Expansion of the Cell Plate in Plant Cytokinesis Requires a Kinesin-like Protein/MAPKKK Complex

The tobacco mitogen-activated protein kinase kinase kinase NPK1 regulates lateral expansion of the cell plate at cytokinesis. Here, we show that the kinesin-like proteins NACK1 and NACK2 act as activators of NPK1. Biochemical analysis suggests that direct binding of NACK1 to NPK1 stimulates kinase activity. NACK1 is accumulated specifically in M phase and colocalized with NPK1 at the phragmoplast equator. Overexpression of a truncated NACK1 protein that lacks the motor domain disrupts NPK1 concentration at the phragmoplast equator and cell plate formation. Incomplete cytokinesis is also observed when expression of NACK1 and NACK2 is repressed by virus-induced gene silencing and in embryonic cells from Arabidopsis mutants in which a NACK1 ortholog is disrupted. Thus, we conclude that expansion of the cell plate requires NACK1/2 to regulate the activity and localization of NPK1.