Analyze The Data

Motor proteins are generally considered to move cellular components along microtubules. However, some kinesin family motors may have other functions as described below.

a. In one set of experiments, microtubules were observed in a microscope chamber in which the solution could be rapidly exchanged. Part (a) of the accompanying figure depicts the effect on microtubule length as a function of time when the tubulin and buffer in the chamber were replaced first by an identical tubulin and buffer solution and then by buffer alone. Part (b) depicts the results when the tubulin and buffer solution in the chamber was replaced with a solution containing tubulin and the M-type kinesin motor protein Kin I. What conclusions can you draw from these experiments?

Time

Time b. In another assay, taxol-stabilized microtubules were incubated alone (Ctrl), with kinesin heavy chain (KHC) and ATP, with Kin I and ATP, or with Kin I and AMPPNP, and then centrifuged to pellet polymerized tubulin. Supernatant (s) and pellet (p) fractions were then separated by SDS-PAGE, and the gel was stained to reveal the position of tubu-lin, as shown in part (c). What conclusions can you draw from these experiments? What is the significance of the nu-cleotide present for Kin I activity? Would you expect ATP to be present in the experiments depicted in part (b) of the figure? Why or

Tubulin

Kin I + tubulin

why not?

ATP Ctrl

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