Molecular Biology of the Cell. 4th version.
Actin-based Motor Proteins Are Members of the Myosin Superfamily
The primary motor protein recognized was skeletal muscle myosin, which is accountable for producing the power for muscle contraction. This myosin, referred to as myosin II (see under) is an elongated protein that’s shaped from two heavy chains and two copies of every of two mild chains. Every of the heavy chains has a globular head area at its N-terminus that accommodates the force-generating equipment, adopted by a really lengthy amino acid sequence that types an prolonged coiled-coil that mediates heavy chain dimerization (Determine 16-51). The 2 mild chains bind near the N-terminal head area, whereas the lengthy coiled-coil tail bundles itself with the tails of different myosin molecules. These tail-tail interactions outcome within the formation of huge bipolar “thick filaments” which have a number of hundred myosin heads, oriented in reverse instructions on the two ends of the thick filament (Determine 16-52).
Every myosin head binds and hydrolyses ATP, utilizing the vitality of ATP hydrolysis to stroll towards the plus finish of an actin filament. The opposing orientation of the heads within the thick filament makes the filament environment friendly at sliding pairs of oppositely oriented actin filaments previous one another. In skeletal muscle, through which fastidiously organized actin filaments are aligned in “thin filament” arrays surrounding the myosin thick filaments, the ATP-driven sliding of actin filaments leads to muscle contraction (mentioned later). Cardiac and easy muscle include myosins which can be equally organized, though they’re encoded by totally different genes.
When a muscle myosin is digested by chymotrypsin and papain, the pinnacle area is launched as an intact fragment (referred to as S1). The S1 fragment alone can generate filament sliding in vitro, proving that the motor exercise is contained utterly inside the head (Determine 16-53).
It was initially thought that myosin was current solely in muscle, however within the 1970’s, researchers discovered {that a} comparable two-headed myosin protein was additionally current in nonmuscle cells, together with protozoan cells. At about the identical time, different researchers discovered a myosin within the freshwater amoeba Acanthamoeba castellanii that was unconventional in having a motor area much like the pinnacle of muscle myosin however a totally totally different tail. This molecule appeared to operate as a monomer and was named myosin I (for one-headed); the traditional myosin was renamed myosin II (for two-headed).
Subsequently, many different myosin sorts have been found. The heavy chains usually begin with a recognizable myosin motor area on the N-terminus, after which diverge extensively with a wide range of C-terminal tail domains (Determine 16-54). The brand new varieties of myosins embrace quite a few one-headed and two-headed varieties which can be roughly equally associated to myosin I and myosin II, and the nomenclature now displays their approximate order of discovery (myosin III by way of at the very least myosin XVIII). The myosin tails (and the tails of motor proteins usually) have apparently diversified throughout evolution to allow the proteins to dimerize with different subunits and to work together with totally different cargoes.
Some myosins (corresponding to VIII and XI) have been discovered solely in vegetation, and a few have been discovered solely in vertebrates (IX). Most, nonetheless, are present in all eucaryotes, suggesting that myosins arose early in eucaryotic evolution. The yeast Saccharomyces cerevisiae accommodates 5 myosins: two myosin Is, one myosin II, and two myosin Vs. One can speculate that these three varieties of myosins are vital for a eucaryotic cell to outlive and that different myosins carry out extra specialised features in multicellular organisms. The nematode C. elegans, for instance, has at the very least 15 myosin genes, representing at the very least seven structural lessons; the human genome contains about 40 myosin genes.
The entire myosins besides one transfer towards the plus finish of an actin filament, though they achieve this at totally different speeds. The exception is myosin VI, which strikes towards the minus finish.
The precise features for many of the myosins stay to be decided. Myosin II is at all times related to contractile exercise in muscle and nonmuscle cells. Additionally it is usually required for cytokinesis, the pinching aside of a dividing cell into two daughters (mentioned in Chapter 18), in addition to for the ahead translocation of the physique of a cell throughout cell migration. The myosin I proteins include a second actin-binding web site or a membrane-binding web site of their tails, and they’re usually concerned in intracellular group and the protrusion of actin-rich constructions on the cell floor. Myosin V is concerned in vesicle and organelle transport. Myosin VII is discovered within the inside ear in vertebrates, and sure mutations within the gene coding for myosin VII trigger deafness in mice and people.
