Norris et al. devise a method to control protein complex assembly in cells and use this approach to investigate how kinesin motor proteins coordinate their activities.
Little is known about how multiple motor proteins cooperate to transport cellular cargos, mainly because it is difficult to control the number and arrangement of motors in a given transport complex. Norris et al. designed a plasmid-based system that enables well-defined motor complexes to be assembled in vivo.
The complexes consist of helical scaffolds that can be linked at different positions to specific proteins of interest. When Norris et al. coupled kinesin-1 motors to either end of a 20-nm scaffold, the complex moved similarly to individual kinesin-1 molecules in vitro, indicating that each motor operated largely independently of the other. The motors showed even less cooperativity when they were separated by a longer, more flexible scaffold.
The researchers then paired kinesin-1 molecules with much faster kinesin-3 motors. Instead of moving at fast or intermediate speeds, most of the kinesin-1/kinesin-3 complexes moved at a slow, kinesin-1–like pace in vitro, again suggesting minimal cooperativity. The complexes also moved slowly on stable microtubules in vivo, but they travelled at faster, kinesin-3–like speeds on dynamic microtubules. Cargos might therefore recruit multiple motors in order to optimize their movement along different microtubule populations in different parts of the cell. Another possibility is that multiple motors may be required to generate enough force for cargo transport.
Norris et al.’s protein assembly system should help researchers to address these questions, as well as many other cell biological problems.
Text by Ben Short