Ste20/GCK kinase Nak1/Orb3 polarizes the actin cytoskeleton in fission yeast during the cell cycle.

Polar growth is a crucial process during cell morphogenesis. The microtubule and actin cytoskeletons, and vesicular transport are tightly regulated to direct cellular growth and to generate specific cell forms. We demonstrate here that the Ste20-related protein kinase Nak1/Orb3 is required in fission yeast to polarize the actin cytoskeleton at the tips of the cells and for cell separation, and so is involved in controlling both cell shape and late stages of cytokinesis. The localization of the Nak1/Orb3 kinase to the cell tips, a medial ring and the spindle-pole bodies changes during the cell cycle, and the accumulation of F-actin at the cell tips is dependent on Nak1/Orb3 kinase. The phosphorylation of Nak1/Orb3 is periodic during the cell cycle and could be part of a mechanism that relocalizes a constitutively active kinase from the cell tips to the middle of the cell, thereby coordinating reorganization of the actin cytoskeleton and regulation of cell separation with cell-cycle progression.