There Are Two Forms of Microtubule Motor Proteins: Kinesins and Dyneins
Kinesin is a motor protein that strikes alongside microtubules. It was first recognized within the large axon of the squid, the place it carries membrane-enclosed organelles away from the neuronal cell physique towards the axon terminal by strolling towards the plus finish of microtubules. Kinesin is analogous structurally to myosin II in having two heavy chains and two mild chains per energetic motor, two globular head motor domains, and an elongated coiled-coil accountable for heavy chain dimerization. Like myosin, kinesin is a member of a big protein superfamily, for which the motor area is the one frequent component (Determine 16-55). The yeast Saccharomyces cerevisiae has six distinct kinesins. The nematode C. elegans has 16 kinesins, and people have about 40.
There are at the very least ten households of kinesin-related proteins, or KRPs, within the kinesin superfamily. Most of them have the motor area on the N-terminus of the heavy chain and stroll towards the plus finish of the microtubule. A very fascinating household has the motor area on the C-terminus and walks in the other way, towards the minus finish of the microtubule. Some KRP heavy chains lack a coiled-coil sequence and appear to operate as monomers, analogous to myosin I. Some others are homodimers, and but others are heterodimers. At the least one KRP (BimC) can self-associate by way of the tail area, forming a bipolar motor that slides oppositely oriented microtubules previous each other, a lot as a myosin II thick filament does for actin filaments. Most kinesins carry a binding web site within the tail for both a membrane-enclosed organelle or one other microtubule. Most of the kinesin superfamily members have particular roles in mitotic and meiotic spindle formation and chromosome separation throughout cell division.
The dyneins are a household of minus-end-directed microtubule motors, however they’re unrelated to the kinesin superfamily. They’re composed of two or three heavy chains (that embrace the motor area) and a big and variable variety of related mild chains. The dynein household has two main branches (Determine 16-56). Probably the most historic department accommodates the cytoplasmic dyneins, that are sometimes heavy-chain homodimers, with two giant motor domains as heads. Cytoplasmic dyneins are in all probability present in all eucaryotic cells, and they’re necessary for vesicle trafficking, in addition to for localization of the Golgi equipment close to the middle of the cell. Axonemal dyneins, the opposite giant department, embrace heterodimers and heterotrimers, with two or three motor-domain heads, respectively. They’re extremely specialised for the fast and environment friendly sliding actions of microtubules that drive the beating of cilia and flagella (mentioned later). A 3rd, minor, department shares larger sequence similarity with cytoplasmic than with axonemal dyneins however appears to be concerned within the beating of cilia.
Dyneins are the biggest of the recognized molecular motors, and they’re additionally among the many quickest: axonemal dyneins can transfer microtubules in a take a look at tube on the exceptional charge of 14 μm/sec. Compared, the quickest kinesins can transfer their microtubules at about 2–3 μm/sec.
The Structural Similarity of Myosin and Kinesin Signifies a Frequent Evolutionary Origin – “how is protein like a car engine”
The motor area of myosins is considerably bigger than that of kinesins, about 850 amino acids in contrast with about 350. The 2 lessons of motor proteins observe alongside totally different filaments and have totally different kinetic properties, and so they don’t have any identifiable amino acid sequence similarities. Nevertheless, dedication of the three-dimensional construction of the motor domains of myosin and kinesin has revealed that these two motor domains are constructed round almost equivalent cores (Determine 16-57). The central force-generating component that the 2 varieties of motor proteins have in frequent contains the location of ATP binding and the equipment essential to translate ATP hydrolysis into an allosteric conformational change. The variations in area dimension and within the alternative of observe will be attributed to giant loops extending outward from this central core. These loops embrace the actin-binding and microtubule-binding websites, respectively.
