Determination of the handedness of the crossbridge helix of Limulus thick filaments.

Thick filaments, isolated in their long conformation from unstimulated Limulus telson muscles, were shadowed with platinum or platinum-carbon and examined using electron microscopy and optical diffraction techniques. All filaments showed evidence of a right-handed surface helix, which had a major repeat at approx. 43 nm. In fortuitously oriented specimens the subunits, presumably crossbridges, which comprised the helical strands were clearly delineated. Optical transforms obtained from images of shadowed filaments confirmed the helical repeat at approx. 43 nm and could be readily interpreted as patterns expected from a one-surface view of the four-stranded filament structure we have previously reported. The striking resemblance between optically filtered images of shadowed filaments and the computed reconstruction of the one-surface filament further confirm our model for the myosin lattice of the Limulus thick filament.