At the nanoscale of cells and molecules, movement is effected by much different forces from those in the macroscopic world. For example, the high protein concentration (200-300 mg/ml) of the cytoplasm prevents organelles and vesicles from diffusing faster than 100 ^m/3 hours. Even a micrometer-sized bacterium experiences a drag force from water that stops its forward movement within a fraction of a nanometer when it stops actively swimming. To generate the forces necessary for many cellular movements, cells depend on specialized enzymes commonly called motor proteins. These mechanochemical enzymes convert energy released by the hydrolysis of ATP or from ion gradients into a mechanical force.
Motor proteins generate either linear or rotary motion (Table 3-2). Some motor proteins are components of macro-
molecular assemblies, but those that move along cytoskeletal fibers are not. This latter group comprises the myosins, ki-nesins, and dyneins—linear motor proteins that carry attached "cargo" with them as they proceed along either microfilaments or microtubules (Figure 3-22a). DNA and RNA polymerases also are linear motor proteins because they translocate along DNA during replication and transcription. In contrast, rotary motors revolve to cause the beat of bacterial flagella, to pack DNA into the capsid of a virus, and to synthesize ATP. The propulsive force for bacterial swimming, for instance, is generated by a rotary motor protein complex in the bacterial membrane. Ions flow down an electrochemical gradient through an immobile ring of proteins, the stator, which is located in the membrane. Torque generated by the stator rotates an inner ring of proteins and the attached flagellum (Figure 3-22b). Similarly, in the mitochondrial ATP synthase, or F0F1 complex, a flux of ions across the inner mitochondrial membrane is transduced by the F0 part into rotation of the y subunit, which projects into a surrounding ring of a and p subunits in the F1 part. Interactions between the y subunit and the p subunits directs the synthesis of ATP (Chapter 8).
From the observed activities of motor proteins, we can infer three general properties that they possess:
■ The ability to transduce a source of energy, either ATP or an ion gradient, into linear or rotary movement
■ The ability to bind and translocate along a cytoskeletal filament, nucleic acid strand, or protein complex
■ Net movement in a given direction
The motor proteins that attach to cytoskeletal fibers also bind to and carry along cargo as they translocate. The cargo in muscle cells and eukaryotic flagella consists of thick filaments and B tubules, respectively (see Figure 3-22a). These motor proteins can also transport cargo chromosomes and membrane-limited vesicles as they move along microtubules or microfilaments (Figure 3-23).
▲ FIGURE 3-22 Comparison of linear and rotary molecular motors. (a) In muscle and eukaryotic flagella, the head domains of motor proteins (blue) bind to an actin thin filament (muscle) or the A tubule of a doublet microtubule (flagella). ATP hydrolysis in the head causes linear movement of the cytoskeletal fiber (orange) relative to the attached thick filament or B tubule of an adjacent doublet microtubule. (b) In the rotary motor in the bacterial membrane, the stator (blue) is immobile in the membrane. Ion flow through the stator generates a torque that powers rotation of the rotor (orange) and the flagellum attached to it.
TABLE 3-2 Selected Molecular Motors
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