An necessary clue to how the central core is concerned in power technology has come from the remark that the motor core additionally bears some structural resemblance to the nucleotide binding web site of the small GTPases of the Ras superfamily. As mentioned in Chapter 3 (see Determine 3-74), these proteins exhibit distinct conformations of their GTP-bound (energetic) and GDP-bound (inactive) types: cell “switch” loops within the nucleotide-binding web site are in shut contact with the γ-phosphate within the GTP-bound state, however these loops swing out when the hydrolyzed γ-phosphate is launched. Though the main points of the motion are totally different for the 2 motor proteins, and ATP fairly than GTP is hydrolyzed, the comparatively small structural change within the energetic web site—the presence or absence of a terminal phosphate—is equally amplified to trigger a rotation of a special a part of the protein. In kinesin and myosin, a change loop interacts extensively with these areas of the protein concerned in microtubule and actin binding, respectively, permitting the structural transitions brought on by the ATP hydrolysis cycle to be relayed to the polymer-binding interface. The relay of structural adjustments between the polymer-binding web site and the nucleotide hydrolysis web site appears to work in each instructions, because the ATPase exercise of motor proteins is strongly activated by binding to their filament tracks.
Motor Proteins Generate Pressure by Coupling ATP Hydrolysis to Conformational Adjustments
Though the cytoskeletal motor proteins and GTP-binding proteins each use structural adjustments of their nucleoside-triphosphate-binding websites to provide cyclic interactions with a companion protein, the motor proteins have an additional requirement: every cycle of binding and launch should propel them ahead in a single path alongside a filament to a brand new binding web site on the filament. For such unidirectional movement, a motor protein should use the vitality derived from ATP binding and hydrolysis to power a big motion in a part of the protein molecule. For myosin, every step of the motion alongside actin is generated by the swinging of an 8.5-nm-long α helix, or lever arm (see Determine 16-57), which is structurally stabilized by the binding of sunshine chains. On the base of this lever arm subsequent to the pinnacle, there’s a piston-like helix that connects actions on the ATP-binding cleft within the head to small rotations of the so-called converter area. A small change at this level can swing the helix like an extended lever, inflicting the far finish of the helix to maneuver by about 5.0 nm. These adjustments within the conformation of the myosin are coupled to adjustments in its binding affinity for actin, permitting the myosin head to launch its grip on the actin filament at one level and snatch maintain of it once more at one other. The complete mechanochemical cycle of nucleotide binding, nucleotide hydrolysis, and phosphate launch (which causes the “power stroke”) produces a single step of motion (Determine 16-58). Within the myosin VI subfamily of myosins, which transfer backward (towards the minus finish of the actin filament), the converter area in all probability lies in a special orientation, in order that the identical piston-like motion of the small helix causes the lever arm to rotate in the other way.
In kinesin, as a substitute of the rocking of a lever arm, the small actions of change loops on the nucleotide-binding web site regulate the docking and undocking of the motor head area to an extended linker area that connects this motor head at one finish to the coiled-coil dimerization area on the different finish. When the entrance (main) kinesin head is certain to a microtubule earlier than the facility stroke, its linker area is comparatively unstructured. On the binding of ATP to this certain head, its linker area docks alongside the facet of the pinnacle, which throws the second head ahead to a place the place will probably be capable of bind a brand new attachment web site on the protofilament, 8 nm nearer to the microtubule plus finish than the binding web site for the primary head. The nucleotide hydrolysis cycles within the two heads are carefully coordinated, in order that this cycle of linker docking and undocking can enable the two-headed motor to maneuver in a hand-over-hand (or head-over-head) stepwise method (Determine 16-59A).
The coiled-coil area appears each to coordinate the mechanochemical cycles of the 2 heads (motor domains) of the kinesin dimer and to find out its directionality of motion. Recall that whereas most members of the kinesin superfamily, with their motor domains on the N-terminus, transfer towards the plus finish of the microtubule, a number of superfamily members have their motor domains on the C-terminus and transfer towards the minus finish. Because the motor domains of those two varieties of kinesins are basically equivalent, how can they transfer in reverse instructions? The reply appears to lie in the best way through which the heads are linked. In high-resolution photographs of forward-walking and backward-walking members of the kinesin superfamily certain to microtubules, the heads which can be connected to the microtubule are basically indistinguishable, however the second, unattached heads are oriented very in a different way. This distinction in tilt apparently biases the subsequent binding web site for the second head, and thereby determines the directionality of motor motion (Determine 16-60).
Though each myosin and kinesin endure analogous mechanochemical cycles, the precise nature of the coupling between the mechanical and chemical cycles is totally different within the two instances (see Determine 16-60). For instance, myosin with none nucleotide is tightly certain to its actin observe, in a so-called “rigor” state, and it’s launched from this observe by the affiliation of ATP. In distinction, kinesin types a rigor-like tight affiliation with a microtubule when ATP is certain to the kinesin, and it’s hydrolysis of ATP that promotes launch of the motor from its observe.
