MinireviewFunctions of B56-containing PP2As in major developmental and cancer signaling pathways
Introduction
The reversible phosphorylation of proteins, regulated by opposing functions of protein kinases and protein phosphatases, is one of the major mechanisms that control the stability, localization, and function of numerous proteins and is essential for all aspects of biology. During phosphorylation, protein kinases transfer phosphate groups from ATP to the hydroxyl side chain of three amino acid residues: serine (Ser), threonine (Thr), and tyrosine (Tyr). In humans, the majority of phosphorylation occurs on Ser and Thr residues and is catalyzed by more than 400 Ser/Thr protein kinases (Manning et al., 2002). These kinases often recognize specific peptide sequences and exhibit distinct substrate specificities. In contrast to the large number of protein kinases, there are only a dozen of genes that encode catalytic subunits of phosphoprotein phosphatases (PPPs). Five of them encode catalytic subunits of PP1 (Sasaki et al., 1990) and PP2A (Stone et al., 1987), the two most abundant Ser/Thr protein phosphatases. Catalytic subunits of PP1 and PP2A have fairly broad substrate specificity and are capable of dephosphorylating many phophoproteins in vitro. In the cell, however, PP1 and PP2A catalytic subunits are always associated with other subunits to form multimeric enzymes. These subunits often modify the substrate specificity and catalytic activity of the enzyme and localize catalytic subunits to specific intracellular compartments or protein complexes.
In the case of PP2A, PP2A exists predominantly as a heterotrimer. The holoenzyme consists of a catalytic subunit (C), scaffold subunit (A), and variable regulatory subunit (B). C subunits are encoded by two distinct genes, Cα and Cβ (Stone et al., 1987). These two proteins are abundantly expressed and make up to 0.1% of total cellular proteins (Ruediger et al., 1991). A subunits, also known as PR65, are encoded by two distinct genes as well (Hemmings et al., 1990). While Aα and Aβ share 87% identity, Aα is found in ~ 90% of PP2A holoenzymes; only 10% of PP2A holoenzymes contain the Aβ subunit. The diversity of PP2A holoenzymes derives largely from B subunits. The human genome contains at least 15 regulatory subunits of PP2A, falling into four regulatory subunit families. These include B/B55/PR55, B′/B56/PR61, B″/PR72, and B‴/PR93/PR110. In addition, many PP2A regulatory subunits are alternatively spliced/translated. This ensures assembly of a large number of distinct PP2A heterotrimeric holoenzymes. Interestingly, formation of the heterotrimeric complex not only affects the substrate specificity and subcellular localization of PP2A, but also regulates the stability of PP2A subunits. In Drosophila S2 cells, knockdown of B subunits accelerates turnover of A and C subunits, and vice versa (Li et al., 2002, Silverstein et al., 2002). Similarly, mammalian C and most B (B/B55/PR55 and B′/B56/PR61) subunits are stable only when they complex with the A subunit (Chen et al., 2005, Li and Virshup, 2002, Sablina et al., 2007, Strack et al., 2004, Strack et al., 2002). Monomeric subunits are degraded rapidly through the ubiquitin/protesome protein degradation pathway (Strack et al., 2004, Strack et al., 2002). Interestingly, it has been noted that the stability of yeast PP2A subunits is not linked to heterotrimer formation (Gentry and Hallberg, 2002, Wei et al., 2001, Wu et al., 2000).
The purpose of this review is to provide a focused discussion on the structure, intracellular localization, and functions of B′/B56/PR61-containing PP2A holoenzymes, with an emphasis on their functions in major developmental and cancer pathways. Due to the scope of the paper, post-translational modification of A or C subunits will not be discussed, although it plays essential roles in regulating PP2A holoenzymes. We must also omit some important functions of B56-containing PP2As, for example, regulation of circadian rhythms (Sathyanarayanan et al., 2004), some important transcription factors (Firulli et al., 2003), and nutrient signaling by B56-containing PP2As (Yan et al., 2010). Due to the page limit, we have to apologize to some colleagues for not discussing their elegant studies on phosphorylation of B56 (Ahn et al., 2007, Letourneux et al., 2006, Margolis et al., 2006, Ruvolo et al., 2008, Usui et al., 1998). Readers are referred to a number of recent reviews for information about other forms of PP2A and other protein phosphatases (Eichhorn et al., 2009, Janssens et al., 2008, Shi, 2009, Virshup and Shenolikar, 2009). From here on, B′/B56/PR61 will be referred to as B56.
