Contractile protein that plays a role in heart development and function (By similarity). Following phosphorylation, plays a role in cross-bridge cycling kinetics and cardiac muscle contraction by increasing myosin lever arm stiffness and promoting myosin head diffusion; as a consequence of the increase in maximum contraction force and calcium sensitivity of contraction force. These events altogether slow down myosin kinetics and prolong duty cycle resulting in accumulated myosins being cooperatively recruited to actin binding sites to sustain thin filament activation as a means to fine-tune myofilament calcium sensitivity to force (PubMed:15331360). During cardiogenesis plays an early role in cardiac contractility by promoting cardiac myofibril assembly (By similarity). Myosin is a hexamer of 2 heavy chains and 4 light chains (By similarity). Interacts with MYOC (By similarity). Abundantly expressed in both cardiac and slow skeletal muscle (soleus), with no detectable expression in fast skeletal muscle (vastus lateralis) or non-muscle tissue. N-terminus is methylated by METTL11A/NTM1. Phosphorylated by MYLK3 and MYLK2; promotes cardiac muscle contraction and function (By similarity) (PubMed:17885681). Dephosphorylated by PPP1CB complexed to PPP1R12B (PubMed:16431080). The phosphorylated form in adult is expressed as gradients across the heart from endocardium (low phosphorylation) to epicardium (high phosphorylation); regulates cardiac torsion and workload distribution (By similarity). This chain binds calcium.