Discoidin I is implicated in cell-substratum attachment and ordered cell migration of Dictyostelium discoideum and resembles fibronectin.

All three forms of discoidin I, an endogenous N-acetylgalactosamine-binding lectin from D. discoideum, contain the amino acid sequence gly-arg-gly-asp also found in fibronectin and implicated in its attachment to cells. Synthetic peptides containing these and adjacent amino acids of discoidin I block organized streaming during aggregation of D. discoideum and, at higher concentrations, block cell attachment and spreading on a plastic surface and formation of fruiting bodies. Pure discoidin I (with or without N-acetylgalactosamine) and univalent anti-discoidin I also block formation of streams during aggregation. Two mutants of D. discoideum with low levels of discoidin I apparently reflect the deficiency of this endogenous lectin by failing to form streams or to spread on plastic and by a partial failure to enter aggregates. Together, the results indicate that discoidin I functions like fibronectin to promote cell attachment and spreading as well as ordered cellular migration during morphogenesis.