Molecular Biology Of The Cell 6th Edition by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter

Exercise 7

Titin, which has a molecular weight of about is the largest polypeptide yet described. Titin molecules extend from muscle thick filaments to the Z disc; they are thought to act as springs to keep the thick filaments centered in the sarcomere. Titin is composed of a large number of repeated immunoglobulin (Ig) sequences of 89 amino acids, each of which is folded into a domain about 4 nm in length (Figure Q3-2A). You suspect that the springlike behavior of titin is caused by the sequential unfolding (and refolding) of indi- vidual Ig domains. You test this hypothesis using the atomic force microscope, which allows you to pick up one end of a protein molecule and pull with an accurately measured force. For a fragment of titin containing seven repeats of the Ig domain, this experiment gives the sawtooth force-ver- sus-extension curve shown in Figure Q3-2 b. If the experi- ment is repeated in a solution of 8 M urea (a protein dena- turant), the peaks disappear and the measured extension becomes much longer for a given force. If the experiment is repeated after the protein has been cross-linked by treat- ment with glutaraldehyde, once again the peaks disappear but the extension becomes much smaller for a given force. a. Are the data consistent with your hypothesis that titin's springlike behavior is due to the sequential unfold- ing of individual Ig domains? Explain your reasoning. b. Is the extension for each putative domain-un- folding event the magnitude you would expect? (In an extended polypeptide chain, amino acids are spaced at intervals of 0.34 nm.)