Motor Proteins

Abstract

Myosin motors power movements on actin filaments, whereas dynein and kinesin motors power movements on microtubules. The mechanisms of these motor proteins differ, but, in all cases, ATP hydrolysis and subsequent release of the hydrolysis products drives a cycle of interactions with the track (either an actin filament or a microtubule), resulting in force generation and directed movement.

Figure 1.

Domain structure of members of the myosin superfamily of proteins. This blueprint of the myosins shows the conserved motor domain and the divergence in other domains that mediate movement amplification, self-association, and cargo binding. CaM, calmodulin; IQ, isoleucine glutamine.

Figure 2.

Structure of the myosin motor. (A) Ribbon diagram of the first myosin state (ATP state) for which there was a high-resolution crystal structure (Rayment et al. 1993). The structure represents the S1 fragment of skeletal muscle myosin II, which precedes the coiled coil. In different colors are the major subdomains of the motor and the myosin lever arm with its associated light chains. (B) Diagrammatic interpretation of the subdomains that move relative to each other as relatively rigid bodies as myosin undergoes state transitions during its ATPase cycle. The converter subdomain is the most flexible and mobile of the subdomains and amplifies the movements of the SH1 helix and relay into large movements of the lever arm, of which it is the most proximal component. The color-coded loops and connectors are individually identified at the bottom of the figure. The double-headed arrow indicates closure of the cleft between the upper and lower 50K subdomains that occur on actin binding, which results in a strong binding interface with actin and loss of the ATP hydrolysis products (P_i followed by ADP). Rebinding of ATP following the dissociation of ADP reopens this cleft, causing myosin to dissociate from actin. N term, amino terminal; ELC, essential light chain; 50K, 50-kDa subdomain; RLC, regulatory light chain.

Figure 3.

Myosin ATPase cycle on actin. The myosin states that bind weakly to actin and, thus, do not bear load or generate force and movement, are shown as noncolored. The myosin states that bind strongly to actin and generate force and movement are colored in shades of blue, becoming darker as the binding affinity increases. The binding of ATP to the rigor state (1) terminates the power stroke by causing myosin to dissociate from actin, forming the postrigor (PR) state (2). The repriming of the lever arm, known as the recovery stroke (3), occurs when myosin dissociated from actin in the PR state (or ATP state) undergoes an isomerization and hydrolyzes ATP to form the pre–power stroke state (PPS). Once the ATP is hydrolyzed, weak binding of the PPS myosin to actin can trigger a transition to the phosphate release (P_iR) state (4), which likely involves some movement of the lever arm and formation of higher-affinity, stereo-specific binding to actin. Release of phosphate then drives the reaction forward (essentially unidirectional) with a large movement of the lever arm to form the strong ADP state (5). A further isomerization of the motor leads to the release of phosphate and is coupled to a further movement of the lever arm, completing the myosin power stroke on actin (6).

Figure 4.

Reversal of directionality by repositioning the myosin lever arm. On the left, the movement of the myosin V lever arm on two actin monomers of a filament is depicted. The rigor position is depicted as an opaque arm, whereas the swing began near the pre–power stroke position (PPS), indicated by a transparent lever arm, on actin. The converter subdomain, which is the last subdomain of the motor, as well as the first component of the lever arm, is indicated in red. On the right, the myosin VI motor and its two calmodulins are indicated in rigor on actin. The converter is indicated in green, and note that its lever arm points in the opposite direction (toward the pointed end) of the actin filament as compared with myosin V. In the middle is a comparison of the high-resolution structures of the myosin V and VI converters, shown in red and green, respectively. Note that an insert (insert 2, shown in dark purple) immediately following the last helix of the myosin VI converter redirects the lever arm and binds a calmodulin (CaM; pale purple). This insert is solely responsible for reversing the direction of myosin VI.

Figure 5.

Structure and mechanochemistry of the kinesin superfamily. (A) Kinesin superfamily members share a conserved motor domain but diverge in domains mediating self-association, such as coiled-coil domains and cargo-binding domains. (B) Ribbon diagram of the crystal structure of the kinesin-1 motor domain (from Kull et al. 1996). (C) Kinesin-1 is formed from the homodimerization of two heavy chains. The motor domains are regulated by autoinhibition; the tail domain binds to and inhibits the motor domains. Autoinhibition is relieved by association with specific binding partners. Kinesin-5 (Eg5) forms a bipolar, tetrameric motor. (D) Kinesin-1 moves processively along the microtubule through the coordinated stepping of the two motor domains. Binding of ATP (T) induces a high-affinity association of the head with the microtubule; release of the products of ATP hydrolysis, ADP (D) and P_i, allows dissociation of the head from the microtubule track. (Modified from Milic et al. 2014.)

Figure 6.

Structure and mechanochemistry of dynein. (A,B) The dynein heavy chain, the motor subunit of dynein family motors, is a ∼500-kDa polypeptide with a conserved structure in which an amino-terminal tail domain is followed by a linker domain and six concatenated AAA domains. AAA1 is the site of catalytic ATP hydrolysis; nucleotide binding to other AAA domains might exert a regulatory effect on motor properties. The microtubule-binding domain is located at the tip of the protruding stalk. C-term, carboxyl terminal. (C) Ribbon diagram of the crystal structure of cytoplasmic dynein-2 (Schmidt et al. 2015). (D) ATPase pathway for dynein (D), emphasizing the role of nucleotide binding and hydrolysis in the transition between states that bind either strongly or weakly to the microtubule (M). (Modified from Holzbaur and Johnson 1989, and Imamula et al. 2007.) (E) The bend in the linker element that spans the AAA-ring of dynein plays a key role in the structural transitions between the apo (no nucleotide bound) and pre–power stroke states (modified from Carter et al. 2016).