SAPCD2 phosphorylation modulates astral microtubule stability to control spindle orientation.

Proper spindle orientation is essential for organogenesis and tissue homeostasis. Cortical tethering complexes, such as the Gαi-LGN-NuMA complex, anchor astral microtubules to guide spindle orientation. However, how astral microtubule stability is coupled to mitosis-dependent regulatory mechanisms remains unclear. Here, we identified suppressor APC domain-containing 2 (SAPCD2) as a cell cycle-regulated factor that functionally links astral microtubule stability to spindle orientation control, utilizing non-polarized HeLa and HaCaT cell culture models. Live-cell imaging revealed that siRNA-mediated depletion of SAPCD2 induces spindle misorientation, leading to mitotic delay and aberrant cell division. Mechanistically, SAPCD2 acts as a previously uncharacterized microtubule-associated protein that promotes microtubule stability, whereas CDK1-mediated phosphorylation at S157, together with phosphorylation at S276, generates a mitosis-specific phosphorylated isoform that destabilizes astral microtubules. Importantly, disruption of SAPCD2 phosphorylation at both S157 and S276 fails to rescue the spindle misorientation caused by SAPCD2 depletion. These findings deepen our understanding of how mitosis-dependent dynamic phosphorylation of microtubule-associated proteins orchestrates astral microtubule behavior to ensure accurate spindle orientation.