Talin is a multidoinain cytoskeletal protein containing discrete binding sites for acidic phospholipids, [I-integrin, actin and vinculin. Hence, it is thought to link microfilaments to the cytoplasmic membrane in cell-matrix adhesion sites, and this should critically depend on talin structure. To obtain more information on the latter, we used energy-filtered transmission electron microscopy of negatively stained talin purified from chicken smooth muscle. We show that in buffers of physiological ionic strength. talin adopts an elongated shape (56 +-7 nm in length), consisting of a series of globular masses. While these compact elements, arranged like beads on a string, were of rather uniform dimensions (3.8 nm in diameter), their center-to-center spacings varied, indicating the flexibility of the connecting strands. The ends of the elongated molecules frequently formed loops. The images obtained are consistent with the assumption that, under the conditions used, the majority of the talin molecules are monomeric. A minor fraction appeared as dimers, composed of two chains only partially intertwined, thus giving rise to Y-shaped particles. Electron micrographs revealed that the biochemically defined 50-kDa N-terminal talin head domain is composed of two globular subunits, while chemical cross-linking provided evidence that the Cterminal 220-kDa fragment is solely responsible for dimerization. These results imply that in the dimeric molecules, the polypeptide chains are arranged in parallel, in contrast to what has been described for human-platelet talin. In buffers of low ionic strength (0.02 M instead of 0.15 M KCI), the molecules collapsed into a compact shape. By showing the high flexibility and versatility of its morphology, our data favour the concept of talin as an important resilient link in microfilament-plasma-membrane attachment.Keywcri-ds: cytoskeleton; talin; structure ; electron microscopy; electron-spectroscopic imaging.Talin is a major component of cell-matrix junctions, concentrated at the cytoplasmic face of focal contacts, dense plaques of smooth muscle and myotendinous junctions (Burridge and Connell. 1983a.b;Tidball et al., 1986). The amino acid sequence of mouse talin consists of 2541 amino acids, from which the molecular mass of the protein was determined as 269 854 Da (Rees et al., 1990). The molecule can be cleaved by thrombin between residues 433 and 434, yielding a 47-kDa N-terminal and a 200-kDa C-terminal fragment (Fox et al., 1985 ;Beckerle et al., 1987). The N-terminal fragment, which has sequence similarities to the membrane-binding domains of protein band 4.1 and ezrin (Rees et al.. 1990). can interact with liposomes containing acidic phospholipids (Niggli et al., 1994; Tempe1 et al., 1995), while the C-terminal portion binds to the cytosolic domains of integrin (Horwitz et al., 1986). vinculin (Burridge and Mangeat, 1984;Gilmore et al.. 1993) and actin (Muguruma et al., 1990;Kaufmann et al., 1991: Goldmann and essential element in the establishment of discrete cell-matrix junctio...