Diverse motors. Skeletal muscle, eukaryotic cilia, and bacterial flagella use different strategies for the conversion of free energy into coherent motion. Compare and contrast these motility systems with respect to (a) the free-energy source and (b) the number of essential components and their identity.

You call that slow? At maximum speed, a kinesin molecule moves at a rate of 6400 Å per second. Given the dimensions of the motor region of a kinesin dimer of approximately 80 Å, calculate its speed in “body lengths” per second. To what speed does this body-length speed correspond for an automobile 10 feet long?

Heavy lifting. A single myosin motor domain can generate a force of approximately 4 piconewtons (4 pN). How many times its “body weight” can a myosin motor domain lift? Note that 1 newton = 0.22 pounds (100 g). Assume a molecular mass of 100 kDa for the motor domain.

Compare and contrast. Describe two similarities and two differences between actin filaments and microtubules.

Lighten up. What is the primary role of the light chains in myosin? In kinesin?

Rigor mortis. Propose an explanation for the fact that the body stiffens after death.

Now you see it, now you don’t. Under certain stable concentration conditions, actin monomers in their ATP form will polymerize to form filaments that disperse again into free actin monomers over time. Explain.

Helicases as motors. Helicases can use single-stranded DNA as tracks. Consider a helicase that moves one base in the 3′ → 5′ direction in each cycle. Assuming that the helicase can hydrolyze ATP at a rate of 50 molecules per second in the presence of a single-stranded DNA template, calculate the velocity of the helicase in micrometers per second. How does this velocity compare with that of kinesin?

New moves. When bacteria such as E. coli are starved to a sufficient extent, they become nonmotile. However, when such bacteria are placed in an acidic solution, they resume swimming. Explain.

Going straight. Suppose you measure the mean distance that an E. coli bacterium moves along a straight path before it tumbles over a period of time. Would you expect this distance to change in the presence of a gradient of a chemoattractant? Would it be longer or shorter?

Hauling a load. Consider the action of a single kinesin molecule in moving a vesicle along a microtubule track. The force required to drag a spherical particle of radius a at a velocity v in a medium having a viscosity η is

F = 6πηav

Suppose that a 2-μm-diameter bead is carried at a velocity of 0.6 μm s⁻¹ in an aqueous medium (η = 0.01 poise = 0.01 g cm⁻¹ s⁻¹).

Unusual strides. A publication describes a kinesin molecule that is claimed to move along microtubules with a step size of 6 nm. You are skeptical. Why?

The sound of one hand clapping. KIF1A is a motor protein that moves toward the plus end of microtubules as a monomer. KIF1A has only a single motor domain. What additional structural elements would you expect to find in the KIF1A structure?

Building blocks. Actin filaments, microtubules, and bacterial flagella are all built from small subunits. Describe three advantages of assembling long filamentous structures from subunits rather than from single, long proteins.

Backward rotation. On the basis of the proposed structure in Figure 35.29 for the bacterial flagellar motor, suggest a pathway for transmembrane proton flow when the flagellar motor is rotating clockwise rather than counterclockwise.

Smooth muscle. Smooth muscle, in contrast with skeletal muscle, is not regulated by a tropomyosin–troponin mechanism. Instead, vertebrate smooth-muscle contraction is controlled by the degree of phosphorylation of its light chains. Phosphorylation induces contraction, and dephosphorylation leads to relaxation. Like that of skeletal muscle, smooth-muscle contraction is triggered by an increase in the cytoplasmic calcium ion level. Propose a mechanism for this action of calcium ion on the basis of your knowledge of other signal-transduction processes.

Myosin V. An abundant myosin-family member, myosin V is isolated from brain tissue. This myosin has a number of unusual properties. First, on the basis of its amino acid sequence, each heavy chain has six tandem binding sites for calmodulin-like light chains. Second, it forms dimers but not higher-order oligomers. Finally, unlike almost all other myosin-family members, myosin V is highly processive.

The rate of ATP hydrolysis by myosin has been examined as a function of ATP concentration, as shown in graph A.