Title: Kinesin's Light Chains Inhibit the Head- and Microtubule-Binding Activity of its Tail
Abstract: Kinesin-1 comprises two heavy chains (KHCs) and two light chains (KLCs). The KHC tail inhibits ATPase activity by interacting directly with the enzymatic KHC heads, and the inhibitory segment of the tail also binds to microtubules. We have discovered a novel role for the KLCs in regulating the head- and microtubule-binding activities of the kinesin-1 tail. We show that KLCs reduce the affinity of the head-tail interaction over ten-fold. Functional assays confirm that the KLCs attenuate tail-mediated inhibition of kinesin-1 activity. We also show that KLCs block tail-microtubule binding. Inhibition of head-tail binding requires both steric and electrostatic factors. Inhibition of tail-microtubule binding is largely electrostatic and is more pronounced at physiological pH (pH 7.4) than under acidic conditions (pH 6.6). Full inhibition requires a negatively-charged linker region in the KLCs, between its KHC-interacting and cargo-binding domains. Our data support a model wherein KLCs promote activation of kinesin-1 for cargo transport by suppressing both the head-tail and tail-microtubule interactions. Additionally, KLC-mediated inhibition of tail-microtubule binding may influence kinesin-1's emerging role in microtubule sliding and cross-linking.