The chromokinesin Kid forms a homodimer and moves processively along microtubules

During prometaphase in mitosis, chromosomes are pushed toward the spindle equator. The chromokinesin Kid moves chromosomes along spindle microtubules during prometaphase. Kid has long been considered as a monomeric and non-processive motor, different from typical kinesins that use two motor domains to transport cargos. However, this notion raises a question about how Kid transports chromosomes along microtubules. In this study, we demonstrate that the full-length Kid forms a homodimer and moves processively along microtubules. Both human and Xenopus Kid move along microtubules approximately at 70 nm/sec. We found Kid movement is characterized by frequent diffusive motions during the processive movement. Both human and Xenopus Kid, in their full-length forms, are eluted in dimer fractions in the size exclusion chromatography analysis but mostly dissociated into monomers in mass photometry analysis. A conserved coiled-coil domain within the stalk region of Kid is not only capable of homodimer formation, but is also required for the processivity of Kid. Furthermore, the stalk domain of Kid could add processive activity to the motor domain of KIF1A, suggesting that the stalk domain of Kid contains a functional neck linker and dimerization capability, a prerequisite for the processivity of kinesin motor domains. These findings collectively suggest the reclassification of Kid as a processive motor that possesses unique features.