Chapter 34: Molecular Motors

Chapter 34: Molecular Motors

• The chapter describes how chemical energy can be used to drive the contraction of muscles or to move cells.
• The authors show how nanometer motions of proteins can be converted into coordinated movements of cells.

• The P-loop NTPase structure is the basis of most molecular-motor proteins.
• A brief description of myosin, kinesin, anddynein are provided.
• The mechanism of myosin's flexing in response to ATP hydrolysis is explained in detail after the subunit structures of all three are shown.
• The authors show the striated appearance of the muscles by showing how thick and thin they are.
• The thick and thin filaments are made of myosin and actin, respectively.
• They show the structure of the F-actin polymer, which is composed of a linear coiled array of G-actin monomers.

• The polarities of the thick and thin filaments within a sarcomere show how coordinated movement can result in the shortening of myofibrils.

• The assembly of myosin-coated beads is moving along the track.
• The dissociation of the S1 heads of myosin with actin and the conformational changes in myosin that are effected by the binding ofATP, its hydrolysis toADP and Pi, and the release of the hydrolysis products suggest how the power stroke occurs.

• Microtubules are found in almost every cell.
• They are composed of a-tubulin and b-tubulin monomers that form large tubular structures.

• The basic macromolecular assembly of the axoneme is contributed in chapter 3.
• Dynein and kinesin interact with microtubules to bend cilia and flagella.

• Dynamic instability of microtubules can be explained by the rapid dissociation of GDP-tubulin from the ends of tubulins.
• The myosin-actin interactions that slide muscle filaments past one another are similar to the vesicle transport in neurons.

• The chapter ends with a description of the flagellar motor ofbacteria and a discussion of how the interplay of two kinds of protein components forming the motor give rise to directional rotation.
• The system can reverse the rotation of the flagella with the help of CheY.
• The examples given in this chapter show how energy can be converted into another form by the regulated activities of proteins.

• You should be able to complete the objectives once you have mastered this chapter.

• List the two sources of energy.

• There is a model that explains the effects of a protons on the flagellar motor.

• The descriptions of the proteins are in the right column.

• The contraction of striated muscle, the beating of flagella or cilia, and the transport of vesicles along microtubules require energy.

• Match the structures with the choices listed in the figure.

• Considering only the power stroke of skeletal muscle contraction and the events that precede and follow it, place the following states or processes that occur in going from the resting state to the contracted state and back again in their proper order.

• In a day, a neuron can move a vesicle from the central cell body to the end of an axon.

• The answer is incorrect because the activity of actin is involved in the formation of F-actin.

• The M lines lie halfway between the Z lines and correspond to the middle of the bare zone where oppositely oriented myosin molecule point toward each other.

• The answer is incorrect because myosin assembles to form the thick filaments.

• Myosin is composed of six polypeptide chains.
• The a-helical coiled coil is formed by two heavy chains interwoven with their C-terminal portions.

• The answer is incorrect because the cross-bridges are broken.
• Answer (e) is incorrect because, at any instant, numerous cross-bridges will be in all stages of forming and breaking because the process is dynamic and asynchronous.

• Although the whole molecule is required for muscle contraction, the activities of the S1 head domains provide the power generation.
• The energy transduction is based on the changes in the head domains and their affinities for actin, ATP,ADP, and Pi.

• The answer is incorrect because the microtubules lack the a-helical coiled-coil structure of myosin and the intermediate filaments.
• The answer is incorrect because dynein movements lead to the bending of the axoneme.

• The tracts along which the cells can move are provided by the microtubules.
• The way myosin acts to power the movements is similar to the way kinesin acts.

• The flagella are formed from flagellin subunits, whereas the fla gella are composed of microtubules.
• The internally generated force causes the flagella to bend.
• A motor is attached to one of the flagella's ends.

• The bacterium would start swimming because protons would move from the higher concentration outside the cell through the flagella, causing their counterclockwise rotation.

• The answer is incorrect because CheY causes the flagella to fall.

• There is a relationship between adenylate kinase and myosin.

• An experiment with actin labeled in the g phosphoryl group with 32P would show that it stimulates the myosin.

• Decide if each of the following will stay the same or decrease in size.
• The sliding-filament model is applicable.
• Refer to page 957 for more information.

• In a sarcomere, six thin filaments surround each thick one in a hexagonal array.
• The ratio between thin and thick muscles has been found to be double that of resting muscles.

• Chronic respiratory disorders and infertility are caused by this.

• The two feet of kinesin can be seen walking down the "tightrope" of the microtubule.
• The "hand-overhand" model is called the "inchworm" model.
• The left foot is always in front, and the right foot is always in front, as if they were walking with one foot.

