A new model for the surface arrangement of myosin molecules in tarantula thick filaments

Three-dimensional reconstructions of the negatively stained thick filaments of tarantula muscle with a resolution of 50 Angstrom have previously sugge sted that the helical tracks of myosin heads are zigzagged, short diagonal ridges being connected by nearly axial links. However, surface views of low er contour levels reveal an additional J-shaped feature approximately the s ize and shape of a myosin head. We have modelled the surface array of myosin heads on the filaments using a s a building block a model of a two-headed regulated myosin molecule in whi ch the regulatory light chains of the two heads together form a compact hea d-tail junction. Four parameters defining the radius, orientation and rotat ion of each myosin molecule were varied. In addition, the heads were allowe d independently to bend in a plane perpendicular to the coiled-coil tail at three sites, and to tilt with respect to the tail and to twist at one of t hese sites. After low-pass filtering, models were aligned with the reconstr uction, scored by cross-correlation and refined by simulated annealing. Comparison of the geometry of the reconstruction and the distance between d omains in the myosin molecule narrowed the choice of models to two main cla sses. A good match to the reconstruction was obtained with a model in which each ridge is formed from the motor domain of a head pointing to the bare zone together with the head-tail junction of a neighbouring molecule. The h eads pointing to the Z-disc intermittently occupy the J-position. Each moto r domain interacts with the essential and regulatory light chains of the ne ighbouring heads. A near-radial spoke in the reconstruction connecting the backbone to one end of the ridge can be identified as the start of the coil ed-coil tail. (C) 2000 Academic Press.