Intracellular Cargo Transport by Single-Headed Kinesin Motors

Abstract

Kinesins are cytoskeletal motor proteins that transport cargoes along microtubules in eukaryotic cells. Motors in the kinesin superfamily share a highly conserved structure containing two motor domains that dimerize through a coiled-coil stalk. The canonical view is that dimerization is required for kinesin's processive motility and force generation, as the two motor domains of a dimer step along the microtubule lattice in a tightly coordinated manner. However, whether dimerization is required for intracellular transport remains unknown. Here, we address this issue using a combination of in vitro and cellular assays to directly compare dimeric motors across the kinesin-1, -2, and -3 families to their monomeric forms. Surprisingly, we find that monomeric motors across different kinesin families are able to work in teams to drive peroxisome dispersion in cells. However, peroxisome transport requires minimal force output, and we find that most monomeric motors are significantly less efficient at dispersion of the Golgi complex, a high-load cargo. Strikingly, monomeric versions of the kinesin-2 family motors KIF3A and KIF3B are able to drive Golgi dispersion in cells, and teams of monomeric KIF3B motors can generate up to 11 pN of force in an optical trap. The ability of KIF3B to work in teams enabled us to test the impact of monomer length on collective cargo transport and force generation. We demonstrate that increasing the motor-to-cargo distance results in a decreased efficiency of cellular cargo transport and a decreased speed and force output in vitro. Together, these results suggest that dimerization of kinesin motors is not required for intracellular transport; however, it enables motor-to-motor coordination and high force generation regardless of motor-to-cargo distance. Dimerization is thus critical for cellular transport events that require an ability to generate or withstand high forces. Our findings lend insight into the minimal requirements and mechanical modulators of collective kinesin cargo transport.