Kinetic Adaptations of Myosins for Their Diverse Cellular Functions

Abstract

Members of the myosin superfamily are involved in all aspects of eukaryotic life. Their function ranges from the transport of organelles and cargos to the generation of membrane tension, and the contraction of muscle. The diversity of physiological functions is remarkable, given that all enzymatically active myosins follow a conserved mechanoenzymatic cycle in which the hydrolysis of ATP to ADP and inorganic phosphate is coupled to either actin-based transport or tethering of actin to defined cellular compartments. Kinetic capacities and limitations of a myosin are determined by the extent to which actin can accelerate the hydrolysis of ATP and the release of the hydrolysis products and are indispensably linked to its physiological tasks. This review focuses on kinetic competencies that - together with structural adaptations - result in myosins with unique mechanoenzymatic properties targeted to their diverse cellular functions.

Keywords: actin; allostery; cation; cytoskeleton; mechanoenzymology; molecular motor; muscle; myosin; transporter.

Fig. 1

Myosin Topology. A) A generic myosin heavy chain consists of a motor domain (lilac), a light chain binding neck domain (pistachio) and a class-specific tail domain (blue). The motor domain is catalytically active and largely determines the interaction signatures of the motor with actin in a nucleotide-dependent manner. The neck domain and the associated light chains function as amplifiers for mechanical force. The tail domains may contain a large collection of functional subdomains which establish the interaction with binding partners and cargos, target a myosin to specific subcellular compartments (3). The tail domain may also dimerize two myosin heavy chains and promote their oligomerization. For kinetic studies, monomeric myosin subfragment-1 (S1)-like constructs comprising the myosin motor domain and the light chain binding neck region are commonly used. S1-like constructs are in most cases constitutively active. To study regulatory aspects of the kinetic and functional activity of dimeric myosins, the heavy meromyosin (HMM) fragment comprising the motor domain, the neck domain, and parts of the coiled-coil forming tail domain are used. The schemes are not drawn to scale. B) 3-dimentional model of a myosin showing the globular motor domain (lilac), the light chain (lemon) binding neck domain (pistachio) and the tail domain (blue). For illustration purposes, the structures of the motor domain of myosin-1C (lilac, PDB entry 4BYF) and the light chain binding tail domain of myosin-1C (pistachio, PDB entry 4R8G) were merged. The neck domain of myosin-1C binds three calmodulins as light chains as shown in lemon. The tail domain harbors a C-terminal extended PH domain (blue). C) Overview of the myosin nucleotide binding pocket (PDB entry 2XEL) in which a salt bridge is formed between the nucleotide switches switch-1 and switch-2. The respective amino acids and the nucleotide are shown in stick representation. The cofactor Mg²⁺ is shown as green sphere. D) Sequence alignment of selected myosins from classes-1, -2, -5, -6 that follow consensus switch-1 and switch-2 sequences. Catalytically inactive class-18 myosins have highly degenerate switch-1 and switch-2 consensus sequences that do not allow for the formation of a salt bridge between both nucleotide switches. Abbreviations used are as follows: Dd M1E: Dictyostelium discoideum myosin-1E, Hs NM2A: Homo sapiens nonmuscle myosin-2A; Dd M2: Dictyostelium discoideum nonmuscle myosin-2; Gg M5A: Gallus gallus myosin-5A; Ss M6: Sus scrofa myosin-6; Mm M18A; Mus musculus myosin-18A; Dm M18: Drosophila melanogaster myosin-18.

