Processivity of the monomeric KLP-6 kinesin and a Brownian ratchet model with symmetric potentials

Most kinesin molecular motors dimerize to move processively and efficiently along microtubules; however, some can maintain processivity even in a monomeric state. Previous studies have suggested that asymmetric potentials between the motor domain and microtubules underlie this motility. In this study, we demonstrate that the kinesin-3 family motor protein KLP-6 can move along microtubules as a monomer upon release of autoinhibition. This motility can be explained by a change in length between the head and tail, rather than by asymmetric potentials. Using mass photometry and single-molecule assays, we confirmed that activated full-length KLP-6 is monomeric both in solution and on microtubules. KLP-6 possesses a microtubule-binding tail domain, and its motor domain does not exhibit biased movement, indicating that the tail domain is crucial for the processive movement of monomeric KLP-6. We developed a mathematical model to explain the unidirectional movement of monomeric KLP-6. Our model concludes that a slight conformational change driven by neck-linker docking in the motor domain enables the monomeric kinesin to move unidirectionally if a second microtubule-binding domain exists.