The variable subunit associated with protein phosphatase 2A0 defines a novel multimember family of regulatory subunits

Abstract

Two protein phosphatase 2A (PP2A) holoenzymes were isolated from rabbit skeletal muscle containing, in addition to the catalytic and PR65 regulatory subunits, proteins of apparent molecular masses of 61 and 56 kDa respectively. Both holoenzymes displayed low basal phosphorylase phosphatase activity, which could be stimulated by protamine to an extent similar to that of previously characterized PP2A holoenzymes. Protein micro-sequencing of tryptic peptides derived from the 61 kDa protein, termed PR61, yielded 117 residues of amino acid sequence. Molecular cloning by enrichment of specific mRNAs, followed by reverse transcription-PCR and cDNA library screening, revealed that this protein exists in multiple isoforms encoded by at least three genes, one of which gives rise to several splicing variants. Comparisons of these sequences with the available databases identified one more human gene and predicted another based on a rabbit cDNA-derived sequence, thus bringing the number of genes encoding PR61 family members to five. Peptide sequences derived from PR61 corresponded to the deduced amino acid sequences of either alpha or beta isoforms, indicating that the purified PP2A preparation was a mixture of at least two trimers. In contrast, the 56 kDa subunit (termed PR56) seems to correspond to the epsilon isoform of PR61. Several regulatory subunits of PP2A belonging to the PR61 family contain consensus sequences for nuclear localization and might therefore target PP2A to nuclear substrates.

GSK-3β-dependent phosphorylation of adenomatous polyposis coli gene product can be modulated by β-catenin and protein phosphatase 2A complexed with Axin

“…Therefore, it remains to be clari®ed which subunit of PP2A, the C or A subunit, interacts with the central region of Axin. To the dimeric core of the C and A subunits, a variable third regulatory (B, B'/B56, or PR72/PR59) subunit binds, and it modulates the enzymatic activity of PP2A (Usui et al, 1988;Zolnierowicz et al, 1996). It is intriguing to speculate that the third subunit regulates the interaction of PP2A with Axin and that Axin itself might be another regulatory protein for PP2A.…”

“…In the reciprocal experiment, little endogenous Axin was co-precipitated with FLAG-PP2Ac in L cells (data not shown). This result suggests that only a small fraction of PP2Ac binds to Axin because PP2Ac has several binding proteins (Zolnierowicz et al, 1996). Alternatively, the catalytic subunit of PP2A alone may not be su cient for the complex formation of PP2A with Axin.…”

Functional interaction between a novel protein phosphatase 2A regulatory subunit, PR59, and the retinoblastoma-related p107 protein

“…The most prevalent isoform, called PR55, B55, or simply B subunit, is encoded by at least three related genes (Mayer et al, 1991;Pallas et al, 1992). Five genes have been cloned that fall into one closely related gene family of PR61 subunits (also called B56, or B' subunit) (McCright and Virshup, 1995;Tehrani et al, 1996;Zolnierowicz et al, 1996). One gene that gives rise to two di erentially spliced mRNAs encodes two B-subunits, PR72 (also called B'') and PR130 (Hendrix et al, 1993a).…”

Phosphorylation and activation of phosphodiesterase type 3B (PDE3B) in adipocytes in response to serine/threonine phosphatase inhibitors: deactivation of PDE3B in vitro by protein phosphatase type 2A

“…It is likely that the PP2A peaks consist of different PP2A holoenzymes and that the broadness of the second peak could indicate that it consists of more than one PP2A isoform. Three families of trimeric holoenzymes, which differ by the presence of variable subunits (B, Bh and Bd), as well as a dimeric PP2A # composed of PP2Ac and the A structural subunit, have been described to exist in mammalian cells, with differential expression in different tissues [24][25][26][27][28]30,50]. It cannot be totally excluded that some of the PDE3B phosphatase activity seen in fractions 20-22 is due to PP1.…”

“…So far, no information on the presence of PP4, PP5, PP6 and PP7 in adipocytes has been reported. The catalytic subunits of PP1, PP2A and PP2B exist in i o as dimeric or trimeric complexes with variety of regulatory subunits [24][25][26][27][28]30]. Incubation of isolated rat adipocytes with okadaic acid, an inhibitor of PP2A and PP1, has been shown to lead to the activation of PDE3B [31], suggesting that one or both of these phosphatases are involved in determining the phosphorylation and activity state of PDE3B.…”