Microtubule Motility on Fixed and Diffusive Motor Proteins*

Motor proteins transporting cargo by walking on microtubules (MTs). MTs are part of the cytoskeleton and are important in cell division, giving shape to the cell membrane. Here we investigate motor-based transportation and behavior of MTs. In a MT gliding assay, motor proteins adhere to the glass while MTs glide on them. MT behavior in gliding assay experiments with high MT concentration is compared with behavior on diffusing motor proteins and at similar concentration. An interesting twist on the standard gliding assay is to attach the motors to a lipid bilayer and let them diffuse on the membrane. Using fluorescence recovery after photobleaching (FRAP), we measured motor diffusion constants. We demonstrate the MT “snuggling” effect, i.e. they spontaneously align locally while gliding on the membrane-bound motor proteins. When MTs encounter each other for gliding assays, they simply cross over in the absence of a crowding agent. Furthermore, we observe that MTs exhibit collective motion, much like bird flocking, with diffusing kinesins, which is not replicable with fixed kinesins.