A structural model for actin-induced nucleotide release in myosin

Abstract

Myosins are molecular motor proteins that harness the chemical energy stored in ATP to produce directed force along actin filaments. Complex communication pathways link the catalytic nucleotide-binding region, the structures responsible for force amplification and the actin-binding domain of myosin. We have crystallized the nucleotide-free motor domain of myosin II in a new conformation in which switch I and switch II, conserved loop structures involved in nucleotide binding, have moved away from the nucleotide-binding pocket. These movements are linked to rearrangements of the actin-binding region, which illuminate a previously unobserved communication pathway between the nucleotide-binding pocket and the actin-binding region, explain the reciprocal relationship between actin and nucleotide affinity and suggest a new mechanism for product release in myosin family motors.

Myosin cleft movement and its coupling to actomyosin dissociation

“…This approach has been refined to the point of identifying a potential hinge point around switch 1 at the myosin nucleotide site that allows the upper 50K domain to rotate towards the lower 50K domain 7 . Recent crystallographic studies support this concept 6,8,9 . While structural evidence is mounting in favour of the cleft closure model, it is important to test this idea in solution.…”

“…2 However, while crystal structures of myosin motor domains have been obtained for a number of nucleotide-bound states, the nature of the actin binding interaction has not been observed directly to date 2,3,8,10,11 . Residues involved in actin binding were identified by fitting the envelope of the myosin crystal structure into the electron micrograph density of decorated actin filaments.…”

Molecular engineering of myosin

“…Based on structural models of myosin motor domains in different nucleotide states, it has been suggested that myosins amplify the 'open' to 'closed' transition of switch-2 to a swinging motion of the adjacent lever-arm from an initial 'up' to a 'down' position (Geeves & Holmes 1999). Movement of switch-1 is communicated by the three edge b-strands of the central b-sheet to the actin-binding residues of the upper and lower 50 K domains (myosin subdomains), leading to changes of up to 20 Å in the distance between elements of the actinbinding region that are positioned in the upper and lower 50 K domains (Coureux et al 2003;Reubold et al 2003). Additionally, the conformational changes that are associated with the transition of the switch motifs are tightly coordinated and integrated.…”

How Linear Motor Proteins Work

“…Binding of the myosin motor domain to actin induces a reversal of the sequence of conformational changes that are induced by ATP binding. Strong actin binding induces closing of the large cleft between U50 and L50 [16]; this leads to a distortion of the central β-sheet, the outward movement of the nucleotide binding loops disrupts the coordination of the Mg 2+ ion and thereby ADP binding [17,18]. The loss of the Mg 2+ -ion coordination induced by actin-binding is similar to the effect of GTPase exchange factors on the release of GDP by small G-proteins.…”

“…The loss of the Mg 2+ -ion coordination induced by actin-binding is similar to the effect of GTPase exchange factors on the release of GDP by small G-proteins. Therefore, actin can be viewed as an ADP-exchange factor for myosin [12,18]. Concomitant with the transition from the ATP-bound state to the rigor-like state, the lever arm swings from its UP position to the DOWN position [19].…”