Section snippets
Structure of B56 regulatory subunits
All five mammalian B56 family members, including B56α (PPP2R5A), B56β (PPP2R5B), B56δ (PPP2R5D), B56ε (PPP2R5E), and B56γ (PPP2R5C), were discovered in the mid 1990s (Csortos et al., 1996, McCright et al., 1996a, McCright et al., 1996b, McCright and Virshup, 1995, Tehrani et al., 1996). Later, B56 subunits in other species were identified. Unlike vertebrates (from zebrafish to mammals), lower organisms such as fission yeast (Tanabe et al., 2001, Tehrani et al., 1996) and Drosophila (Berry and
Intracellular localization of B56-containing PP2A
An important function of B56 subunits is to localize PP2A holoenzymes to specific intracellular compartments. Since monomeric B56s are degraded rapidly in vivo (Chen et al., 2005, Li and Virshup, 2002, Sablina et al., 2007, Strack et al., 2004, Strack et al., 2002), localization of B56s reflects the intracellular distribution of B56-containing PP2A holoenzymes. During mitosis, B56 family members can be detected at centromeres from the prophase to metaphase (Kitajima et al., 2006, Riedel et al.,
Functions of B56s in major developmental pathways
Currently, functions of B56s during mammalian embryonic development remain unclear. While all five B56s are expressed in embryonic day 7 mouse embryos (Martens et al., 2004), their spatial expression patterns have not been investigated. In addition, none of B56 genes has been knocked out in mouse. Our knowledge about functions of B56s during development was derived from a few studies performed in lower organisms. These studies, together with studies in tissue culture cells, demonstrate that
Functions of B56s in tumorigenesis
Tumorigenesis by definition is uncontrolled cell growth, which is often caused by genetic mutations or exposure to carcinogens that inactivate the DNA damage response and the mitotic checkpoint functions. During malignant transformation, many cancer cells are characteristic of chromosomal instability. Increasing data suggests that B56-containing PP2As are involved in DNA damage response and the mitotic checkpoint regulation.
Remaining questions
One and a half decades of B56 studies has led to many exciting discoveries. While many important functions of B56s have already been uncovered, we are still far from a thorough understanding of this important PP2A regulatory subunit family. Many questions remain, ranging from structural to functional aspects. For example, more structural analysis is needed to understand the dynamics of B56-containing PP2A holoenzyme assembly and to identify potential regulatory mechanisms. Existing data
Conflict of interest statement
The authors declare that there are no conflicts of interest.
Acknowledgements
JY is supported by grant 1R01GM093217-01A1 from NIGMS. CP is supported by grant 1R01AG031833 from NIA.
References (117)
Wnt/PCP signaling: a veritable polar star in establishing patterns of polarity in embryonic tissues
Semin Cell Dev Biol
(2006)- et al.
Cyclin G2 associates with protein phosphatase 2A catalytic and regulatory B′ subunits in active complexes and induces nuclear aberrations and a G1/S phase cell cycle arrest
J Biol Chem
(2002) - et al.
Multiple protein phosphatases are required for mitosis in Drosophila
Curr Biol
(2007) - et al.
High complexity in the expression of the B′ subunit of protein phosphatase 2A0. Evidence for the existence of at least seven novel isoforms
J Biol Chem
(1996) - et al.
Protein phosphatase 2A regulatory subunits and cancer
Biochim Biophys Acta
(2009) - et al.
PKA, PKC, and the protein phosphatase 2A influence HAND factor function: a mechanism for tissue-specific transcriptional regulation
Mol Cell
(2003) - et al.
Nuclear export and centrosome targeting of the protein phosphatase 2A subunit B56alpha: role of B56alpha in nuclear export of the catalytic subunit
J Biol Chem
(2010) - et al.
PP2A holoenzyme assembly: in cauda venenum (the sting is in the tail)
Trends Biochem Sci
(2008) - et al.
Hedgehog signaling in development and cancer
Dev Cell
(2008) - et al.
