How kinesin's two heads coordinate their ATPase cycles during processive movement also remains an important unresolved question in the motility mechanism. If the nucleotide- and microtubule-binding states of the two heads are completely unsynchronized, then kinesin would not be able to achieve tight chemomechanical coupling (each ATP hydrolysis leading to a step) and possibly its high processivity (Valentine and Gilbert, 2007). To coordinate the activities of the two heads and keep them out of phase, it is believed that a chemical or structural transition in one head is inhibited until the partner head proceeds through a critical step in its cycle (referred to as a “gating” mechanism).