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

Exercise 5

Discuss the following problems.
-The movements of single motor-protein molecules can be analyzed directly. Using polarized laser light, it is possible to create interference patterns that exert a cen- trally directed force, ranging from zero at the center to a few piconewtons at the periphery (about 200 nm from the center). Individual molecules that enter the interference pattern are rapidly pushed to the center, allowing them to be captured and moved at the experimenter's discretion. Using such "optical tweezers," single kinesin mol- ecules can be positioned on a microtubule that is fixed to a coverslip. Although a single kinesin molecule cannot be seen optically, it can be tagged with a silica bead and tracked indirectly by following the bead (Figure Q16-3A). In the absence of ATP, the kinesin molecule remains at the center of the interference pattern, but with ATP it moves toward the plus end of the microtubule. As kinesin moves along the microtubule, it encounters the force of the inter- ference pattern, which simulates the load kinesin carries during its actual function in the cell. Moreover, the pres- sure against the silica bead counters the effects of Brown- ian (thermal) motion, so that the position of the bead more accurately reflects the position of the kinesin molecule on the microtubule. A trace of the movements of a kinesin molecule along a microtubule is shown in Figure Q16-3B.