One-dimensional diffusion on microtubules of particles coated with cytoplasmic dynein and immunoglobulins

We characterized and compared the diffusion of beads coated with proteins such as cytoplasmic dynein, α-casein, and some immunoglobulins on microtubules. Such weak binding interactions could be common and convenient for concentrating proteins at the surface of cytoplasmic structures such as microtubules. In studying the motile behavior of anionic latex beads coated with limiting dilutions of cytoplasmic dynein, we observed that in addition to active movement, 20-50% of the beads moved back and forth in a random manner. The random movement was inhibited by depletion of ATP or addition of ADP or AMP-PNP. Mean-square-displacement analysis showed that the movement is a one-dimensional diffusion along the microtubule axis with a diffusion coefficient of 2.16 x 10^-10 cm²/sec. Histogram analysis of off-axis movements suggested that ~60% of the diffusing beads followed the path of a single microtubule protofilament. Beads coated with proteins such as α-casein or a monoclonal immunoglobulin were also observed to diffuse on microtubules with a similar diffusion coefficient to cytoplasmic dynein. However, α-casein or immunoglobulin-bead diffusion was not ATP dependent and did not follow the paths of single protofilaments. Thus, although the environment of the microtubule surface can trap a variety of different protein-coated beads, cytoplasmic dynein's interaction is unusual in its ATP dependence and tracking on a single protofilament, which is consistent with its specific interaction with microtubules. Diffusive interactions could concentrate associating proteins and still allow for freedom of movement.