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

Soluble lectins: a new class of extracellular proteins

Soluble lectins of cellular slime molds and vertebrates are present at extracellular sites in the developing or adult tissues that make them. Some lectins are concentrated around cell groups, as in extracellular matrix or elastic fibers. Others are at the interface between cells and the external environment, as in mucin or slime. Specific glycoproteins, proteoglycans, or polysaccharides that bind these endogenous lectins may also be present at these sites. Interactions between the lectins and glycoconjugates appear to play a role in shaping extracellular environments.

Differential expression of the members of the discoidin I multigene family during growth and development of Dictyostelium discoideum

Discoidin I and II are lectins synthesized during the aggregation of Dictyostelium discoideum amoebae which may play a role in cellular cohesion. Discoidin I was thought to consist of two major polypeptides, but we show that there are three. The N-terminal amino acid sequence of the polypeptides has been predicted by determining part of the nucleotide sequence of their respective mRNAs. We obtained the nucleotide sequences by reverse transcription of the mRNAs, using as primers, fragments derived from the coding region of two cloned discoidin I sequences, and utilizing cross hybridization to the various mRNA species and differences in the length of their 5' noncoding regions to isolate fragments for DNA sequencing. We used primer extension to measure the relative concentration of the three major discoidin I mRNA sequences. We show that during development changes in the abundance of all three mRNA sequences occur coordinately. In cells growing in nutrient broth, however, only two of the three major discoidin I mRNA sequences accumulate, and if such cells are grown to a very high density, both sequences disappear. These results indicate that the coordination of discoidin I gene expression is not obligatory and that the members of this multigene family may differ in the mode of their induction during normal development.

Endogenous lectins in chickens and slime molds: transfer from intracellular to extracellular sites

Endogenous lectins in both cellular slime molds and chicken tissues have been localized primarily intracellularly, in contrast with the predominantly extracellular localization of the glycoproteins, glycolipids, and glycosaminoglycans with which they might interact. Here we present evidence that lectins in both of these organisms may be externalized and become associated with the cell surface and/or extracellular materials. In chicken intestine, chicken-lactose-lectin-II is shown to be localized in the secretory granules of the goblet cells, along with mucin, and to be secreted onto the intestinal surface. In embryonic muscle, chicken-lactose-lectin-I is shown to be externalized with differentiation, ultimately becoming localized on the surface of myotubes and in the extracellular spaces. In a cellular slime mold, Dictyostelium purpureum, externalization of lectin is elicited by either polyvalent glycoproteins that bind the small amount of endogenous cell surface lectin, or by slime mold or plant lectins that bind unoccupied complementary cell surface oligosaccharides. These results suggest that externalization of endogenous lectin may be a response to specific external signals. We conclude that lectins are frequently held in intracellular reserves awaiting release for specific external functions.