REPLACEMENT OF THE PHOSPHOLIPID-ANCHOR IN THE CONTACT SITE-A GLYCOPROTEIN OF D-DISCOIDEUM BY A TRANSMEMBRANE REGION DOES NOT IMPEDE CELL-ADHESION BUT REDUCES RESIDENCE TIME ON THE CELL-SURFACE

The contact site A (csA) glycoprotein of Dictyostelium discoideum, a c ell adhesion molecule expressed in aggregating cells, is inserted into the plasma membrane by a ceramide-based phospholipid (PL) anchor. A c arboxyterminal sequence of 25 amino acids of the primary csA translati on product proved to contain the signal required for PL modification. CsA is known to be responsible for rapid, EDTA-resistant cohesion of c ells in agitated suspensions. To investigate the role of the PL modifi cation of this protein, the anchor was replaced by the transmembrane r egion and short cytoplasmic tail of another plasma membrane protein of D. discoideum. In cells transformed with appropriate vectors, PL-anch ored or transmembrane csA was expressed under the control of an actin promoter during growth and development. The transmembrane form enabled the cells to agglutinate in the presence of shear forces, similar to the PL-anchored wild-type form. However, the transmembrane form was mu ch more rapidly internalized and degraded. In comparison to other cell -surface glycoproteins of D. discoideum the internalization rate of th e PL-anchored csA was extremely slow, most likely because of its exclu sion from the clathrin-mediated pathway of pinocytosis. Thus, our resu lts indicate that the phospholipid modification is not essential for t he csA-mediated fast type of cell adhesion but guarantees long persist ence of the protein on the cell surface.