B cell receptor-associated protein α4 displays rapamycin-sensitive binding directly to the catalytic subunit of protein phosphatase 2A

Abstract

Recently, TAP42 was isolated as a high copy suppressor of sit4 ؊ , a yeast phosphatase related to protein phosphatase 2A (PP2A). TAP42 is related to the murine ␣4 protein, which was discovered independently by its association with Ig-␣ in the B cell receptor complex. Herein we show that a glutathione S-transferase (GST)-␣4 fusion protein bound the catalytic subunit (C) of human PP2A from monomeric or multimeric preparations of PP2A in a ''pull-down'' assay. In an overlay assay, the GST-␣4 protein bound to the phosphorylated and unphosphorylated forms of C that were separated in two-dimensional gels and immobilized on filters. The results show direct and exclusive binding of ␣4 to C. This is unusual because all known regulatory B subunits, or tumor virus antigens, bind stably only to the AC dimer of PP2A. The ␣4-C form of PP2A had an increased activity ratio compared with the AC form of PP2A when myelin basic protein phosphorylated by mitogen-activated protein kinase and phosphorylase a were used as substrates. Recombinant ␣4 cleaved from GST was phosphorylated by p56 lck tyrosine kinase and protein kinase C. A FLAG-tagged ␣4 expressed in COS7 cells was recovered as a protein containing phosphoserine and coimmunoprecipitated with the C but not the A subunit of PP2A. Treatment of cells with rapamycin prevented the association of PP2A with FLAG-␣4. The results reveal a novel heterodimer ␣4-C form of PP2A that may be involved in rapamycin-sensitive signaling pathways in mammalian cells.

Role of Janus kinase-2 in insulin-mediated phosphorylation and inactivation of protein phosphatase-2A and its impact on upstream insulin signalling components

“…Our observations coincide with a number of recent studies documenting PP-2A association with different kinases [12][13][14][15][16][17][18][19][20][21][22]. For example, studies by Heriche et al [12] indicate that casein kinase 2α associates with PP-2A and causes activation of PP-2A.…”

“…For example, PP-2A has been shown to bind the β2-adrenergic receptor [11], casein kinase 2α [12], homeobox gene 11 [13], tau [14], translation termination eukaryotic release factor 1 [15], adenovirus e4orf4 protein [16], α4 protein [17], neurofilament proteins [18,19] and small t and middle t antigens encoded by DNA tumour viruses [20,21]. Recent studies by Westphal et al [22] identified a calmodulin IV (CaMIV)-PP-2A complex in which PP-2A dephosphorylates…”

Target of rapamycin (TOR): an integrator of nutrient and growth factor signals and coordinator of cell growth and cell cycle progression

“…In mammalian cells, a human ortholog of TAP42 has been identified, the B-cell receptor-binding protein alpha 4 (a4), that interacts with the catalytic subunit of PP2A (Murata et al, 1997). As the rapamycin sensitivity of this interaction is controversial, it is not clear whether the mTOR pathway regulates formation of a phosphatase/Tap42-like complex in mammals, as in yeast (Murata et al, 1997;Nanahoshi et al, 1998).…”

Functional analysis of the PP2A subfamily of protein phosphatases in regulating Drosophila S6 kinase

“…Mammalian cells express a protein (α4/mTap42) that has 23% amino acid sequence identity with yeast Tap42. The α4/ mTap42 protein interacts directly with the catalytic subunits of PP2A, PP4 and PP6 [12][13][14][15] and modulates their enzyme activity [16]. The α4/mTap42 protein has also been reported to interact with mammalian S6K [17].…”