A TALIN HOMOLOG OF DICTYOSTELIUM RAPIDLY ASSEMBLES AT THE LEADING-EDGE OF CELLS IN RESPONSE TO CHEMOATTRACTANT

In an attempt to identify unknown actin-binding proteins in cells of D ictyostelium discoideum that may be involved in the control of cell mo tility and chemotaxis, monoclonal antibodies were raised against prote ins that had been enriched on an F-actin affinity matrix. One antibody recognized a protein distinguished by its strong accumulation at the tips of filopods. These cell-surface extensions containing a core of b undled actin filaments are rapidly protruded and retracted by cells in the growth-phase stage. The protein of 269 kD turned out to resemble mouse fibroblast talin (Rees et al., 1990) in its primary structure. T he fit is best among the first 400-amino acid residues of the NH2-term inal region where identity between the two proteins is 44% and the las t 200-amino acid residues of the COOH-terminal region with 36% identit y. In the elongated cells of the aggregation stage the Dictyostelium t alin is accumulated at the entire front where also F-actin is enriched . Since this protein exists in a soluble state in the cytoplasm, mecha nisms are predicted that cause accumulation at sites of the cell where a front is established. Evidence for receptor-mediated accumulation w as obtained by local stimulation of cells with cAMP. When a new front was induced by the chemoattractant, the talin accumulated there within half a minute, indicating a signal cascade in Dictyostelium responsib le for assembly of the talin beneath sites of the plasma membrane wher e chemoattractant receptors are strongly activated. The ordered assemb ly of the talin homologue together with actin and a series of other pr oteins is considered to play a key role in chemotactic orientation.