KAP, the Accessory Subunit of Kinesin-2, Binds the Predicted Coiled-Coil Stalk of the Motor Subunits

Kinesin-2 is an anterograde motor involved in intraflagellar transport and certain other intracellular transport processes. It consists of two different motor subunits and an accessory protein KAP (kinesin accessory protein). The motor subunits were shown to bind each other through the coiled-coil stalk domains, while KAP was proposed to bind the tail domains of the motor subunits. Although several genetic studies established that KAP plays an important role in kinesin-2 functions, its exact role remains unclear. Here, we report the results of a systematic analysis of the KAP binding sites by using recombinant Drosophila kinesin-2 subunits as well as the endogenous proteins. These show that at least one of the coiled-coil stalks is sufficient to bind the N-terminal region of DmKAP. The soluble complex involving the recombinant kinesin-2 fragments is reconstituted in vitro at high salt concentrations, suggesting that the interaction is primarily nonionic. Furthermore, independent distant homology modeling indicated that DmKAP may bind along the coiled-coil stalks through a combination of predominantly hydrophobic interactions and hydrogen bonds. These observations led us to propose that KAP would stabilize the motor subunit heterodimer and help assemble a greater kinesin-2 complex in vivo.