Motor-molecule

Nishizaka T, Miyata H, Yoshikawa H, Ishiwata S and Kinosita K 1995 Unbinding force of a single motor molecule of muscle measured using optical tweezers Nature 377 251-4... 

Dynein, kinesin, and myosin are motor proteins with ATPase activity that convert the chemical bond energy released by ATP hydrolysis into mechanical work. Each motor molecule reacts cyclically with a polymerized cytoskeletal filament in this chemomechanical transduction process. The motor protein first binds to the filament and then undergoes a conformational change that produces an increment of movement, known as the power stroke. The motor protein then releases its hold on the filament before reattaching at a new site to begin another cycle. Events in the mechanical cycle are believed to depend on intermediate steps in the ATPase cycle. Cytoplasmic dynein and kinesin walk (albeit in opposite... 

The structural hypothesis, which was formulated in response to observations that axonal transport rate components move as discrete waves, each with a characteristic rate and a distinctive composition, can explain the coherent transport of functionally related proteins and is consistent with the relatively small numbers of motor molecules in neurons. The only assumption is that the number of elements that can interact with transport motor complexes is limited, and this requires appropriate packaging of the transported material. Different rate components result from packaging of transported material into different, cytologically identifiable, structures. In fact, the faster rates reflect the transport of proteins preassembled as membranous organelles, including vesicles and... 

FIGURE 28-5 Schematic illustration of the movement of cytoskeletal elements in slow axonal transport. Slow axonal transport represents the movement of cytoplasmic constituents including cytoskeletal elements and soluble enzymes of intermediary metabolism at rates of 0.2-2 mm/day which are at least two orders of magnitude slower than those observed in fast axonal transport. As proposed in the structural hypothesis and supported by experimental evidence, cytoskeletal components are believed to be transported down the axon in their polymeric forms, not as individual subunit polypeptides. Cytoskeletal polypeptides are translated on cytoplasmic polysomes and then are assembled into polymers prior to transport down the axon in the anterograde direction. In contrast to fast axonal transport, no constituents of slow transport appear to be transported in the retrograde direction. Although the polypeptide composition of slow axonal transport has been extensively characterized, the motor molecule(s) responsible for the movement of these cytoplasmic constituents has not yet been identified. 

Transport of MBOs to their various destinations is typically mediated by MTs and motor molecules. Membrane and secretory proteins become associated with membranes either during or immediately following their synthesis, and then maintain this association throughout their lifetime in the cell. For example, inhibiting synthesis of either protein or phospholipid leads to a proportional decrease in the amount of both protein and phospholipid... 

Although many motor proteins are found in nervous tissue, there are few instances in which we hilly understand their cellular functions. The proliferation of different motor molecules and the existence of numerous isoforms raises the possibility that some physiological activities require multiple motors. There may be cases in which motors serve a redundant role to ensure that the physiological activity is maintained in the event of a loss of one motor protein. Finally, the existence of so many different types of motor molecule suggests that novel physiological activities requiring molecular motors may be as yet unrecognized. 

N. J. Cordova, B. Ermentrout, and G. E Oster, Dynamics of single-motor molecules the thermal ratchet model, Proc. Natl. Acad. Set USA 89, 339-343 (1992). 

Kinesins and other molecular motors. Before considering further how the myosin motor may work, we should look briefly at the kinesins, a different group of motor molecules,1683 which transport various cellular materials along microtubule "rails." They also participate in organization of the mitotic spindle and other microtubule-dependent activities.168a/b/C See Section C,2 for further discussion. More than 90 members of the family have been identified. Kinesin heads have much shorter necks than do the myosin heads. 

Fig. 19-4).212b However, single kinesin heads, which lack the coiled-coil neck region, have a duty ratio of <0.45. The movement is nonpro-cessive.213 The Ned motor is also nonprocessive.214-216 As mentioned previously, the Ned and kinesin motor domains are at opposite ends of the peptide chain, and the motors move in opposite directions along microtubules.217 218 The critical difference between the two motor molecules was found in the neck domains, which gave rise to differing symmetries in the two heads.219 The latter are shown in Fig. 19-20, in which they have been docked onto the tubulin protofilament structure.

For example, polysaccharides are not likely to provide motor molecules, and proteins are not well suited to carry genetic information. Particular features of nucleic acids and proteins make them well suited for the functions they serve in life on Earth. An understanding of these features provides an intellectual framework for thinking about alternative macromolecules in other forms of life. 

Maughan, D.W., and J.O. Yigoreaux (1999). An integrated view of insect flight muscle genes, motor molecules, and motion. News Physiol. Sci. 14 87-92. 

DNA unzipping the strands, and the ribosomal motors which pull the RNA and the nascent polypeptide through the ribosome. In contrast, myosins whose functional form is filaments, such as myosin II, need not exhibit this property any given myosin molecule need not be attached to the actin filament because attachment and orientation of the myosin filament to the actin filament will be maintained so long as one or two myosins anywhere in the filament remain attached. Which features of these motor molecules determine the presence or absence of proces-sivity remains a mystery. 

Ludwig J, Oliver D, Frank G, Klocker N, Gummer AW, et al. 2001. Reciprocal electromechanical properties of rat prestin The motor molecule from rat outer hair cells. Proc Natl Acad Sci USA 98 4178-4183. 

Fig. 2.11. One step of the Brownian stepper. The step is induced by the binding of a fuel molecule X according to a rate process with a rate 7(f) which is proportional to the concentration [X](t) of the fuel molecules. Afterwards the motor molecule undergoes conformational changes, thereby releasing the used fuel X and advancing by one step Length [15]...

Other single molecule techniques involve direct observation of motor molecules or of SI myosin fragments tagged with highly fluorescent labels. ... 

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