Molecular motor protein. Molecular model of a two-headed motor protein, Myosin V. Motor proteins convert chemical energy into mechanical movements in response to specific chemical stimuli. Myosins are a large family of motor proteins that are responsible for muscle contraction in eukaryotic tissues. Most myosin molecules are composed of a head, neck, and tail domain. Each domain serves a specific purpose in the action of myosin on actin filaments in muscles to bring about contractions.