Fig. 1.

3D reconstructions from electron micrographs of negatively stained or frozen-hydrated thick filaments for smooth, skeletal, and cardiac muscles of different invertebrate and vertebrate animals. According to the presence of interacting-heads motif (IHM) on the 3D reconstructions for each species, the IHM seem to be present in the bilaterians metazoans. These thick filaments exhibits three, four, or seven helices of IHMs. All these 3D reconstructions show densities corresponding to IHMs (red circles) approximately parallel to the filament axis except the IHM for indirect flight muscle (IFM) of the insect Lethocerus, which is approximately perpendicular. Bare zone location at the top. Zebrafish 3D reconstruction reproduced with permission from González-Solá et al. (2014) and the Biophysical Journal. Images of metazoans from Wikipedia (S. mansoni: David Williams, Illinois State University, Scallop: Manfred Heyde, Lethocerus: Richard Orr, H. sapiens: Michelangelo Buonarroti, The Creation of Adam c. 1512)