The FLA3 KAP subunit is required for localization of kinesin-2 to the site of flagellar assembly and processive anterograde intraflagellar transport.

Intraflagellar transport (IFT) is a bidirectional process required for assembly and maintenance of cilia and flagella. Kinesin-2 is the anterograde IFT motor, and Dhc1b/Dhc2 drives retrograde IFT. To understand how either motor interacts with the IFT particle or how their activities might be coordinated, we characterized a ts mutation in the Chlamydomonas gene encoding KAP, the nonmotor subunit of Kinesin-2. The fla3-1 mutation is an amino acid substitution in a conserved C-terminal domain. fla3-1 strains assemble flagella at 21 degrees C, but cannot maintain them at 33 degrees C. Although the Kinesin-2 complex is present at both 21 and 33 degrees C, the fla3-1 Kinesin-2 complex is not efficiently targeted to or retained in the basal body region or flagella. Video-enhanced DIC microscopy of fla3-1 cells shows that the frequency of anterograde IFT particles is significantly reduced. Anterograde particles move at near wild-type velocities, but appear larger and pause more frequently in fla3-1. Transformation with an epitope-tagged KAP gene rescues all of the fla3-1 defects and results in preferential incorporation of tagged KAP complexes into flagella. KAP is therefore required for the localization of Kinesin-2 at the site of flagellar assembly and the efficient transport of anterograde IFT particles within flagella.