Protein phosphatase 2A (PP2A) comprises a family of serine/threonine phosphatases into the phosphoprotein phosphatase (PPP) family.
The PP2A core enzyme consists of a 65 kDa scaffolding protein known as A or PR65 subunit and a 36 kDa catalytic subunit or C subunit. To gain full activity towards specific substrates, the PP2A core enzyme associates with a variable regulatory subunit to form a heterotrimeric holoenzyme. The variable regulatory subunits have four subfamilies: B (PR55), B′ (B56 or PR61), B″ (PR72), and B″′ (PR93/PR110), the B subunits determine the substrate specificity as well as the spatial and temporal functions of PP2A. The two catalytic metal ions located at bottom of PP2A's active site have been recently identified as manganese, which had previously been suggested as potential candidates.
The PP2A scaffolding subunit contains 15 tandem repeats of a conserved 39-residue sequence known as a HEAT (huntingtin-elongation-A subunit-TOR) motif (Hemmings et al., 1990,Walter et al., 1989). These 15 HEAT repeats are organized into an extended, L-shaped molecule (Groves et al., 1999). The catalytic subunit recognizes one end of the elongated scaffolding subunit by interacting with the conserved ridges of HEAT repeats 11–15 (Ruediger et al., 1992,Ruediger et al., 1994,Xing et al., 2006). Formation of the PP2A core enzyme results in significant bending of the HEAT repeats 12–15 towards the amino terminus of the scaffolding subunit (Xing et al., 2006). The scaffolding subunit interacts with B subunit toward the carboxyl terminus.
Structure of the Protein Phosphatase 2A Holoenzyme Yanhui Xu,Yongna Xing,Yu Chen,Yang Chao,Zheng Lin,Eugene Fan,Jong W. Yu,Stefan Strack,Philip D. Jeffrey,and Yigong Shi
Regulation of PP2A
Although there are only two C subunit isoforms of PP2A, the number of potential combinatorial associations of the diﬀerent A and B regulatory subunits is very large. Given the occurrence of two A, two C, and eight B a total of about 75 diﬀerent dimeric and trimeric PP2A holoenzymes can be generated. This specific holoenzyme composition provides many possibilities for regulation. First, due to their specific cellular and subcellular localizations, the third subunits can target the PPase to diﬀerent tissues and cellular compartments. Secondly, the presence of diﬀerent regulatory subunits has been shown to determine the substrate specificity of PP2A holoenzymes. Thirdly,the presence or absence of additional subunits can modulate theresponse to agents that modify PP2A activity (such as protamine and heparin) and aﬀects the catalytic activity of PP2A towards the same substrate.
PP2A and the cell cycle: phosphatase negatively regulates entry into mitosis
PP2A is required to maintain MPF (M-phase-promoting factor) in its inactive precursor form. The regulatory functions of PP2A at the G\M transition are summarized in Figure. However, it is clear that, in order to full these functions, the activity of PP2A has to be tightly regulated as well. In particular, for progression into mitosis PP2A has to be inactivated, and at the exit from mitosis it has to be re-activated.
The interaction of PP2A with viral proteins reveals its role in cell transformation
Protein phosphatase 2A: a highly regulated family of serine/threonine phosphatases implicated in cell growth and signalling Veerle JANSSENS and Jozef GORIS
Cellular transformation by the small DNA tumour viruses simian virus 40 (SV40) and polyoma virus depends on the expression of the so-called tumour antigens, which form multiple complexes with cellular proteins involved in signal transduction and growth control, in order to change their normal functions. PP2A is another important cellular target for these viral antigens. Polyoma small t and middle T, as well as SV40 small t, form stable complexes with PP2A by displacing the A subunit [Pallas, D. C., Shahrik, L. K., Martin, B. L., Jaspers, S., Miller, T. B., Brautigan, D. L.and Roberts, T. M. (1990) Polyoma small and middle T antigens and SV40 small]. PP2A is a tumor suppressor protein. Mutations in both the α and β isoforms of the scaffolding subunit have been found in cancer. Glu64 of Aα was mutated to Asp in lung cancer and to Gly in breast cancer (Ruediger et al., 2001b).
The interaction of PP2A with SV40 small t alters the substrate specificity of PP2A and inhibits PP2A enzyme activity towards some substrates the viral antigens, particularly SV40 small t, target PP2A to overcome its negative role in some signalling pathways leading to increased cell proliferation. Another mechanism by which SV40 small t promotes cell growth and transformation is by stimulating PKC activity, resulting in MEK activation and nuclear factor-jB (NF-jB)-dependent transactivation. Following inhibition of PP2A, PKCf and NF-jB apparently become constitutively active. A third transcriptional target of small t (in addition to AP-1 and NF-jB) is CREB (cAMP regulatory element binding protein). Small t inhibits dephosphorylation of PKA-phosphorylated CREB and thereby stimulates CREB-dependent transactivation