Take two pathways — Cdc42–Par6–protein kinase Cξ (PKCξ), and glycogen synthase kinase-3β (GSK-3β)–β-catenin–adenomatous polyposis coli (Apc) — that have previously, but independently, been implicated in the control of cell polarity and what have you got? A single mechanism to spatially control cell polarity, report Alan Hall and Sandrine Etienne-Manneville in Nature.

What lay downstream of Par6–PKCξ in cell-polarity control was unknown, so Hall and Etienne-Manneville studied GSK-3β phosphorylation. Using a scratch-induced cell migration assay, they noticed an increase in the levels of GSK-3 phosphorylated at serine 9 soon after scratching. Anti-phospho-GSK-3 (Ser9) staining occurred predominantly at the leading edge of migrating cells, colocalizing with Cdc42, where Par6 and PKCξ have previously been observed. They also showed that GSK-3β and PKCξ exist in a complex, which dissociates during scratch-induced migration, and that the same complex might also contain Par6. As phosphorylated GSK-3β couldn't be detected in PKCξ precipitates, phosphorylation probably causes GSK-3β to dissociate from the complex.

Inhibiting Cdc42 or PKCξ prevented GSK-3β phosphorylation, whereas transfection of Par6 or PKCξ induced GSK-3β phosphorylation. Phosphorylation of GSK-3β on Ser9 inhibits its catalytic activity, and when the authors microinjected a non-phosphorylatable, constitutively active mutant of GSK-3β (GSK-3 S9A) into leading-edge cells, they saw that it blocked centrosome reorientation, a measure of polarity.

As in the Wnt pathway, inactivation of GSK-3β results in the accumulation of β-catenin, but during astrocyte migration, β-catenin accumulated at the leading edge and not in the nucleus. Although β-catenin is not required for polarity, Apc, another target of GSK-3β in the Wnt pathway, is required. Two to four hours after scratch-induced migration, in a Cdc42-, PKCξ- and phospho-Ser9-GSK-3β-dependent manner, Apc associated with the plus ends of microtubules at the leading edge. Apc binds to microtubules directly or indirectly through the microtubule-binding protein EB1, but EB1 localization at microtubules was independent of Cdc42, PKCξ and GSK-3β. Deletion of the carboxy-terminal domain of Apc containing the microtubule- and EB1-binding sites inhibited centrosome reorientation.

So the spatially restricted association of Apc with the plus ends of microtubules — crucial for establishing polarity — arises from the action of Cdc42 on Par6–PKCξ, which, in turn, results in GSK-3β phosphorylation and its inactivation. GSK-3β might affect several other microtubule-associated proteins, too.