GSK3β phosphorylation modulates CLASP-microtubule association and lamella microtubule attachment

Polarity of the microtubule (MT) cytoskeleton is essential for many cell functions. Cytoplasmic linker-associated proteins (CLASPs) are MT-associated proteins thought to organize intracellular MTs and display a unique spatio-temporal regulation. In migrating epithelial cells, CLASPs track MT plus ends in the cell body but bind along MTs in the lamella. In this study, we demonstrate that glycogen synthase kinase 3 beta (GSK3 beta) directly phosphorylates CLASPs at multiple sites in the domain required for MT plus end tracking. Although complete phosphorylation disrupts both plus end tracking and association along lamella MTs, we show that partial phosphorylation of the identified GSK3 beta motifs determines whether CLASPs track plus ends or associate along MTs. In addition, we find that expression of constitutively active GSK3 beta destabilizes lamella MTs by disrupting lateral MT interactions with the cell cortex. GSK3 beta-induced lamella MT destabilization was partially rescued by expression of CLASP2 with mutated phosphorylation sites. This indicates that CLASP-mediated stabilization of peripheral MTs, which likely occurs in the vicinity of focal adhesions, may be regulated by local GSK3 beta inactivation.