Processive movement of single kinesins on crowded microtubules visualized using quantum dots

Kinesin‐1 is a processive molecular motor transporting cargo along microtubules. Inside cells, several motors and microtubule‐associated proteins compete for binding to microtubules. Therefore, the question arises how processive movement of kinesin‐1 is affected by crowding on the microtubule. Here we use total internal reflection fluorescence microscopy to image in vitro the runs of single quantum dot‐labelled kinesins on crowded microtubules under steady‐state conditions and to measure the degree of crowding on a microtubule at steady‐state. We find that the runs of kinesins are little affected by high kinesin densities on a microtubule. However, the presence of high densities of a mutant kinesin that is not able to step efficiently reduces the average speed of wild‐type kinesin, while hardly changing its processivity. This indicates that kinesin waits in a strongly bound state on the microtubule when encountering an obstacle until the obstacle unbinds and frees the binding site for kinesin's next step. A simple kinetic model can explain quantitatively the behaviour of kinesin under both crowding conditions.