Cyclin-dependent kinases (CDKs) control cell cycle progression through timely coordinated phosphorylation events. Two kinesin-like proteins that interact with CDKA;1 were identified and designated KCA1 and KCA2. They are 81% identical and have a similar three-partite domain organization. The N-terminal domain contains an ATP and microtubule-binding site typical for kinesin motors. A green fluorescent protein (GFP) fusion of the N-terminal domain of KCA1 decorated microtubules in Bright Yellow-2 cells, demonstrating microtubule-binding activity. During cytokinesis the full-length GFP-fusion protein accumulated at the midline of young and mature expanding phragmoplasts. Two-hybrid analysis and coimmunoprecipitation experiments showed that coiled-coil structures of the central stalk were responsible for homo- and heterodimerization of KCA1 and KCA2. By western-blot analysis, high molecular mass KCA molecules were detected in extracts from Bright Yellow-2 cells overproducing the full-length GFP fusion. Treatment of these cultures with the phosphatase inhibitor vanadate caused an accumulation of these KCA molecules. In addition to dimerization, interactions within the C-terminally located tail domain were revealed, indicating that the tail could fold onto itself. The tail domains of KCA1 and KCA2 contained two adjacent putative CDKA;1 phosphorylation sites, one of which is conserved in KCA homologs from other plant species. Site-directed mutagenesis of the conserved phosphorylation sites in KCA1 resulted in a reduced binding with CDKA;1 and abolished intramolecular tail interactions. The data show that phosphorylation of the CDKA;1 site provokes a conformational change in the structure of KCA with implications in folding and dimerization.