Importin-β Directly Regulates the Motor Activity and Turnover of a Kinesin-4

Spatiotemporal regulation of kinesins is essential for microtubule-dependent intracellular transport. In plants, cell wall deposition depends on the FRA1 kinesin, whose abundance and motility are tightly controlled to match cellular growth rate. Here, we show that an importin-β, IMB4, regulates FRA1 activity in a developmental manner. IMB4 physically interacts with a PY motif in the FRA1 motor domain and inhibits its motility by preventing microtubule binding, while also protecting FRA1 against proteasome-mediated degradation, thus providing a mechanism to couple the motility and stability of FRA1. This regulatory mechanism is likely to be broadly applicable, based on the conservation of the PY motif in the motor domains of plant and animal kinesins and the direct interaction of multiple plant kinesins with IMB4. Together, our data establish IMB4 as a multi-functional regulator of FRA1 and reveal a mechanism for how plants control the magnitude of cargo transport needed for cell wall assembly. [Display omitted] •The importin-β IMB4 directly binds to a PY motif in the FRA1 kinesin motor domain•IMB4 protects FRA1 from proteasome-mediated degradation in a developmental manner•IMB4 also inhibits the microtubule binding ability of the FRA1 motor domain•The IMB4 binding PY motif is conserved in multiple kinesin families How the activity of kinesins is regulated to meet cellular needs remains poorly understood. Ganguly et al. show that the motility and proteasome-mediated degradation of a kinesin-4 is regulated in a developmental manner by the binding of an importin-β to a conserved PY motif in the kinesin motor domain.