SUMMARYSensory cilia are assembled and maintained by kinesin-2-dependent intraflagellar transport (IFT). We investigated if two C. elegans α- and β-tubulin isotypes, identified via mutants that lack their cilium distal segments, are delivered to their assembly sites by IFT. Mutations in conserved residues in both tubulins destabilize distal singlet microtubules (MTs). One isotype, TBB-4, assembles into MTs at the tips of the axoneme core and distal segments, where the MT tip-tracker, EB1, is found, and localizes all along the cilium, whereas the other, TBA-5, concentrates in distal singlets. IFT assays, FRAP analysis and modeling suggest that the continual transport of sub-stoichiometric numbers of these tubulin subunits by the IFT machinery can maintain sensory cilia at their steady state length.

An outstanding unresolved question is how does the mitotic spindle utilize microtubules and mitotic motors to coordinate accurate chromosome segregation during mitosis? This process depends upon the mitotic motor, kinesin-5, whose unique bipolar architecture, with pairs of motor domains lying at opposite ends of a central rod, allows it to crosslink microtubules within the mitotic spindle and to coordinate their relative sliding during spindle assembly, maintenance and elongation. The structural basis of kinesin-5’s bipolarity is, however, unknown, as protein asymmetry has so far precluded its crystallization. Here we use electron microscopy of single molecules of kinesin-5 and its subfragments, combined with hydrodynamic analysis plus mass spectrometry, circular dichroism and site-directed spin label electron paramagnetic resonance spectroscopy, to show how a staggered antiparallel coiled-coil ‘BASS’ (bipolar assembly) domain directs the assembly of four kinesin-5 polypeptides into bipolar minifilaments.

Highlights d Ciliary tip turnaround is visualized using single-molecule fluorescence microscopy d IFT dynein, OSM-3, and CHE-11 (IFT-A) reverse direction without pause d In contrast, OSM-6 (IFT-B) pauses for several seconds at the ciliary tip d IFT trains disassemble into at least IFT-A, IFT-B, IFT-dynein, and OSM-3 at the tip

Molecular motor proteins fulfill the critical function of transporting organelles and other building blocks along the biopolymer network of the cell's cytoskeleton, but crowding effects are believed to crucially affect this motor-driven transport due to motor interactions. Physical transport models, like the paradigmatic, totally asymmetric simple exclusion process (TASEP), have been used to predict these crowding effects based on simple exclusion interactions, but verifying them in experiments remains challenging. Here, we introduce a correlation imaging technique to precisely measure the motor density, velocity, and run length along filaments under crowding conditions, enabling us to elucidate the physical nature of crowding and test TASEP model predictions. Using the kinesin motor proteins kinesin-1 and OSM-3, we identify crowding effects in qualitative agreement with TASEP predictions, and we achieve excellent quantitative agreement by extending the model with motor-specific interaction ranges and crowding-dependent detachment probabilities. These results confirm the applicability of basic nonequilibrium models to the intracellular transport and highlight motor-specific strategies to deal with crowding.

Heterotrimeric kinesin-2 motors [1; 2] transport intraflagellar transport (IFT)-particles from the base to the tip of the axoneme to assemble and maintain cilia [3; 4; 5; 6; 7; 8; 9; 10]. These motors are distinct in containing two non-identical motor subunits together with an accessory subunit [1; 11; 12; 13; 14; 15]. We evaluated the significance of this organization by comparing purified wild type kinesin-2 holoenzymes with mutant trimers containing only one type of motor domain. In motility assays, wild type kinesin-2 moved microtubules (MTs) at a rate intermediate between the rates supported by the two mutants. Interestingly, one of the mutants, but not the other mutant or the wild-type protein, was observed to drive a persistent counter-clockwise rotation of the gliding MTs. Thus one of the two motor domains of heterotrimeric kinesin-2 exerts torque as well as axial force as it moves along a MT, which may allow kinesin-2 to control its circumferential position around a MT doublet within the cilium.