The 48-kDa alternative translation isoform of PP2A:B56epsilon is required for Wnt signaling during midbrain–hindbrain boundary formation
J Biol Chem
(2009)
PP2A:B56epsilon, a substrate of caspase-3, regulates p53-dependent and -independent apoptosis during development
J Biol Chem
Rec8 phosphorylation by casein kinase 1 and Cdc7-Dbf4 kinase regulates cohesin cleavage by separase during meiosis
Dev Cell
PP2A regulates BCL-2 phosphorylation and proteasome-mediated degradation at the endoplasmic reticulum
J Biol Chem
A functional genomics analysis of the B56 isoforms of Drosophila protein phosphatase 2A
Mol Cell Proteomics
Wnt/beta-catenin signaling: components, mechanisms, and diseases
Dev Cell
Role for the PP2A/B56delta phosphatase in regulating 14-3-3 release from Cdc25 to control mitosis
Cell
Genomic organisation, chromosomal localisation tissue distribution and developmental regulation of the PR61/B′ regulatory subunits of protein phosphatase 2A in mice
J Mol Biol
Identification of a new family of protein phosphatase 2A regulatory subunits
J Biol Chem
Assignment of human protein phosphatase 2A regulatory subunit genes b56alpha, b56beta, b56gamma, b56delta, and b56epsilon (PPP2R5A-PPP2R5E), highly expressed in muscle and brain, to chromosome regions 1q41, 11q12, 3p21, 6p21.1, and 7p11.2 -- > p12
Genomics
The B56 family of protein phosphatase 2A (PP2A) regulatory subunits encodes differentiation-induced phosphoproteins that target PP2A to both nucleus and cytoplasm
J Biol Chem
Mechanistic insight into how Shh patterns the vertebrate limb
Curr Opin Genet Dev
Developmental roles and clinical significance of hedgehog signaling
Curr Top Dev Biol
Genomic organization and mapping of the gene encoding the PP2A B56gamma regulatory subunit
Genomics
Cyclin G recruits PP2A to dephosphorylate Mdm2
Mol Cell
Hedgehog signalling: how to get from Smo to Ci and Gli
Trends Cell Biol
A PP2A regulatory subunit regulates C. elegans insulin/IGF-1 signaling by modulating AKT-1 phosphorylation
Cell
Neural and head induction by insulin-like growth factor signals
Dev Cell
A positive role for the PP2A catalytic subunit in Wnt signal transduction
J Biol Chem
The IGF pathway regulates head formation by inhibiting Wnt signaling in Xenopus
Dev Biol
Inhibition of B56-containing protein phosphatase 2As by the early response gene IEX-1 leads to control of Akt activity
J Biol Chem
PP2A:B56epsilon is required for eye induction and eye field separation
Dev Biol
Regulation of convergence and extension movements during vertebrate gastrulation by the Wnt/PCP pathway
Semin Cell Dev Biol
PKR regulates B56(alpha)-mediated BCL2 phosphatase activity in acute lymphoblastic leukemia-derived REH cells
J Biol Chem
The tumor suppressor PP2A Abeta regulates the RalA GTPase
Cell
Posttranslational regulation of Drosophila PERIOD protein by protein phosphatase 2A
Cell
p53-dependent apoptosis pathways
Adv Cancer Res
Serine/threonine phosphatases: mechanism through structure
Cell
Wnt signaling and dorso-ventral axis specification in vertebrates
Curr Opin Genet Dev
Protein phosphatase 2A holoenzyme assembly: identification of contacts between B-family regulatory and scaffolding A subunits
J Biol Chem
Critical role for protein phosphatase 2A heterotrimers in mammalian cell survival
J Biol Chem
Molecular cloning of a 74-kDa regulatory subunit (B″ or delta) of human protein phosphatase 2A
FEBS Lett
Molecular heterogeneity of the cDNA encoding a 74-kDa regulatory subunit (B″ or delta) of human protein phosphatase 2A
FEBS Lett
PP2A is required for centromeric localization of Sgo1 and proper chromosome segregation
Dev Cell
Protein kinase A activates protein phosphatase 2A by phosphorylation of the B56delta subunit
Proc Natl Acad Sci USA
Phosphorylation status of the SCR homeodomain determines its functional activity: essential role for protein phosphatase 2A,B′
EMBO J
Molecular basis for PP2A regulatory subunit B56alpha targeting in cardiomyocytes
Am J Physiol Heart Circ Physiol
Making a grade: sonic hedgehog signalling and the control of neural cell fate
EMBO J
Cancer-associated PP2A Aalpha subunits induce functional haploinsufficiency and tumorigenicity
Cancer Res
Structural and biochemical insights into the regulation of protein phosphatase 2A by small t antigen of SV40
Nat Struct Mol Biol
Crystal structure of a protein phosphatase 2A heterotrimeric holoenzyme
Nature
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