Increases in cyclin A/Cdk activity and in PP2A-B55 inhibition by FAM122A are key mitosis-inducing events

Entry into mitosis has been classically attributed to the activation of a cyclin B/Cdk1 amplification loop via a partial pool of this kinase becoming active at the end of G2 phase. However, how this initial pool is activated is still unknown. Here we discovered a new role of the recently identified PP2A-B55 inhibitor FAM122A in triggering mitotic entry. Accordingly, depletion of the orthologue of FAM122A in C. elegans prevents entry into mitosis in germline stem cells. Moreover, data from Xenopus egg extracts strongly suggest that FAM122A-dependent inhibition of PP2A-B55 could be the initial event promoting mitotic entry. Inhibition of this phosphatase allows subsequent phosphorylation of early mitotic substrates by cyclin A/Cdk, resulting in full cyclin B/Cdk1 and Greatwall (Gwl) kinase activation. Subsequent to Greatwall activation, Arpp19/ENSA become phosphorylated and now compete with FAM122A, promoting its dissociation from PP2A-B55 and taking over its phosphatase inhibition role until the end of mitosis. Synopsis PP2A-B55 inhibition at the G2-M transition is essential for mitotic entry, but Gwl and cyclin B1 kinases activating the inhibitor Arpp19 are still inactive in late G2. This work reveals FAM122A as an additional phosphatase inhibitor acting prior to phospho-Arpp19. Inhibition of PP2A-B55 at late G2 is promoted by FAM122A, which does not require phosphorylation to bind this phosphatase target. FAM122A is essential for mitotic entry in vivo in C. elegans . FAM122A-dependent inhibition of PP2A-B55 triggers mitotic entry by permitting cyclin A/Cdk-dependent phosphorylation events and Gwl-dependent Arpp19 phosphorylation. Once phosphorylated, phospho-Arpp19 replaces FAM122A from PP2A-B55 and takes over inhibition of this phosphatase for the remainder of mitosis. Structural models indicate that phospho-S67-Arpp19 may interact with ions in the phosphatase catalytic subunit, thereby mediating tighter association that displaces FAM122A. The sequential inhibition of PP2A-B55 by FAM122A and Greatwall substrate Arpp19 is crucial to trigger cyclin B/Cdk1 activation and a correct mitosis.