Figure 4.

Highly processive movement of KK6 in the low-salt buffer. (A) Intensity profile of the moving fluorescent spots. Red rectangles show the results with KK6, and black line shows the best-fit Gaussian distribution. Blue line shows the result with dimeric construct K381 (10), which has two peaks (arrows) corresponding to singly and doubly labeled molecules. KK6 showed only one peak corresponding to the single-fluorescent molecule, indicating that KK6 moving on the MT is a monomer in this condition. (B) Distribution of the duration of the movement of KK6. Line shows the best-fit curve with the mean duration of the movement τ_mec = 1.2 s. (C and D) Typical traces of the movement of CK4 in the standard buffer (C) and KK6 in the low-salt buffer (D). Positive displacement indicates the displacement toward the MT plus end. Note the fluctuating nature of the movement and the bias toward the MT plus end. (E and F) Mean displacement (E) and MSD (F) of KK6 movement (red) plotted against time. Red lines show the best-fit curves with the mean velocity v = 120 nm/s and the diffusion coefficient D = 30,000 nm²/s. Blue lines show the best-fit curves of CK4 (v = 160 nm/s, D = 44,000 nm²/s). Dotted lines in F show the diffusion term (MSD − v²t²) to illustrate the difference in the diffusion coefficient.