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Fig. 5. Rapid exchange of EGFP-TPPP/p25 on microtubules demonstrated by FRAP. (A-C) NRK cells were transfected with pEGFP-TPPP/p25. The experimental region (white square shown in B) containing a thick microtubule bundle was bleached with a high-intensity laser beam. Fluorescent intensities were measured over the blue rectangles (rectangle 1 resides within the bleached area; rectangle 2 is in a control region; values from rectangle 3 were used for background correction). Insets from B and C are shown at 1 and 19 seconds, respectively. Fluorescence of the bleached region reappeared nearly completely during the observation period of 20 seconds indicating the very rapid exchange of TPPP/p25 on the microtubule bundle (compare the intensities of the enlarged insets for rectangle 1 below A and C). (D) The intensities of rectangle 1 compared to the bleached area ({diamondsuit}), and rectangle 2 compared to the control area ({blacksquare}) over the course of the experiment.