• A single kinesin molecule can move a single vesicle from the nucleus of a nerve cell to the end of its axon.
• Each step in the kinesin cycle uses one ATP.

• The nerve cell is 6 feet long.

• The tail region of myosin, kinesin, anddynein can be thought of as aProtein-binding region.
• The role of each of the three proteins in tissues can be changed by the specificity of the protein-binding domain.

• The reasons for your choices should be given.

• The P-loop area is where the ATP binding occurs.
• The change in myosin is amplified by nearby structures like the relay helix and the lever arm.
• The equilibrium between the two is very old.
• Myosin should be somewhat younger because he is confined to the eukaryotes.

• CHAPTER 34 face and charged residues on the opposite face allow two helices in two myosin molecule to form a long coiled-coil rod.

• Measure the amount of labeled inorganicphosphate liberated as a function of time without actin by incubating the labeled ATP with myosin.
• Add actin.
• The amount of labeled Pi should be burst.

• An illustration of uncontracted and partially contracted sarcomeres.

• An illustration of a fully contracted sarcomere.

• The ends of the thin filaments have the wrong polarity to interact with an adjacent thick one in the region where they override one another in a fully contracted sarcomere.

• Defective dynein can affect the cilia of the respiratory tract, as well as prevent particles from being swept out of the lungs.

• The rate of cell division for cancer cells is higher than normal.
• In the presence of vincristine or vinblastine, cell division is retarded.
• Cancer cells are more sensitive to drugs when they grow fast.

• The hip and knee joints are used to allow normal human walking.
• The "hand-over-hand" model requires kinesin to turn 180 degrees with each step, whereas the "inchworm" model allows kinesin to move with very little rotation.
• Scientists attached a length of microtubule to the top of kinesin and took photographs that showed the microtubule was aimed in the same direction.
• The "inchworm" model appears to be the correct one.
• The "hand-over-hand" version was shown in early editions.

• The m/step is 2.3 x108.

• 4 6 is 1 m / cm.

• The striated muscle is formed by myosin binding other myosin molecules.
• striated muscle would not form without this self-aggregating property of myosin.
• The tail regions of dynein bind the surface of a vesicle.
• Changing their tail regions would change what is being transported.
• The ciliary dynein must have different tail regions so that it can transport vesicles and the cellular one can have tubulin in cilia.

• X and Y are likely to be attracted to each other.
• Smooth swimming can be achieved because of the decrease in clockwise rotation caused by the addition of X.
• A decrease in counterclockwise rotation is caused by the addition of Y.

In grams, the force is (0.22 lbs) x (4 lbs) x (454 lbs) x (10 lbs)

• The force lifted per myosin molecule is obtained by dividing the above two numbers.

• There are two consequences to the rapid decrease in the level of ATP.
• The sarcoplasmic reticulum is no longer functioning because the Ca2+-ATPase pumps are no longer working.
• Myosin can interact with actin through high Ca2+.
• A lot of S1 heads will be associated with actin.
• The actomyosin complexample is locked in the contracted state in the absence of ATP.

• The critical concentration is lower for actin-ATP than it is for actin-ADP.
• The actin-ATP will eventually depolymerize, as the boundATP becomes hydrolyzed to the ADP, if the concentrations of actin-ATP and actin-ADP are in between.

• The processes could play active roles.

• It is reasonable to predict that a conventional kinesin with a motor domain from ncd will continue to move in the plus direction.

• The total distance between the bases is measured along the spine.
• A movement along 50 bases over a distance of 600 A would correspond to a movement along 50 bases at a rate of 600 A/s.
• The rate of movement is about one-tenth that of kinesin.

• The bacterial flagellar rotation can be driven by the flow of protons from the acidic solution.

• Dividing erg per mole by Avogadro's number gives us 8.3 x 10-13 erg/molecule.
• The work performed in moving the 2-mm-diameter bead could yield less energy than the work done in the hydrolysis of 40 ATP molecule.
• A single kinesin motor can provide more than enough free energy to power the transport of micrometersize cargos.

• The actual distance between equivalent binding sites on tubulin subunits would be different if the step size was only 6 nm.

• When the motor domain needed to detach, one or more additional tether domains might allow the KIF1A to remain bound to a microtubule.
• A change in tether and motor attachement could allow theProtein to be processive.

• When the direction of flagellar rotation is reversed, the direction of the flow of protons would change.

• Ca2+-calmodulin stimulates myosin light-chain ki nase.
• Phosphoryl groups are removed from myosin by a Ca2+-independent phosphatase.

• Between the two myosin heads there would be different cycles of binding, ATP hydrolysis,ADP release, and movement.

• Processivity could be achieved if one head remained attached when the other was released or moved.