Thus, cytoskeletal motor proteins work in a fashion extremely analogous to GTP-binding proteins, besides that in motor proteins the small protein conformational adjustments (a number of tenths of a nanometer) related to nucleotide hydrolysis are amplified by particular protein domains—the lever arm within the case of myosin and the linker within the case of kinesin—to generate large-scale (a number of nanometers) conformational adjustments that transfer the motor proteins stepwise alongside their filament tracks. The analogy between the GTPases and the cytoskeletal motor proteins has just lately been prolonged by the remark that one of many GTP-binding proteins—the bacterial elongation issue G—interprets the chemical vitality of GTP hydrolysis into directional motion of the mRNA molecule on the ribosome.
Motor Protein Kinetics Are Tailored to Cell Capabilities
The motor proteins within the myosin and kinesin superfamilies exhibit a exceptional range of motile properties, nicely past their alternative of various polymer tracks. Most strikingly, a single dimer of standard kinesin strikes in a extremely processive vogue, touring for tons of of ATPase cycles alongside a microtubule with out dissociating. Skeletal muscle myosin II, in distinction, can’t transfer processively and makes only one or a number of steps alongside an actin filament earlier than letting go. These variations are essential for the motors’ varied organic roles. A small variety of kinesin molecules should be capable of transport a mitochondrion all the best way down a nerve cell axon, and subsequently require a excessive degree of processivity. Skeletal muscle myosin, in distinction, by no means operates as a single molecule however fairly as a part of an enormous array of myosin II molecules. Right here processivity would truly inhibit organic operate, since environment friendly muscle contraction requires that every myosin head carry out its energy stroke after which shortly get out of the best way, to keep away from interfering with the actions of the opposite heads connected to the identical actin filament.
There are two causes for the excessive diploma of processivity of kinesin motion. The primary is that the mechanochemical cycles of the 2 motor heads in a kinesin dimer are coordinated with one another, in order that one kinesin head doesn’t let go till the opposite is poised to bind. This coordination permits the motor protein to function in a hand-over-hand vogue, by no means permitting the organelle cargo to diffuse away from the microtubule observe. There isn’t a obvious coordination between the myosin heads in a myosin II dimer. The second purpose for the excessive processivity of kinesin motion is that kinesin spends a comparatively giant fraction of its ATPase cycle tightly certain to the microtubule. For each kinesin and myosin, the conformational change that produces the force-generating working stroke should happen whereas the motor protein is tightly certain to its polymer, and the restoration stroke in preparation for the subsequent step should happen whereas the motor is unbound. However as we now have seen in Determine 16-59, myosin spends solely about 5% of its ATPase cycle within the tightly certain state and is unbound the remainder of the time.
What myosin loses in processivity it positive factors in pace; in an array through which many motor heads are interacting with the identical actin filament, a set of linked myosins can transfer their filament a complete distance equal to twenty steps throughout a single cycle time, whereas kinesins can transfer solely two. Thus, myosins can sometimes drive filament sliding far more quickly than kinesins, although they hydrolyze ATP at comparable charges and take molecular steps of comparable size.
Inside every motor protein class, motion speeds range extensively, from about 0.2 to 60 μm/sec for myosins, and from about 0.02 to 2 μm/sec for kinesins. These variations come up from a fine-tuning of the mechanochemical cycle. The variety of steps that a person motor molecule can absorb a given time, and thereby the speed, will be elevated by both rising the motor protein’s intrinsic ATPase charge or lowering the proportion of cycle time spent certain to the filament observe. Furthermore, the scale of every step will be modified by both altering the size of the lever arm (for instance, the lever arm of myosin V is about thrice longer than the lever arm of myosin II) or the angle by way of which the helix swings (Determine 16-61). Every of those parameters varies barely amongst totally different members of the myosin and kinesin households, comparable to barely totally different protein sequences and constructions. It’s assumed that the conduct of every motor protein, whose operate is decided by the id of the cargo connected by way of its tail-domain, has been fine-tuned throughout evolution for pace and processivity based on the precise wants of the cell.
“how is protein like a car engine”