Information about the Structure of the vertebrate striated muscle thick filarrient backbone is important for Understanding the arrangement of both the rod portion of the myosin molecule and the accessory proteins associated with the backbone region of the filament. Although models of the backbone have been proposed. direct data on the Structure of the backbone is limited. III this Study. we provide evidence that electron micrographs of isolated negatively stained cardiac thick filaments contain significant information about the filament backbone. Computed Fourier transforms from isolated cardiac thick filaments show meridional (or near meridional) reflections on the 10th and 11th layer lines that are particularly strong. Comparison or Fourier filtrations of the filaments that exclude. or include, these reflections, provide evidence that these reflections originate at least in part front a series of striations on the backbone at a similar to4 nm spacing. The striations are likely to result either from the packing of the myosin rods, or from proteins such as titin associated with the filament backbone. (C) 2005 Elsevier Inc. All rights reserved.