EVIDENCE FOR A CONFORMATIONAL CHANGE IN ACTIN INDUCED BY FIMBRIN (N375) BINDING

Fimbrin belongs to a superfamily of actin cross-linking proteins that share a conserved 27-kD actin-binding domain. This domain contains a t andem duplication of a sequence that is homologous to calponin. Calpon in homology (CH) domains not only cross-link actin filaments into bund les and networks, but they also bind intermediate filaments and some s ignal transduction proteins to the actin cytoskeleton. This fundamenta l role of CH domains as a widely used actin-binding domain underlines the necessity to understand their structural interaction with actin. U sing electron cryomicroscopy, we have determined the three-dimensional structure of F-actin and F-actin decorated with the NH2-terminal CH d omains of fimbrin (N375). In a difference map between actin filaments and N375-decorated actin, one end of N375 is bound to a concave surfac e formed between actin subdomains 1 and 2 on two neighboring actin mon omers. In addition, a fit of the atomic model for the actin filament t o the maps reveals the actin residues that line, the binding surface. The binding of N375 changes actin, which we interpret as a movement of subdomain 1 away from the bound N375. This change in actin structure may affect its affinity for other actin-binding proteins and may be pa rt of the regulation of the cytoskeleton itself. Difference maps betwe en actin and actin decorated with other proteins provides a way to loo k for novel structural changes in actin.