Fass and C3/C4 contribute to the activities and levels of the protein phosphatase 2A catalytic subunit pool and regulate mitotic events in Arabidopsis

The serine-threonine protein phosphatase PP2A holoenzymes regulate many eukaryotic cellular processes. They consist of catalytic (C), regulatory (B) and scaffolding (A) subunits. C3-C4 and their interacting B" (Fass) subunits of PP2A were in the focus of this study. Our tools were Arabidopsis wild-type plants and phosphatase mutants-fass (B"), c3c4 and microcystin-LR (MCY-LR), a specific inhibitor of PP2A/PP1. We have studied their role in the regulation of protein phosphatase activities and expression. In relation to this, we have focused on their involvement in mitotic activity and histone H3 phosphorylation. We show here for the first time that besides C3-C4, B"/Fass subunit has a crucial role in the maintenance of normal protein phosphatase activities and levels in Arabidopsis. fass mutants show increased activities of PP2A and PP1 (a phosphatase related to PP2A) and increased expression of PP2A/C. Both overall PP2A and PP1 activities are inhibited in c3c4. Fass and C3-C4 are involved in the regulation of mitotic activity. C3-C4 have less direct influence on the onset of mitosis, but Fass is involved in the regulation of metaphase-anaphase transition. In wild-type plants, MCY-LR inhibited PP2A/C activities, related to alterations of mitotic events. Distinct effects of MCY-LR in mutants also reveal the role of Fass/C3-C4 in the biochemical functionality of PP2A. Fass and C3-C4 regulate the phosphorylation of histone H3, but this effect is not related to the regulation of mitotic onset in Arabidopsis. This study gives new insights into the regulatory roles of Fass and C3-C4 subunits during mitosis in plants.