A comparison of the atomic model of F-actin with cryo-electron micrographs of actin and decorated actin.

We compare the atomic model calculated from the crystal structure and the X-ray fiber diagram of orientated F-actin1) with the 3-D reconstructions produced from cryo-electron microscopy of actin2). Out to 30A resolution the two structures are essentially identical. Furthermore, by combining the atomic model with the reconstruction of S1-decorated actin filaments2) one can establish the nature of the actin binding site for myosin in the rigor complex. Each myosin head binds to two actin molecules on two distinct sites. Some of the actin residues involved in each of these binding sites can be identified. Furthermore, the atomic model of actin may be combined with the reconstruction of the S1 decorated thin filament to establish the tropomyosin binding site in the rigor complex. This result is compared with the model of tropomyosin-actin derived from an analysis of the X-ray fibre diagram of a reconstituted thin filament and are shown to be very similar.