A NOVEL DICTYOSTELIUM CELL-SURFACE PROTEIN IMPORTANT FOR BOTH SELL ADHESION AND CELL SORTING

A mutant of Dictyostelium that is aberrant in the process of tip forma tion (dtfA(-): defective in tip formation A) has been isolated by gene tagging. The dtfA gene is predicted to encode a protein of 163 kDa, T here are no extensive sequence homologies between DTFA and previously identified proteins, but four short N-terminal sequence motifs show pa rtial homology to repeats found in mammalian mucins, Immunofluorescenc e reveals a lattice-like arrangement of DTFA protein at the cell surfa ce. When developing on a bacterial lawn, cells of the mutant strain (d tfA- cells) aggregate to form tight mounds, but development then becom es arrested, When developed in the absence of nutrients, a fraction of dtfA(-) cells complete development, but there is a long delay at the tight mound stage and the culminants that eventually form are aberrant . In such dtfA- mounds the prestalk cells fail to move to the apex on cue and so tip formation is delayed. dtfA- cells also show a condition al defect in early development, in that they are unable to aggregate w hen plated at low density, In addition dtfA- cells do not agglomerate efficiently when shaken in suspension. In combination, these results s uggest that DTFA may form part of a cell-cell adhesion system that is needed both for optimal aggregation and for efficient cell sorting dur ing multicellular development, The DTFA protein also appears to be imp ortant during cell growth, because cytokinesis is defective and the ac tin cytoskeleton aberrant in growing dtfA- cells.