Abstract
Protein phosphatase 1 (PP1) is a eukaryotic Ser/Thr protein phosphatase, and mediates diverse cellular processes via the association of a catalytic subunit (PP1c) with multiple regulatory subunits that determine the catalytic activity, the subcellular localization, and the substrate specificity. However, no regulatory subunit of PP1 has been identified in plants so far. Here we report the identification of inhibitor-3 (Inh3) as a regulatory subunit of PP1 in Arabidopsis. Inh3 bound to PP1c through the RVxF motif, a consensus PP1c-binding sequence, and inhibited the PP1c catalytic activity. Inh3 was localized in both the nucleus and cytoplasm, and colocalized with PP1c in these compartments. Disruption mutants of Arabidopsis INH3 arrested the embryo development at the globular stage and caused embryo lethality. Transformation of the embryo-lethal inh3 mutant with INH3 complemented the phenotype, whereas mutation of the RVxF motif did not. Furthermore, reduction of INH3 expression by RNAi led to a reduction in fertility. These results define Inh3 as the first regulatory subunit of PP1 in plants, and suggest that Inh3 plays a crucial role in the early embryogenesis in Arabidopsis.
