Fig. 3.

Identification of MyBP-C. (A, B) Interleaved stereo images showing part of the averaged thick filament tomogram between stripes 7 and 9 (purple colored mesh, with density thresholded to optimize visualization of MyBP-C). The location of actin filaments has been drawn in (white) as they are suppressed by the averaging protocol (which is centered on the myosin filament), due to disorder in the filament lattice presumably caused by variable amounts of compression occurring during the specimen preparation procedures (including freezing, freeze-substitution, dehydration, and sectioning), and also by shrinkage effects due to electron radiation. Actin filaments are clearly visualized at these positions when a lower threshold is used (Fig. 4). Blue and yellow show fitting of heads (crowns 2 and 3 from stripe 3 and crown 1 from stripe 4) into the reconstruction (see text). (C–E) Transverse views at stripes 7 to 9, with actin positions depicted in white circles; contact between MyBP-C and actin is seen at each level. In (D and E), stripe 4, crown 1 density is shown in yellow and blue [cf. (A, B) above]. In (B), (C) and (D) curved, white lines depict possible paths of MyBP-C, which appear to approach and make contact with actin filaments anticlockwise (stripe 7), clockwise (stripe 8), and in both directions (stripe 9). (F) Model of possible MyBP-C arrangement. A myosin filament is shown surrounded by six actin filaments, with two possible paths of MyBP-C connecting them via the N terminus. C-terminal MyBP-C domains, which may run axially along the filament, are not shown as they would be obscured (see Fig. S5).