Figure 3.

Surface views of oblique section image reconstructions (OSRs) from IHM treated with adenylyl-imidodiphosphate (AMPPNP), a non-hydrolysable analog of ATP at 23 °C (on the left in gold color) and in rigor (on the right in copper color), superposition of rigor and AMPPNP in the middle. (A) Myac layers; (B) actin layers; (C) transverse view of unflared-X layer (AMPPNP on the left) and flared-X layer (rigor on the right). The region in the myac layer of (A) and the region of the actin layer of (B) contained in the transverse view in (C) are marked by the Wedgewood blue and teal green backgrounds respectively. L, Lead bridge; R, rear bridge; T, troponin. Triangles in (B) highlight the triangular shape of rigor lead bridges. Lead bridges of rigor and AMPPNP do not coincide exactly in shape so that in the superimposed region in the center, extra mass in rigor shows as copper color. The troponin density is more pronounced in rigor, possibly because the thin filaments are better ordered. In (B) the cross-bridge pair that produces the two arms of the unflared-X are labeled 1 and 2. These same cross-bridges are similarly labeled in (C). In (C) the rigor rear bridge is located at the far surface of the flared-X layer (denoted by the arrow). The extra density provided by the rigor rear bridge results in the close proximity of the vertex arms in projections of flared-X. Reprinted from [28] with permission from Elsevier.