Figure 2.

(A) Surface rendering of the white-framed area in Fig. 1. Light gray, unaveraged tomogram; dark gray, averaged filaments replacing or superimposed on the same filaments of the original tomogram averaged only along the y-axis. In the superimposed region, wherever the unaveraged tomogram displays mass that does not appear in the average, the extra mass is shown in light gray. M, myosin filament; A, actin filament; T, troponin complex. A 14.5-nm repeat of crossbridge collars is visible on the thick filament even without averaging. The troponin landmark, best detected in the averages, is identified every 38.7 nm along thin filaments. Target zones in glycol-PNP fall midway between troponin sites. These are the same levels (A, dotted lines at lower left; single arrowheads at lower center and right) occupied by the regular crossbridge attachments noted in Fig 1. The narrow zone of regular attachment indicates that target zones in glycol-PNP are more axially restricted than rigor, apparently because rear bridge targets are not favored. Target zone bridges vary in angle around 90° (reflected by the angle of white arrows). Nontarget zone bridges (*) attach between target zone levels and their structure is highly variable. Extended stretches that lack bridges are fairly common (#). Actin filaments are often off center where asymmetry of crossbridge attachments is pronounced. Z-line is at the bottom. (B) Target zone crossbridge attachments mapped onto white silhouette of A. Horizontal black bars mark levels where semiregular near-90° crossbridges attach to target zones, whose locations were determined by the position of troponin in the average. Horizontal gray stripes mark levels of target zones, positioned midway between troponin bumps, in axial average on left. Black lines represent the actual attachments at or near those actin target zones. Most of the time, these lines fall into the gray regions, showing that the rows of 38.7-nm crossbridges attach in the rigor lead bridge target zone. However, a few attachments fall outside the target zones (rows 4 and 8). Registration of axial average with underlying and adjacent filaments is computed, not manual. The gray stripes fit well to levels of individual target zones across the adjacent tomogram, but for best reading of troponin repeat and actin azimuth on single filaments (as in Fig. 5), the local average of that filament is used.