ASK1 controls spindle orientation and positioning by phosphorylating EB1 and stabilizing astral microtubules.

Orientation and positioning of the mitotic spindle are involved in dictating cell division axis and cleavage site, and play important roles in cell fate determination and tissue morphogenesis. However, how spindle movement is controlled to achieve a defined alignment within the dividing cell is not fully understood. Here, we describe an unexpected role for apoptosis signal-regulating kinase 1 (ASK1) in regulating spindle behavior. We find that ASK1 is required for proper mitotic progression and daughter cell adhesion to the substratum. ASK1 interacts with end-binding protein 1 (EB1) and phosphorylates EB1 at serine 40, threonine 154 and threonine 206, enhancing its binding to the plus ends of astral microtubules. Consequently, astral microtubules are stabilized and therefore capable of mediating spindle interaction with the cell cortex, a requirement for spindle movement. These findings reveal a previously undiscovered function of ASK1 in cell division by regulating spindle orientation and positioning, and point to the importance of protein phosphorylation in the regulation of spindle behavior.