Molecular Characterization and Evolution of the Protein Phosphatase 2A B′ Regulatory Subunit Family in Plants

Abstract

Type 2A serine/threonine protein phosphatases (PP2A) are important components in the reversible protein phosphorylation events in plants and other organisms. PP2A proteins are oligomeric complexes constituted by a catalytic subunit and several regulatory subunits that modulate the activity of these phosphatases. The analysis of the complete genome of Arabidopsis allowed us to characterize four novel genes, AtBЈ⑀, AtBЈ, AtBЈ, and AtBЈ, belonging to the PP2A BЈ regulatory subunit family. Because four genes of this type had been described previously, this family is composed of eight members. Reverse transcriptase-polymerase chain reaction experiments showed that AtBЈ⑀ mRNAs are present in all Arabidopsis tissues analyzed, and their levels do not respond significantly to heat stress. Expressed sequence tags corresponding to AtBЈ, AtBЈ, and AtBЈ have been identified, indicating that the new genes are actively transcribed. The genomic organization of this family of PP2A regulatory subunits is reported, as well as its chromosomal location. An extensive survey of the family has been carried out in plants, characterizing BЈ subunits in a number of different species, and performing a phylogenetic study that included several BЈ regulatory proteins from animals. Our results indicate that the animal and plant proteins have evolved independently, that there is a relationship between the number of BЈ isoforms and the complexity of the organism, and that there are at least three main subfamilies of regulatory subunits in plants, which we have named ␣, , and .Reversible protein phosphorylation is widely accepted as a major mechanism for the control of biological processes in eukaryotic cells. In plants, reversible protein phosphorylation is involved in processes such as hormonal, pathogenic, or environmental stress responses (Mumby and Walter, 1993;Smith and Walker, 1993; Garbers et al., 1996;Schö ntal, 1998; Janssens and Goris, 2001). In this context, Ser/Thr protein phosphatases (PPs) are important regulatory components of many signal transduction pathways (Ingebritsen and Cohen, 1983a;Schö ntal, 1998). Several Ser/Thr phosphatases, grouped into different categories, have been identified in a variety of plant species. Specifically, homologs of the 1, 2A, and 2C types of animal PPs have been described in plants (Rodríguez, 1998; Lin et al., 1999; Meek et al., 1999). All these types of PPs are distinguished by their different sensitivity to inhibitors and their divalent cation requirements, and are structurally different (for review, see Mumby and Walter, 1993).Type 2A phosphatases (PP2A) are oligomeric enzymes with no obvious requirements for ions or cofactors, and are implicated in a variety of cellular processes (Mumby and Walter, 1993; Janssens and Goris, 2001). In general, the native forms of PP2A proteins exist as oligomeric complexes, constituted by a catalytic subunit (PP2Ac), and one or more regulatory subunits named A and B. Thus, PP2A proteins can be heterodimers, consisting of a PP2Ac catalytic subunit and a type A...

A PP6-Type Phosphatase Holoenzyme Directly Regulates PIN Phosphorylation and Auxin Efflux in Arabidopsis

“…Protein phosphorylation by kinases and dephosphorylation by phosphates represent a major mechanism regulating eukaryotic cell signaling (Terol et al, 2002). Protein phosphatases can be classified into different groups based on their sequence, structure, and catalytic mechanism (Moorhead et al, 2007).…”

“…Besides PID, two other AGC3 kinases, including WAVY ROOT GROWTH1 (WAG1) and WAG2 were also reported to phosphorylate PIN proteins (Dhonukshe et al, 2010). Overexpression of PID, WAG1, or WAG2 leads to comparable root phenotypes, including root meristem collapse and agravitropic root growth, as a result of the basal-to-apical shift of PIN1, PIN2, and PIN4 localization (Dhonukshe et al, 2010).Protein phosphorylation by kinases and dephosphorylation by phosphates represent a major mechanism regulating eukaryotic cell signaling (Terol et al, 2002). Protein phosphatases can be classified into different groups based on their sequence, structure, and catalytic mechanism (Moorhead et al, 2007).…”

“…Protein phosphatases can be classified into different groups based on their sequence, structure, and catalytic mechanism (Moorhead et al, 2007). In general, the PP2A heterotrimeric holoenzyme consists of a catalytic C subunit, a type A regulatory subunit, and a type B regulatory subunit (Terol et al, 2002). Whereas the A regulatory subunits are composed of tandem HEAT repeats that form a hook-like architecture for binding the catalytic and regulatory B subunits, and are hence also known as the scaffold subunits or structural subunits, the type B regulatory subunits of PP2A are diverse (Farkas et al, 2007;Janssens et al, 2008).…”

Protein phosphatase 2A regulatory subunits affecting plant innate immunity, energy metabolism, and flowering time – joint functions among B'η subfamily members

“…4,5 The B' family is divided into 3 subfamilies a, h, and k, where the h subfamily comprises the close homologs B'h, B'g, B'u, and B'z. 4,6 Sequence alignments show 83% identity and 89% similarity for the B'h and B'u pair, and 81% identity and 88% similarity for the B'g and B'z pair. The sequence homology conservation is mostly located in the core region and comprises the distinct domains that are responsible for the interaction with the AC heterodimer.…”

“…The sequence homology conservation is mostly located in the core region and comprises the distinct domains that are responsible for the interaction with the AC heterodimer. [6][7][8] Subcellular targeting of members of the h subfamily has been investigated using transient expression of fluorescent tagged proteins in isolated protoplasts and leaf tissue. 7 B'h was reported to be in nucleus and nucleolus, B'g in nucleus and cytosol, B'z in mitochondria, and B'u in the peroxisomes.…”