Cells lacking SadA move faster but with less path persistence.

Using a clever genetic screen, Fey et al., reporting on page 1109, have identified the first cell–substrate adhesion molecule in the social amoeba Dictyostelium discoideum. The protein shares some structural features with adhesion molecules in higher eukaryotes, suggesting that the relatively simple amoeba will be a useful model system for understanding the contributions of substrate adhesion to cell movement.

The authors generated a panel of Dictyostelium insertional mutants, and then screened the transformants by repeatedly transferring them to new culture dishes. Only mutants that lacked the ability to attach to the dish were transferred. This screen identified nine independent substrate adhesion deficient (sad) mutants, one of which has now been characterized. sadA defines a novel gene encoding a molecule critical for cell–substrate adhesion in vegetative cells. GFP-tagged SadA protein localizes to the plasma membrane. In addition to multiple membrane-spanning regions, the protein's predicted structure includes three EGF-like domains, suggesting that these domains may be an evolutionarily conserved feature of adhesion molecules.

Mammalian cell–substrate adhesion is more complex than one molecule, but the identification of nine genes essential for adhesion in Dictyostelium is an encouraging start. The authors are now characterizing the other Sad genes in an effort to determine how their gene products interact. ▪