Isolectins from Dictyostelium purpureum. Purification and characterization of seven functionally distinct forms.

Purpurin, the lectin from Dictyostelium purpureum, has been resolved into seven tetrameric isolectins by polyacrylamide gel electrophoresis at pH 8.9. The isolectins are assembled from four distinct subunits resolved by electrophoresis in sodium dodecyl sulfate and by tryptic peptide mapping. Two of the subunits combine randomly with each other to form mixed tetramers (I4, I3II1, I2II2, I1II3, II4) in binomial proportions. The other two subunits (III and IV) form only homotetramers. The isolectins can be functionally discriminated and separated on the basis of their relative affinities for columns derivatized with complementary saccharides. On the basis of relative sensitivity to hapten inhibitors of hemagglutination, isolectins III4 and IV4 are distinct from each other and from isolectins composed of subunits I and II. However, isolectins of I and II are not distinguishable on the basis of hemagglutination inhibition. None of the subunits are glycosylated, and all form tetramers with molecular weights of approximately 88,000. The existence of multiple functionally distinct forms suggests that lectin function in cellular slime molds may be more complex than presently envisioned.