Full-length myosin Va exhibits altered gating during processive movement on actin

Myosin Va (myoV) is a processive molecular motor that transports intracellular cargo along actin tracks with each head taking multiple 72-nm hand-over-hand steps. This stepping behavior was observed with a constitutively active, truncated myoV, in which the autoinhibitory interactions between the globular tail and motor domains (i.e., heads) that regulate the full-length molecule no longer exist. Without cargo at near physiologic ionic strength (100 mM KCl), full-length myoV adopts a folded (approximately 15 S), enzymatically-inhibited state that unfolds to an extended (approximately 11 S), active conformation at higher salt (250 mM). Under conditions favoring the folded, inhibited state, we show that Quantum-dot-labeled myoV exhibits two types of interaction with actin in the presence of MgATP. Most motors bind to actin and remain stationary, but surprisingly, approximately 20% are processive. The moving motors transition between a strictly gated and hand-over-hand stepping pattern typical of a constitutively active motor, and a new mode with a highly variable stepping pattern suggestive of altered gating. Each head of this partially inhibited motor takes longer-lived, short forward (35 nm) and backward (28 nm) steps, presumably due to globular tail-head interactions that modify the gating of the individual heads. This unique mechanical state may be an intermediate in the pathway between the inhibited and active states of the motor.