-dependent cytotoxic effect on human
Burkitt lymphoma Raji cells (which have high surface expression of Gb3) but had no such effect on
erythroleukemia K562 cells (which do not express Gb3). The cytotoxic effect of MytiLec was specifically blocked by the co-presence of an α-galactoside. MytiLec treatment of Raji cells caused increased binding of anti-
annexin V antibody and incorporation of
propidium iodide, which are indicators of cell membrane inversion and perforation. MytiLec is the first reported
lectin having a primary structure with the highly novel triple tandem-repeat domain and showing transduction of apoptotic signaling against
Burkitt lymphoma cells by interaction with a
glycosphingolipid-enriched microdomain containing Gb3.