Fig. 2

The myosin kinetic and mechanic cycle and kinetic concepts. A) Consensus scheme of the myosin and actomyosin ATPase cycle in conjunction with a mechanical model. The upper part represents the ATPase cycle in the absence of actin. The lower part represents the ATPase in the presence of actin. The main flux through the pathway is described as a generic series of sequential steps including (I) ATP binding, (II) ATP-induced dissociation of myosin from actin, (III) ATP hydrolysis, (IV) Rebinding of M.ADP.P_i to actin sets the starting point for the mechanical interaction. The release of the hydrolysis products (V) P_i and (VI) ADP are linked to conformational changes in the motor domain that result in the translocation of myosin on the actin filament. The (V) P_i release is rate-limiting in low duty ratio myosins to extend the time myosin spends in the weak actin-binding states. The release of (VI) ADP is slow and rate-limiting in high duty ratio myosins to extend the time the motor spends in the strong actin-binding states. The main flux through the pathway is indicated by black font color. A green box indicates strong actin-binding states, a blue box weak actin-binding states. Abbreviations used: A: actin; M: myosin; P_i: inorganic phosphate. In the structural models, actin is represented in grey, the myosin motor and neck region in cherry and the light chains in orange color. B) Schematic representation of the duty ratio, the time myosin spends in the strong actin-binding states. The duty ratio is a function of the actin and ATP concentration. Both define the relative distribution of the strong and weak actin-binding states in the actomyosin ATPase cycle (20, 23). Nonprocessive myosins have a low duty ratio and spend a short fraction of the kinetic cycle bound to actin and a large fraction of time in the weak actin-binding states (left panel). Conversely, processive myosins spend a short fraction of the overall kinetic cycle in the weak actin-binding states and a long fraction in the strong actin-binding states (middle panel). Conditional processive myosins are kinetically nonprocessive myosins that can alter their kinetic properties to increase the time they spend strongly attached to actin to become processive (right panel). Weak actin-binding states are colored lilac, strong actin binding states in pistachio.

Fig. 3

Myosin classification. The classification of myosin motors into five classes includes type I (fast movers), type II (force holders), type III (strain sensors), type IV (gated/processive myosins), and type V (catalytically inactive) myosins. The arrows indicate changes in the myosin type that may be induced by oligomerization of myosin-2 molecules into filaments or by increases in the free Mg²⁺ concentration for some class-5 myosins. The lack of detailed kinetic transient-kinetic information only allows for a provisional classification of class-9 and -19 myosins. Abbreviations used: Ac M1A: Acanthamoeba castellanii myosin-1A; Ac M1B: Acanthamoeba castellanii myosin-1B; Ac M2: Acanthamoeba castellanii myosin-2; Bt M2 (card): Bos taurus cardiac myosin-2; Bt M2 (slow): Bos taurus slow muscle myosin-2; Bt M10: Bos taurus myosin-10; Cc M11: Chara corallina myosin-11; Dd M1B: Dictyostelium discoideum myosin-1B; Dd M1D: Dictyostelium discoideum myosin-1D; Dd M1E: Dictyostelium discoideum myosin-1E; Dd M2: Dictyostelium discoideum myosin-2; Dd M5B: Dictyostelium discoideum myosin-5B; Dm M2 (IF): Drosophila melanogaster indirect flight muscle myosin-2; Dm M2: Drosophila melanogaster nonmuscle myosin-2; Dm M5: Drosophila melanogaster myosin-5; Dm M7A: Drosophila melanogaster myosin-7A; Dm M7B: Drosophila melanogaster myosin-7B; Dm M18: Drosophila melanogaster myosin-18; Dm M20: Drosophila melanogaster myosin-20; Gg M1A: Gallus gallus myosin-1A; Gg M2 (sm): Gallus gallus smooth muscle myosin-2; Gg M5A: Gallus gallus myosin-5A; Hs M1E: Homo sapiens myosin-1E; Hs M2 (IIa): Homo sapiens striated muscle myosin-IIa; Hs M2 (IIb): Homo sapiens striated muscle myosin-IIb; Hs M2 (IId): Homo sapiens striated muscle myosin-IId; Hs M2 (EO): Homo sapiens extraocular muscle myosin-2; Hs NM2A: Homo sapiens nonmuscle myosin-2A; Hs NM2B: Homo sapiens nonmuscle myosin-2B; Hs NM2C: Homo sapiens nonmuscle myosin-2C; Hs M3A: Homo sapiens myosin-3A; Hs M3B: Homo sapiens myosin-3B; Hs M5B: Homo sapiens myosin-5B; Hs M5C: Homo sapiens myosin-5C; Hs M6: Homo sapiens myosin-6; Hs M7A: Homo sapiens myosin-7A; Hs M7B: Homo sapiens myosin-7B; Hs M18A: Homo sapiens myosin-18A; Lp M3: Limulus polyphemus myosin-3; Mm M7B; Mus musculus myosin-7B; Mm M18A; Mus musculus myosin-18A; Mm M19; Mus musculus myosin-19; Nt: Nicotiana tabacum myosin-11; Oc M2 (sk): Oryctolagus cuniculus skeletal muscle myosin-2; Oc M2 (soleus): Oryctolagus cuniculus soleus muscle myosin-2; Rr M1B: Rattus rattus myosin-1B; Rr M1C: Rattus rattus myosin-1C; Rr M9B: Rattus rattus myosin-9B; Ss M6: Sus scrofa myosin-6.