Diversity in subcellular targeting of the PP2A B'η subfamily members

Protein phosphatase 2A (PP2A) is a serine/threonine-specific phosphatase comprising a catalytic subunit (C), a scaffolding subunit (A), and a regulatory subunit (B). The B subunits are believed to be responsible for substrate specificity and localization of the PP2A complex. In plants, three families of B subunits exist, i.e. B (B55), B', and B''. Here, we report differential subcellular targeting within the Arabidopsis B'eta subfamily, which consists of the close homologs B'eta, B'theta, B'I-3 and B'I . Phenotypes of corresponding knockouts were observed, and particularly revealed delayed flowering for the B'I center dot knockout. The B' subunits were linked to fluorescent tags and transiently expressed in various tissues of onion, tobacco and Arabidopsis. B'I center dot and B'I-3 targeted the cytosol and nucleus. B'I localized to the cytoplasm and partly co-localized with mitochondrial markers when the N-terminus was free. Provided its C-terminus was free, the B'I subunit targeted peroxisomes. The importance of the C-terminal end for peroxisomal targeting was further confirmed by truncation of the C-terminus. The results revealed that the closely related B' subunits are targeting different organelles in plants, and exemplify the usage of the peptide serine-serine-leucine as a PTS1 peroxisomal signaling peptide.