Figure 2.

Cardiac myosin integration into the sarcomere.A, diagram of the sarcomere, drawn roughly to scale. Bands and discs observed by electron microscopy are labeled. B, cartoon showing a zoomed-in view of the sarcomere within the C-zone. Elements are drawn roughly to scale. Myosin heads in the super relaxed state (SRX) are bound to the thick filament backbone, whereas myosin heads in the disordered relaxed state (DRX) are freed from the backbone. C, schematic structure of myosin in the autoinhibited, interacting heads motif (IHM) associated with the SRX (left and center) and the DRX (right). In the IHM, the blocked head (yellow) interacts with the free head (green). It has been proposed that the IHM is stabilized by cMyBP-C (blue), where phosphorylation of cMyBP-C reduces its interaction with myosin (center). Phosphorylation of the RLC (pink) and mechanical forces each promote the formation of the DRX; however, it is important to note that they are not required for the formation of the DRX. Note that RLC phosphorylation can occur on either one or both of the heads.