Why motor proteins team up - Intraflagellar transport in C. elegans cilia

Inside the cell, vital processes such as cell division and intracellular transport are driven by the concerted action of different molecular motor proteins. In C. elegans chemosensory cilia, 2 kinesin-2 family motor proteins, kinesin-II and OSM-3, team up to drive intraflagellar transport (IFT) in the anterograde direction, from base to tip, whereas IFT dynein hitchhikes toward the tip and subsequently drives IFT in the opposite, retrograde direction, thereby recycling both kinesins. While it is evident that at least a retrograde and an anterograde motor are necessary to drive IFT, it has remained puzzling why 2 same-polarity kinesins are employed. Recently, we addressed this question by combining advanced genome-engineering tools with ultrasensitive, quantitative fluorescence microscopy to study IFT with single-molecule sensitivity.(1,2) Using this combination of approaches, we uncovered a differentiation in kinesin-2 function, in which the slower kinesin-II operates as an 'importer', loading IFT trains into the cilium before gradually handing them over to the faster OSM-3. OSM-3 subsequently acts as a long-range 'transporter', driving the IFT trains toward the tip. The two kinesin-2 motors combine their unique motility properties to achieve something neither motor can achieve on its own; that is to optimize the amount of cargo inside the cilium. In this commentary, we provide detailed insight into the rationale behind our research approach and comment on our recent findings. Moreover, we discuss the role of IFT dynein and provide an outlook on future studies.