Chapter 9

If a myofibril were pulled so that the sarcomeres increased in length by approximately 50 percent, what effect would you expect this to have on the contractile ability of the myofibril? Why? What effects would this have on the \(\mathrm{H}, \mathrm{A},\) and I bands?

What are three different radioactively or fluorescently labeled substances that you might inject into a cell that would label the microtubules of a cell without labeling the other cytoskeletal elements?

Microtubules can be formed in vitro from tubulin that is bound to GTP analogues that (unlike GTP) cannot be hydrolyzed. What properties would you expect these microtubules to possess?

It was noted that a demembranated ciliary or flagellar axoneme is able to beat with a normal frequency and pattern. Can you conclude that the plasma membrane is not important in ciliary or flagellar function?

Because cytoplasmic vesicles are seen to move in both directions within an axon, can you conclude that some microtubules are oriented with their plus end facing the axon terminus and others oriented with the opposite polarity? Why or why not?

Would you agree with the statement that the centrosome plays a key role in determining the rates of lengthening and shortening of the microtubules of an animal cell? Why or why not?

If you were comparing the molecular structure of kinesin and myosin, which are thought to have been derived from a common ancestral protein, which part (heads or tails) would you expect to be most similar between them? Why?

Why do you suppose more can be learned about microtubule dynamics by injecting fluorescent tubulin into a cell than radioactively labeled tubulin? Can you think of a question that would be better answered by radioactively labeled tubulin?

Suppose you discovered that a mouse lacking copies of one of the conventional kinesin genes appeared to show no ill effects and lived to a ripe old age. What could you conclude about the role of this kinesin in intracellular locomotion?

Which type of vertebrate tissue would you expect to be an excellent source of tubulin? of actin? of keratin? Which protein would you expect to be the least soluble and most difficult to extract? What types of protein would you expect as contaminants in a preparation of tubulin? Which in a preparation of actin?

Actin is one of the most evolutionarily conserved proteins. What does this tell you about the structure and function of this protein in eukaryotic cells?

According to data from genome sequences, cytoplasmic dynein is absent in some plants (e.g., Arabidopsis) and present in others (e.g., rice). Does this finding surprise you? What else might you do to confirm or refute such a statement? How is it possible that higher plant cells could operate without cytoplasmic dynein?

Motor proteins are often described as mechanoenzymes. Why? Could this same term be applied to virtually all enzymes? Why or why not?